Hi
SALINITY IN PAMLICO SOUND, NORTH CAROLINA
A SPACE-TIME APPROACH
A Thesis
Presented to
the Faculty of the Department of Geography and Planning
East Carolina University
In Partial Fulfillment
of the Requirements for the Degree
Master of Arts in Geography
by
Jonathan David Phillips
April 1982
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»AST CAROLINA UNIVERSITY
SALINITY IN PAMLICO SOUND, NORTH CAROLINA
A SPACE-TIME APPROACH
by
Jonathan David Phillips
APPROVED BY:
DIRECTOR OF THESIS :r -'?L / ‘rr ' j
Richard A. Stephensoj/
CHAIRMAN OF THE DEPARTMENT OF GEOGRAPHY AND PLANNING
DEAN OF THE GRADUATE SCHOOL
Jo^ph G. "Boyette
Jonathan David Phillips. SALINITY IN PAMLICO SOUND, NORTH CAROLINA:
A SPACE-TIME APPROACH. (Under the direction of Richard A. Stephenson)
Department of Geography and Planning, April, 1982.
The Pamlico Sound estuarine system is a complex one, with human,
physical, and biotic components. These components interact and are
spatially linked with alterations of one component related to the others.
The purpose of this study was to establish the temporal behavior of a
physical component, salinity, and to relate this behavior to human and
biotic components.
From 1948-1981 salinity levels showed evidence of a gradual long
term declining trend throughout most of the study area. This trend is
not explainable in terms of climatic variation or other factors. During
the same time period, anthropic drainage activities increased dramatically
in lands adjacent to the study area. The spatial variation in salinity
trends was found to have an areal association with the level of anth-
ropic drainage impact. Additionally, the salinity and drainage factors
were related, temporally and spatially, to the spatial displacement of
oysters. Oyster displacement is an example of the biotic responses that
often accompany salinity changes. A functional relationship exists be-
tween estuarine salinity, land use in adjacent areas, and estuarine biota.
Spatial linkages exist in that alterations of land by man-made drainage
are related to salinity and biotic changes at other locations.
ACKNOWLEDGEMENTS
The invaluable aid of my thesis advisor, Dr. Richard A. Stephenson,
and committee members. Dr. Donald Steila, Dr. Ennis L. Chestang, and
Dr. Charles W. O'Rear, is gratefully acknowledged. Also invaluable in
the data-gathering stages of this project were Dr. David Phelps, and
the North Carolina Division of Marine Fisheries, especially Jess Haw-
kins, Terry Sholar, and Doug Mumford. Special acknowledgements go to
Dr. Charles Ziehr for aid and encouragement during my time at East
Carolina, to the Phillips family for constant support and encouragement,
and to the Bryan family of Vanceboro, North Carolina, for convincing me
that an ex-newspaper hack could get a Master's degree.
TABLE OF CONTENTS
PAGE
LIST OF TABLES iv
LIST OF FIGURES v
CHAPTER I. INTRODUCTION 1
Problem Statement 2
Salinity as a Spatial Determinant 5
Study Area 7
Methodology 12
CHAPTER II. SALINITY TRENDS 15
General Patterns of Salinity 15
Factors Related to Salinity Distributions 15
Modelling 18
The Salinity Data Set 19
Long Term Salinity Trends 21
Summary 30
CHAPTER III. ENVIRONMENTAL RESPONSES TO SALINITY CHANGES 36
Oysters as an Environmental Indicator 36
Salinity Ranges for Oysters 38
Spatial Displacement of Oysters 39
Prehistoric Oyster Displacement 43
Summary 44
CHAPTER IV. SPATIAL VARIATION OF SALINITY
AND ANTHROPIC DRAINAGE 45
Salinity and Anthropic Drainage 45
Spatial Variation in Salinity Trend 46
Factors Related to Salinity Changes 47
Areal Association of Salinity and Drainage 49
Summary 63
CHAPTER V. CONCLUSIONS 65
Temporal Trends 65
Spatial Relationships 66
Conclusions 67
Future Prospects 71
BIBLIOGRAPHY 73
APPENDIX A 80
LIST OF TABLES
TABLE PAGE
1. PEARSON PRODUCT-MOMENT CORRELATION COEFFICIENTS,
BY SUBAREAS, FOR SALINITY AND TIME 27
2. SALINITY TREND, DRAINAGE AND OYSTER DISPLACEMENT
IN PAMLICO SUBAREAS 58
3. CONTINGENCY TABLES AND TESTS OF SIGNIFICANCE FOR
AREAL ASSOCIATIONS OF SALINITY TREND, ANTHROPIC
DRAINAGE IMPACTS, AND SPATIAL DISPLACEMENT OF OYSTERS 61
LIST OF FIGURES
FIGURE PAGE
1. THE STUDY AREA 8
2. SALINITY TREND LINES, BY SUBAREAS 28
3. SALINITY CORRELLOGRAMS 29
4. SPECTRAL DENSITY PLOTS 31
5. HISTORIC OYSTER RANGES 42
6. ANTHROPIC DRAINAGE WITHIN THE STUDY AREA 53
CHAPTER ONE
INTRODUCTION
Estuaries where fresh water mixes with salt water are among the
most dynamic, fragile, and productive areas on earth. These vital zones
have considerable ecological, economic, recreational, and aesthetic
value (see Clark, 1977; U.S. Department of Interior, 1970; U.S. Fish and
Wildlife Service, 1970). The complexity and fragility of brackish es-
tuaries associated with barrier islands is expressed within an environ-
mental im.pact statement on barrier islands:
. . . the brackish water, is extremely valuable
as a fishery and for its production of vast quanti-
ties of seafood. With its protected waters, its
recreational value is also undoubted, as these
waters support numerous marinas and waterfront sub-
divisions. It must be remembered, however, that
this brackish water is itself an ecosystem of
finely-tuned associations of freshwater, saltwater
transported nutrients and a variety of plant and
animal species. The plants and animals are con-
ditioned to and depend upon the natural ambient
conditions, including the flux of salinity, the
seasonal changes of temperature or dissolved oxy-
gen, and, especially, the protection offered by
the barrier islands from major storm winds and
tides. These water bodies are fragile in that they
do not require too serious an intrusion to upset
the natural dynamic balance. (U.S. Department of
Interior, 1979:99).
One such area is the Pamlico Sound in North Carolina. Intrusions, some
serious, are currently having an impact on the area, and may be in danger
of upsetting this "natural dynamic balance". The purpose of this study
is to examine one such intrusion: Anthropic drainage of adjacent lands,
its effects on salinity, a key factor in estuaries, and the environmental
response to salinity changes.
2
Rapid change in salinity and other variables is the norm in estu-
aries. Mangelsdorf (1967) once estimated that summer salinity gradients
in Chesapeake Bay were so local and transient that a precision of plus or
minus 0.003 parts per thousand would be wasted unless location was speci-
fied to within 40 feet, time of sampling to within 90 seconds, and depth
to within a quarter of an inch, Denman and Platt (1978:227) note that
such natural dynamism is compounded by bad weather, costs, equipment
failures and other factors:
The net result is that a brute-force attack on the
system, where we are armed with the ideal data set,
is impossible. Rather, we must attempt through
judicious experimental design and sampling to iso-
late certain processes or interactions for study.
The potential importance of the problems in the Pamlico area call for
just such an approach.
Problem Statement
The objectives of this study are threefold: (1) To determine the
spatial and temporal patterns of salinity in the western portion of Pamli-
CO Sound, and its estuarine tributaries, (2) to document environmental
response to salinity trends, and (3) to establish a functional relation-
ship, in terms of spatial linkages, between estuarine salinity, environ-
mental response, and land use of adjacent lands (specifically drainage).
Land use in a watershed is linked to water quality via natural and man-
made channels. Salinity and anthropic drainage may be said to be spa-
tially linked if the variation in man-made channels is related to the
variation in salinity of receiving waters.
3
Officials concerned with fisheries and environmental management in
the coastal zone have for some time suspected a general long term de-
crease in salinity in western Pamlico Sound and the Pamlico and Neuse
River estuaries, A relationship with drainage of nearby lowlands for
agriculture and forestry has also been suspected, in addition to rela-
tionships between salinity and estuarine species such as shrimp and
oysters. In 1981, for example, the North Carolina Assistant Secretary
for Natural Resources reported to the State Water Conference that sali-
nity in Pamlico Sound had been declining for 25 years as part of a call
for more applied research in the region (Grigg, 1981). The North Caro-
lina Water Quality Task Force in 1981 received a report on water pro-
blems in the state's estuaries from the North Carolina Office of Coastal
Management (Schmidt, 1981). The study stated that a 10-mile downstream
migration of oysters in the Pamlico River was apparently related to a
declining salinity trend, and also noted a suspected salinity decline
in the Neuse River. The report linked these phenomena to coastal area
land drainage.
Sholar (1980) concluded in a North Carolina Division of Marine Fish-
eries report that there is evidence of a long term salinity decline in
the Pamlico Sound area, possibly accompanied by a biological response.
Another Division of Marine Fisheries official, James T. Brown, stated
in 1978 that freshwater intrusion and declining salinities were a major
problem in North Carolina estuaries because of adverse effects on com-
mercial species (Doucette and Phillips, 1979:5-11).
Other observers have noted the possible problems, including sali-
nity changes, related to man-made drainage activities in the Pamlico
4
area. Heath (1975:75) stated in a U.S. Geological Survey Report that
"the effect of runoff from the region (the Pamlico-Albemarle peninsula)
on the quality of nearby estuarine waters, chiefly Albemarle and Pamli-
CO Sounds, may be the most important problem posed by agricultural
developments." The newsletter of the North Carolina Sea Grant program
also indicated problems in estuaries posed by adjacent or upstream changes
in land use. The director of the program, B.J. Copeland, observed that
large-scale land use changes, such as drainage projects, pose the great-
est current threat to the environmental quality of North Carolina estu-
aries (University of North Carolina Sea Grant, 1980).
Macro-scale studies of spatial linkages of major components of estu-
arine systems have focused on morphological variables rather than on re-
lationships between anthropic, physical, and ecological factors. Research
related to this study exist in three general forms: (1) systematic stu-
dies similar in focus but on a micro-scale of one small estuary, (2)
general studies of salinity regimes in estuaries, and (3) studies of land
use changes and anthropic drainage activities in eastern North Carolina.
The only previous attempt at a macro-scale investigation of salinity
trends in Pamlico Sound is Sholar (1980). This report did not consider
drainage impact, however, and presented no spatial analysis of salinity
data. Sholar's study may be considered the springboard for this study.
The magnitude of the problems involving salinity and land use in
the Pamlico area cannot be fully known for some time. It is clear at
present, however, that there is recognition of problems and potential
problems involving salinity, land use, and environmental quality in
5
Pamlico Sound and other estuaries. This study intends to define vari-
ables relevant to the problems indicated above.
Salinity as a Spatial Determinant
Salinity of water refers to the salt content of water, and is for-
mally defined in this context as the total amount of dissolved seawater
in parts per thousand (ppt) by weight when all the carbonate has been con-
verted to oxide, the bromide and iodide to chloride, and all organic mat-
ter is completely oxidized (Gagliano, ejt 1970B:6). Salinity readings
presented herein are in parts per thousand sodium chloride.
Salinity is related to the distribution, survival, health, and re-
production of estuarine organisms and has been called an "ecological
master factor" (Kinne, 1966). Salinity has historically been used as an
index to the ecology of the coastal zone (Gunter, 1961; Gagliano, et al,
1970B:6). Variations in the number, diversity, individual size, and
spatial distribution of species have been linked to spatial and temporal
variations in salinity with organisms affected by low, high, and unstable
salinity (Gunter, 1956; 1961; Wells, 1961).
The critical role of salinity is now generally accepted as given by
coastal area planners and managers. For example, a study on areas of en-
vironmental concern in North Carolina coastal areas cautions that changes
in freshwater discharge that alter salinity patterns in the state's estu-
aries "will be reflected directly in the functioning of the estuary and
this in turn will control the plant and animal populations in the system"
(Thayer, 1975:64). This is also reflected in other treatments of coastal
zone management (Clark, 1977).
6
More specifically. North Carolina Division of Marine Fisheries stu-
dies have noted the observed and potential effects of salinity changes
on commercial species in western Pamlico Sound (Pate and Jones, 1980;
Sholar, 1980). Numerous studies exist where salinity is used as an
independent variable in studies of species distributions. Examples of
such work within the area of interest include studies of fish distribu-
tions in the Neuse River (Keup and Bayless, 1964), macrobenthos in the
Pamlico River (Tenore, 1970), and the epifauna of the Pamlico (Reed,
1978).
Salinity is also a key physical parameter in estuaries. Water den-
sity, specific gravity, conductivity, and temperature are all related to
salinity. Salinity is thus a major factor in estuarine water circula-
tion (Dyer, 1973). Salinity is also related to various other chemical
components of estuarine waters. Oxygen depletion and occurrence of sedi-
mentary phosphorus, for example, have both been linked to salinity in
the Pamlico River (Davis, et 1978; Upchurch, 1972).
Because of its importance as an estuarine component, salinity is
often used as a boundary criterion when delimiting regions for coastal
area planning, management, and research. Clark (1977) recommends a sali-
nity criterion in delimiting administrative and planning boundaries of
coastal areas. Smith (1977) uses salinity as an indicator of the boun-
dary of the estuary itself.
Salinity has implications for biological distributions and ecologi-
cal functions in estuaries, determining in large measure the biogeography
of the regions, and, by extension, man's use of these areas. When this
is considered along with the role of salinity in water circulation and
7
as a boundary criterion, it is clear that changes in salinity regimes
have serious spatial ramifications.
Study Area
General description
The study area is located in the central coastal plain of North
Carolina. The study area includes the following waters and their adja-
cent lands: Pamlico Sound west of the 75°50' west meridian, and the
Pamlico River upstream to the U.S. 17 bridge at Washington, N.C., the
Neuse River to the U.S. 17 bridge at New Bern, N.C., and the Pungo River
at the U.S. 264 bridge at Leechville, N.C. The area is shown in Figure 1.
Delimitation of the area is based on physiography. The 75°50' west
meridian coincides roughly with Bluff Shoal, which spans Pamlico Sound
from Bluff Point to Ocracoke Island, dividing the sound into eastern and
western portions. The upstream boundaries are at or near the points where
the respective rivers widen, decrease noticeably in velocity, and have
salinity levels high enough at least part of the year to support estuarine
species.
Pamlico Sound is the largest body of water in North Carolina and the
largest water body behind a barrier island system on the United States
coast (National Oceanic and Atmospheric Administration, 1980:91). It is
part of an interconnected system of North Carolina estuaries extending
from the Virginia border to Beaufort, N.C., more than 130 miles to the
south. It is nearly 100 miles from Washington at the head of the Pamlico
River to Cape Natteras at the easternmost boundary of the sound.
The study area was chosen because of the perceived declining sali-
nity problem in the area, and the ecological, economic, recreational, and
FIGURE 1: STUDY AREA
9
aesthetic importance of the area to North Carolinians.
One property of the system related to all the above benefits is the
role of the estuary as a diverse and productive biological zone. Illu-
strative of this is the value of commercial fishing in the region. The
Pamlico area is a nursery for shrimp, croaker, and other commercial spe-
ci es that spawn in the estuary and may be harvested in the estuary or at
sea. Other species, such as oysters, crabs, eels, and brown shrimp, are
harvested directly from the area. Catch statistics show the area leading
North Carolina in brown shrimp, eel, and oyster production, and in the
past Pamlico Sound has yielded more than 80 percent of the state's oysters
(Chestnutt, 1951:155). Belhaven, on the Pungo River, is the largest sin-
gle center for blue crab processing on the east coast. Commercial land-
ings for Hyde, Beaufort, Pamlico, and Craven Counties, bordering the
study area, totalled an estimated $23,578,424 in dockside value in 1979
(N.C. Division of Marine Fisheries, 1980). This represents approximately
47 percent of the dockside value reported for North Carolina in 1979.
Physical setting
The study area is in the South Atlantic Estuarine Region. This region
is characterized by a wide continental shelf, contacting the Gulf Stream
at its margins. A low, sediment-rich coastal plain is dissected by ri-
vers, with drainage systems terminating in islands and salt marshes (U.S.
Department of Interior, 1970:12). Pamlico Sound is in the southernmost
portion of the "embayed" section of the Atlantic Coastal Plain, character-
ized by large drowned estuaries. The North Carolina coastal plain is under-
lain by an eastward-dipping monoclinal wedge of sediments deposited in the
10
Mid-Mesozoic era over basement of crystalline rock (Hardaway, 1980:12).
The region is within the area commonly referred to as the outer coastal
plain or tidewater region, a low-lying area averaging not more than 20
feet in elevation (Stuckey, 1965), The study area is astride several
marine terraces of the Pleistocene age. The most significant of these
features with respect to this study is the Suffolk Scarp, which separates
the Pamlico Terrace from the Talbot Terrace to the west. East of the
scarp, which has an elevation of about 25 feet, lies the lowest land
with the poorest natural drainage (Stuckey, 1965),
According to the classification scheme developed by Pritchard (1967),
Pamlico Sound is a bar-built estuary impounded behind the Outer Banks, a
chain of barrier islands. The region also has characteristics of the
drowned river valley estuary.
Hydrography
Surface waters within the study area are broad and shallow. Nauti-
cal charts show a maximum depth of 25 feet at mean low water with many
areas less than five feet deep. Typical open-water depths in the upper
rivers and bays is six to eight feet with 15 to 20 feet in the lower ri-
vers and midsound areas. Nearshore depths are one to three feet. Shoal-
ing is common with several major shoals all extending generally from the
mainland toward Ocracoke Inlet: Bluff Shoal, tending north-south at the
eastern edge of the study area; Middle Ground, extending southeasterly
from Swan Quarter; and Brant Island Shoal, protruding from Goose Creek
Island parallel to Middle Ground. Charts also indicate shoaling around
most points and meanders in creeks and rivers, at tributary mouths, and
n
in extensive areas around Point of Marsh. Navigation routes include a
portion of the Atlantic Intracoastal Waterway. These routes typically
follow shifting natural channels, and some dredged channels at sediment-
rich creek mouths and in intermittent stretches of the Neuse and Pamlico
Rivers.
Long fetches result in frequent choppy water. Pamlico Sound pro-
per is approximately 65 miles at its longest and 25 miles at its widest
with additional fetches across major tributary reaches and adjacent sounds.
The entire area is well-mixed in general and is considered wind-dominated
with respect to short-term water levels and circulation (Giese, et al,
1979:71-122). Freshwater input is the major long term determinant of
water levels and circulation in the study area. The chief sources are
the Neuse-Trent and Tar-Pamlico (the Tar River becomes the Pamlico River
at Washington, N.C.) River systems. Albemarle Sound also drains into
Pamlico Sound via Roanoke and Croatan Sounds. Including Albemarle Sound
and its tributaries, Pamlico Sound receives drainage from nearly 31,000
square miles. The Neuse-Trent and Tar-Pamlico systems combined drain
12,500 square miles--about one fifth of the land area of North Carolina
(Giese, et. 1979:71). Numerous small, lateral tributaries supply ad-
ditional fresh water to the sound.
The major ocean connection in western Pamlico Sound is Ocracoke
Inlet. To the northeast are Natteras and Oregon Inlets. All are narrow,
shifting outlets. Smaller still are several minor inlets connecting Core
Sound to the Atlantic Ocean south of the study area. Due to these re-
strictive connections and the protection of the islands, nautical charts
indicate a diurnal lunar tidal range of less than half a foot throughout
most of the region.
12
Socioeconomic Setting
The counties surrounding the Pamlico area are predominantly rural.
New Bern, Washington, and Havelock are the only settlements with 1980
census populations greater than 3,000. These are also the only sites
with significant industry with the exception of a large phosphate mining
and processing operation in the vicinity of Aurora. Dominant economic
activities include agriculture (corn, soybeans, tobacco, hogs), forest
products, commercial fishing, and tourism. Although portions of the
area are relatively undeveloped due both to their remoteness and to physi
cal constraints, there are densely-settled areas of recreational and
retirement housing along numerous waterfront areas. Per capita income
has historically been lower in the area than in the rest of North Caro-
lina, and until the 1970s the region experienced a net loss of population
Methodology
The research objectives required collection of three kinds of data
for the post World War II time period: salinity data, information rela-
ting to environmental responses to salinity, and data on land use and
anthropic drainage activities.
Salinity data for the study area are among the best for any area of
North Carolina. Very little salinity data were gathered before 1948, how
ever, and data gathered since then is derived from an uncoordinated hodge
podge of sources. The data is non-continuous, and few researchers used
the same sample stations, sampling frequency, methods, or degree of pre-
cisión. Thousands of salinity readings from 1948-1981 were gathered in
13
1980-1981 from a variety of sourcesJ This data was updated, supple-
merited and organized. The result was a raw data set consisting of
7,805 observations from 22 different sources (Appendix A).
Oyster bed persistence is utilized herein as an indicator of envir-
onmental response. These organisms have been used as an indicator of
estuarine water quality (see Chapter 3), and the fact that oysters are
non-motile and congregate in beds lends them utility in spatial analysis.
Changes in the spatial distribution of oyster beds is considered an indi-
cator of spatial changes in water quality or character. Data on past and
present locations of oyster beds was gathered from field observations,
interviews with area residents, and interviews with veteran oystermen
reported by Sholar (1980). In addition, information on prehistoric spa-
tial displacement of oysters may be inferred from the location of Indian
middens (refuse heaps) containing oyster shells. This raw data was ob-
tained from the East Carolina University Archaeological Laboratory.
Land use information needs focused on drainage of lowlands adjacent
to waters of the study area. Delimitation of drained areas and identifi-
cation of anthropic drainage outlets was achieved through interpretation
of U.S. Geological Survey 7%-minute series topographic maps and U.S. Army
Corps of Engineers color aerial photographs taken in 1978. This was sup-
plemented when required by field observations.
The research strategy requires analysis of data to establish spatial
^Most of the data were originally gathered by Jess Hawkins and
Terry Sholar of the North Carolina Division of Marine Fisheries with ad-
ditional data gathered by the author. Hawkins, Sholar, and the author
cooperated in organizing the raw data set.
14
and temporal variations in salinity and accompanying environmental re-
sponse. With these variations established, the areal association of
drainage practices with salinity variation is examined to determine the
possibility of spatial as well as temporal relationships between the phen-
omena and to determine functional spatial linkages.
CHAPTER TWO
SALINITY TRENDS
General Patterns of Salinity
Salinity in the study area ranges from freshwater (0 ppt) upstream
to near seawater (about 34 ppt) at Ocracoke Inlet. Except for the in-
let, salinity values greater than 30 ppt are rare. A mean salinity as
high as 20 ppt was not found. Mean salinity values, based on all read-
ings in the data set, range from 2.5 ppt in the upper Pamlico and Neuse
River estuaries to 18-19 ppt in some areas of Pamlico Sound.
Horizontal salinity gradients are usually east-west oriented with
salinity generally increasing eastward toward the Atlantic Ocean. This
pattern is modified by wind, circulation patterns, variable inflows from
given sources, spatial variation in inflows, and the rotation of the earth
Wind is especially important. Some researchers have noted a north-south
salinity gradient in Pamlico Sound when strong, prolonged winds blow sa-
line water north from Core Sound or freshwater south from Albemarle Sound
(Roelofs and Bumpus, 1953:185-188; Woods, 1967:108).
Variation in salinity is greatest near freshwater sources and inlets,
but is variable throughout the study area (Roelofs and Bumpus, 1953:185).
A biological study using four sampling stations in the Pamlico River estab
lished, for example, that salinity varied by as much as 13 ppt in conse-
cutive months at the same station. Spatial variation is also apparent,
as upstream and downstream stations varied by as much as 15 ppt on the
same day (Dean, 1973:27).
Vertical salinity stratification is rare, since the study area is
16
shallow and wind-dominated, creating well-mixed conditions. Stratifica-
tion does occasionally occur in the rivers when freshets override denser
"salt wedges" of higher salinity water (Roelofs and Bumpus, 1953:191).
Seasonally, salinity tends to be higher in winter (January and Decern-
ber) and lowest in spring (April). But this is highly variable, and in
various locations in the study area, annual maximum and minimum readings
have been recorded in every month. Salinity patterns generally respond
to precipitation events with a lag time of one to two months (Giese, et
ai, 1979:88; Roelofs and Bumpus, 1953:193-194; Woods, 1967:108). It is
doubtful salinity can be predicted with much precision based on climatic
factors. Wax and others (1978), for example, found salinity patterns in
Louisiana too complex to be explained by weather alone. This has not been
demonstrated for Pamlico Sound, but it seems reasonable to assume for
the Pamlico area.
Factors Related to Salinity Distributions
A number of factors interact to affect the mixing of seawater and
freshwater at a given location in an estuary. This mixing is what deter-
mines salinity levels. Freshwater can be supplied by streamflow, basal
groundwater flow, direct precipitation, overland runoff, and anthropic in-
puts. Seawater enters the bar-built estuary via the inlets, storm over-
wash on the barrier islands, and saltwater intrusion into the groundwater
reservoir. These sources are related to each other and to wind, circu-
lation, and tides.
The spatial and temporal variations in estuarine salinity are such
that attempts at modelling are difficult at best and fruitless at worst.
17
Newbolt and Herbich (1970), in studying salinity chiefly as an ecological
factor, suggest that a five percent margin of error is acceptable for
merely gathering data. Therefore a complex set of interactions deter-
mining the spatial distribution of salinity in an estuary can be, and
probably should be, simplified.
Saltwater intrusion and storm overwash are generally minor factors
influencing estuary-ocean interchange. Marshall (1951:46) observed that
virtually all oceanic exchange in western Pamlico Sound is through Ocra-
coke Inlet. This small, shifting inlet has remained open throughout all
of recorded history (Stick, 1958:8).
Streamflow is by far the dominant freshwater source to the estuary.
Overland runoff directly to the Pamlico estuary rarely occurs because of
the permeable soils and lack of significant relief in the region. Ground-
water flows are also negligible and may be considered, for analytical
purposes, a subsurface extension of streamflow (Gagliano, et 19708:9).
Man-made freshwater inputs and direct precipitation are also negligible
compared to streamflow. The strong relationship of stream discharge and
estuarine salinity has been noted by many investigators (Schroeder, 1978;
Bronfman, 1977; Moskowitz, 1976; Mather, ^ al_, 1973: Gagliano, et al,
1970A; 1970B; Keighton, 1966; Cohen and McCarthy, 1962).
As noted, wind has an extraordinary influence in the study area on
short term water levels and circulation. Salinity at a given point in
the area may be viewed as a result of streamflow, oceanic exchange through
Ocracoke Inlet, and wind-driven currents.
18
Modelling
A number of models exist to model estuarine flows and to predict
water quality parameters, including salinity, under given conditions of
flow, tide, wind, temperature, and various other inputs. Examples of more
general estuary models focusing on salinity include the studies of water
balance in Louisiana estuaries (Gagliano, ^ ll, 1970A), models designed
to predict saltwater intrusion in estuaries in Delaware and Ecuador
(Keighton, 1966; Sanmuganathan, 1979), and climate-based models in the
Delaware estuary (Mather, et 1973).
Several of the numerous models available for estuarine simulation
have been developed in or for waters in eastern North Carolina. Lauria
and O'Melia (1980), for example, developed an engineering model for hy-
drology and nutrient loading in the Pamlico River, while water quality
management models have been developed for the Chowan River (Amein and
Galler, 1979). A number of hydrological models exist which could be ap-
plied to prediction of freshwater inputs to an estuary, including the
Wiser (1976) model, which was tested on the Neuse River basin. None of
the available models, or modified versions based on the same principles,
are applicable to this study for several reasons.
First, many of the models require that the estuary in question be
treated as a one-dimensional flow model. Use of a one-dimensional flow
model is not acceptable in the study area which has a complex multidimen-
sional flow pattern. Application of such models would require disaggre-
gating the study area into numerous sub-basins, then integrating the
disaggregated model—a prohibitively expensive and time consuming task.
19
Second, many models require data that are unavailable and/or would be
prohibitively expensive to gather within the study area, including de-
tailed data on stream discharge, hydrography and channel geometry, and
chemical measures such as specific conductance and viscosity. Even if
such information were available, the area is so large and hydrologically
diverse as to make application difficult.
Pamlico Sound is under strong influence from two major drainage ba-
sins which rise in the Piedmont, numerous minor drainage areas originating
in the Coastal Plain, a number of interconnected estuaries, and the At-
1 antic Ocean. The nature of the study area, lack of available data, and
the prohibitive expense of generating such data makes use of a model,
desirable in theory, virtually impossible at present.
The Salinity Data Set
Salinity data were collected, organized, and set up for computer
retrieval and analysis with cooperation of Jess Hawkins and Terry Sholar
of the North Carolina Division of Marine Fisheries. Consistent data with
regular sampling is not available for North Carolina estuaries. The 7,805
observations were derived from a number of different sources (Appendix A).
The data, from June, 1948 through June, 1981, (the few pre-1948 ob-
servations were dropped from the analysis, and analysis was terminated at
June, 1981) are intermittent. Few researchers through the years used the
same sampling locations, methods, or degree of precision. The following
steps were taken as this information was assembled into a usable data set:
(1) Readings were plotted on a map of the study area with one map for
each month for which salinity data were recorded. In many cases only one
20
value, a surface or midwater reading, was available. Where both surface
and bottom values were given for the sample, the arithmetic mean was plotted
This inconsistency should be minimized by the lack of vertical stratifica-
tion in the well-mixed estuary. At least one researcher began his salinity
observations by taking surface and bottom readings, only to find the dif-
ferences so minute that he began taking surface readings only in the name
of efficiency (Roelofs and Bumpus, 1953:184).
(2) There are a number of methods for measuring and expressing salinity
Observations not expressed in parts per thousand sodium chloride were con-
verted with all observations rounded to tenths.
(3) Sampling has been intermittent since 1948 with no readings at all
collected for 1962-1963, and relatively few in some other periods. Any
available reading was plotted with time specified only by the month.
(4) Sampling locations varied widely over the years. The chosen re-
course to express the areal variation in salinity in a consistent manner was
to superimpose a grid over the study area. The original grid system, used
by Sholar (1980) and retained here, consisted of uniform squares each repre-
senting approximately 41.11 square miles. Readings were also specified ac-
cording to ten larger subareas shown in Figure 1. These subareas are com-
posed of groupings of the smaller, uniform grid cells. These locational
groupings were designed to obtain as complete a time-series record as pos-
sible for as many data cells as possible, while still retaining the spatial
variation in salinity within the study area.
(6) The final raw data set includes 7,769 observations with salinity,
month, year, sample grid cell number, and subarea number indicated for each
observation.
21
For statistical analyses it was found that the most satisfactory manner
of handling this data was to compute mean monthly salinity for each subarea
for each month from 1948-1981. The data set used in statistical analysis
consists of one observation per month from June, 1948 through June, 1981
with month, year, and mean salinity for each subarea specified.
The data base suffers from lack of consistency, and statistical results
must be viewed in light of the data deficiencies. The fact that this limited
data base is the best of its type in North Carolina points to the need for
coordinated, long term water quality and hydrologic data collection.
Long Term Salinity Trends
Statistical methods
The testable hypothesis is that throughout the fluctuation of salinity
within the study area, there is a general, detectable long term decline in
salinity through time. In terms of the slope, b, of a trend line describing
a salinity time series, the hypothesis and null hypothesis (H^) may be
stated:
H : b = 0 (1)
0
b 0 (2)
Simple linear regression is used to test this hypothesis with the F-ratio
significance test used to test whether b (and other coefficients) is signi-
ficantly different from 0 at the 0.05 level. Additionally, autoregression
and spectral analysis are utilized to analyze the salinity data for cyclic
trends. Traditional techniques of time series analysis generally require
continuous data. The salinity data set does not meet the requirements for
most forms of time series analysis, but portions of the data set are usable
for autoregression and spectral analysis.
22
Simple linear regression was utilized due to its relative ease of com-
putation and interpretation and its utility for quantifying the relation-
ship between two variables. Salinity was the dependent variable with the
variable "time" as the independent variable. Each value of time represents
the year with the month expressed as a fraction. For example, January, 1977
would be 1977.0 and December, 1977 would be 1977.92. Several useful sta-
tisties were computed: Pearson's product-moment correlation coefficient
(r) and the coefficient of determination (r ) to test the degree of associ-
ation between the variables, ordinary least squares (OLS) best-fit trend
line parameters to approximate the linear salinity trend, and F-ratios to
test for statistical significance of coefficients and parameters.
Regression and correlation are commonly used techniques and are dis-
cussed in most basic statistics texts. Use of this technique does, however,
violate one of the assumptions (albeit a rather frequently-ignored assump-
tion) required for linear regression. Salinity values are autocorrelated--
in other words, salinity in a given month depends to some extent on the
salinity the previous month, and observations are technically not completely
independent. For each set of OLS residuals from subareas 1-10, the Durbin-
Watson statistic (d) was computed to test for autocorrelation:
^ _ ^t(et - et - 1 (3)
where t is time and the e^'s are OLS residuals. Residuals represent the dif-
ference between observed and predicted values of the dependent variable and
are standardized by dividing each residual by the standard error of estimate.
For each subarea, d values were such that at the 95 percent confidence level.
23
the null hypothesis of no autocorrelation was rejected. Ostrom (1978) dis-
cusses the Durbin-Watson statistic and the Neil-Thagar criteria for estab-
lishing limits.
Autoregression can be used for time-series data where observations oc-
cur at regularly-spaced intervals and where errors (residuals) are posi-
tively autocorrelated. Only subareas one (the upper Pamlico River) and
five (the area of Rose, Swanquarter, and Juniper Bays) had enough continu-
ous data to merit use of this procedure, and then only with the 1970-1981
period considered. The sporadic missing data in these sets of observations
were replaced with the overall mean salinity values for the respective sub-
areas to preserve the time series integrity. Autoregression was utilized
for two purposes: (1) to compare autoregression trend line parameters which
adjust for autocorrelation with OLS parameters to see if the same basic
trends are indicated, and (2) to investigate the possible presence of cyclic
trends in the data by analyzing correllograms produced by autoregression.
The autoregression procedure first estimates OLS regression parameters
and residuals, then computes autocorrelations up to lag q of the residuals.
For example, if q= 28, as in this analysis, and e^ is the residual at time
t, correlations would be computed for e^ with , ^^+2" ’ ‘ ®t+28* ^
of 28 was used because several authors suggest that the number of lags should
be less than 25 percent of the number of observations (Haan, 1977:286). With
144 observations, 28 is approximately 20 percent of n.
A correllogram is obtained by plotting the autocorrelation functions
against the lags. Corellogram analysis can indicate whether there is a deter-
ministic component, such as a seasonal cycle, as well as a stochastic compon-
ent in the time series. Since the lag represents 28 months, seasonal salinity
24
cycles should be detectable. If the autocorrelation r(q) for q suffi-
ciently large is significantly different from zero or if the correllogram
plot oscillates regularly about a line representing a zero autocorrelation,
the possible presence of a deterministic component should be investigated
(Haan, 1977:279-286). Autoregression also computes trend line parameters
which account for the non-independence of observations and that may be
more accurate predictors than OLS parameter estimates. This is of little
interest here, since much of the data is inappropriate for autoregression.
We can compare autoregression parameters with OLS parameters for the two
data groupings used in autoregression.
The OLS trend line for subarea one is described by Y = 201.384 - .lOlX,
while the autoregression trend line is Y = 163.716 - 0.081X. Subarea five
has an OLS equation of Y = 1017.997 - .510X and an autoregression equation
of Y = 986.928 - 0.494X. Though there are significant differences, the
basic trends indicated by autoregression and OLS are similar. For this rea-
son it was considered valid to use Pearson correlations and OLS regression
lines for purposes of comparing salinity trends among the subareas.
Salinity is related to freshwater input and, thus, to precipitation.
Since precipitation is subject to both seasonal and long term cycles, it
may be reasonably suspected that the salinity data should express a cyclic
component. Such a component might well be of longer duration than would be
detectable in a 28-month lag in a corellogram. Spectral analysis was utilized
to detect any cyclic components which might be present in the 12-year spans
of available data and to identify any cycles which may account for large
portions of the variation in salinity.
25
Data requirements for spectral analysis are similar to those for auto-
regression, so the same portions of data were used. These time series were
first approximated by a fourier series. A fourier series is a series of
sine and cosine waves of the form:
f(x) = + a1,cosX + b1,sinX +0 a.2,cos2X + b2.,sin2X . . .a cos nX + b nsin nX (4)n
where a and b are fourier coefficients and X is salinity. Mechanics and theory
of finding fourier series are discussed by Gowar and Baker (1974). Pro-
minent cycles can be identified by plotting the spectral densities of the
series against the frequencies and periods of the spectra. Spectral density
F|^ of X is given by:
J= -P
? X .
where W. are the smoothing weights and I, is the periodogram of X, defined by
J 7 2 ^
= (n/2) (aj^ + bjj ). (6)
A high peak of spectral density with steady ascents and descents of the
plotted points on either side indicates that much of the spectrum is "ex-
plained" by a cycle of the period or frequency corresponding with the peak.
In spectral density plots, a single peak or "spike" indicates dominance of
a single cycle (Haan, 1977:280-286).
Simple linear regressions were calculated with the REGRESSION proce-
dure of the Statistical Package for the Social Sciences (SPSS). Autore-
gression and spectral analysis computations were performed with the AUTOREG
and SPECTRA procedures of the Statistical Analysis System (SAS). Computa-
tional details and documentation are found in the SPSS and SAS manuals (Nie,
et al, 1975; SAS Institute, 1979).
26
Results of simple linear regression
Correlation coefficients for salinity and time, by subareas, are pre-
sented in Table 1. OLS trend lines for subareas with statistically signi-
ficant parameters at the 0.05 level are given in Figure 2.
Correlations are relatively low, indicating absence of a direct,
linear relationship between salinity and time. Upstream areas have no signi-
ficant trend at all, according to the coefficients and trend line parameters,
while subareas two, three, and five through nine show statistically signi-
ficant negative correlations. This indicates that an indistinct negative
relationship may exist in those subareas with salinity showing signs of a
general decline through time. The trend is described by the basic equation
Y = a + bX (7)
for each subarea, where Y is salinity, X is time, a is the intercept con-
stant, and b is the slope of the regression line. These trend lines are
useful for comparing the general salinity trend as shown in Figure 2, but
are not meaningful predictors due to the autocorrelation mentioned previously.
Correlation coefficients and trend lines show evidence of a declining
salinity trend through time in at least seven of ten subareas. This trend
appears strongest and most pronounced along the northeast shore of the study
area from the mouth of the Pamlico River to the eastern boundary; and in an
area near and just south of the mouth of the Neuse River. The declining sa-
linity trend is nonexistent in the upper Pamlico and Neuse River estuaries.
For the seven subareas indicated in Figure 2, the null hypothesis is rejected
and the research hypothesis of significant salinity trends is accepted.
Results of autoregression and spectral analysis
Corellograms for subareas five and one are shown in Figure 3. The
27
Table 1
Pearson product-moment correlation, coefficients, by subareas,
for salinity and time. Significance is determined by F-tests.
Subarea Correlation coefficient Significance
1 Upper Pamlico River 0.02400 0.38306*
2 Mid Pamlico River -.20480 0.00406
3 Confluence of
Pamlico-Pungo Rivers -.22812 0.00101
4 Upper Pungo River omitted due to lack of observât!(
5 Swanquarter, Rose, and
Juniper Bay area -.39707 0.00000
6 NE portion of study area -.53021 0.00000
7 Area between mouths of
Neuse and Pamlico Rivers -.20237 0.00907
8 Neuse River/South River/
West Bay area -.45349 0.00000
9 Mid Neuse River estuary -.23961 0.00017
10 Upper Neuse River estuary 0.14610 0.04925
*Not statistically significant at the 0.05 level.
Source: Computations by the author.
28
T?-)3
Subareas
2 Y = J3120X
3 Y = J0847X
5 Y = .16103X
6 Y = .18297X
7 Y = .08022X
8 Y = .18569X
9 Y = .09381X
Figure 2: Salinity trend lines by subareas
Subarea one
-no.
Figure 3: Salinity correllograms.
30
plots show the autocorrelation functions for both subareas rapidly ap-
preaching zero and oscillating irregularly about the line representing
zero autocorrelation. The correllograms indicate the absence of a deter-
mi ni Stic component and tends to suggest that the salinity time series are
basically stochastic processes in that there are no intrinsic cycles. This
is confirmed by the spectral analysis.
Plots of spectral density against frequency and period for subareas
five and one are shown in Figures 4a-4d. Notable by their absence in all
four plots are prominent "spikes"—portions of the plot where points rise
steadily and steeply. Absence of spikes is indicative of a stochastic pro-
cess without intrinsic deterministic cycles. The highly irregular nature
of the plots for both subareas is indicative of the complex nature of the
salinity trend as numerous irregular fluctuations in the spectrum are de-
tected.
Correllogram and spectral analysis of a limited portion of the salinity
data do not indicate any cycles that might possibly explain the negative
salinity trends indicated by simple linear regression. If these results
may be generalized to the entire data set, it may be said that any long term
salinity trend is not an intrinsic property of the salinity regime itself.
Summary
Simple linear regression of salinity against time indicates a long
term general decline in salinity in much of the study area. This trend is
stronger in some areas and nonexistent in others. The negative trend is
not a strong, linear trend, but statistically significant indications of a
decline are evident.
1Ó
s II
E
G
T
R
12
A
L il
II
owZ
I •
T
Y 8 -
4
«
« «« ««« «««
Il «« lift à «HHI» ««««ilMtt K *
«llkll «HHU ll|»««llKÜMII«ttllllllllll lllt
I I I l
0.59 1.18 1.78 .37 2.96
FREQUENCY FRCM O TO PI
Figure 4a: Spectral density vs. frequency for subarea five U)
16
12
t/JZKO
C OB»
II£
30 60 90 120
PEHIOD (RADIAM3) 150
Figure 4b: Spectral density vs. period for subarea five (B-2 observations, C-3 observations, etc.)
U>
ro
6.0t_
a
p
lËcTR 4.?AL « «Ot*3NIT 3.0Y m« ñ «1.5 H 4t«M •«« « « «II « «««« ««« ft «« «ft* ftftftft * ft««II* •<>«• ««• ««• •JL.
0.59 I.IR 1.V8 2.37 2.96
FREQUENCY FROM 0 TO PI
CJ
CO
Figure 4c: Spectral density vs. frequency for subarea one.
6.(»-
S
P
E
C
T
R «
A
L
II
C
w
2
c/5 ti
I
T
Y 3.0 ë
ti
il
«
««
«
«
• B *
« li
F D » » *
JC B
CDD»
B
J X to
30 60 90 1R0 150
PERIOD (RADIANS)
Figure 4d: Spectral density vs. period for subarea one (B=2 observations, C=3 observations, etc.)
OJ
-P»
35
No evidence was found through autoregression and spectral anlaysis of
cyclic phenomena inherent in the salinity time series which might explain
the negative salinity trends. If the time series is not deterministic,
causes for any negative trend must be the independent factors which deter-
mine salinity in a given location.
Within the study area, salinity at a given location is basically a
result of freshwater input from streams and oceanic exchange via Ocracoke
Inlet modified by wind. If the statistical analysis accurately reflects
actual salinity trends, the trend should be explainable in terms of those
factors.
CHAPTER THREE
ENVIRONMENTAL RESPONSES TO SALINITY CHANGES
Oysters as an Environmental Indicator
Salinity is such a crucial ecological factor in estuaries that any
significant change in salinity regimes will likely be accompanied by
some sort of environmental response. If the quantitative analysis of
the preceding chapter is indicative of actual salinity trends, then we
would expect an observable environmental response. Area residents per-
ceive a change in the species composition of many waters within the study
area with saltwater species replaced by freshwater species. An indicator
of environmental response is needed which is responsive to salinity changes
and the location of which can be reliably determined. The Atlantic Oy-
ster, Crasseostrea virginica, can be used for this purpose.
Oysters are filter feeders, extracting nutrients from the water, and
are highly sensitive to water quality. Unlike fish and other motile spe-
cies, the non-motile oyster must experience prolonged or frequent environ-
mental changes that affect mortality and/or reproduction in order to be
spatially displaced. Unlike floral species, oysters are not quickly or
easily reestablished in a given location after displacement has occurred
(Van Sickle, et 1976).
Oysters meet both criteria for use as an indicator of environmental
response to salinity trends. They are sensitive to salinity variation,
and their location can be reliably determined at a given time. Oysters
live in beds (also called lumps, rocks, and reefs) that are noted in both
their presence and disappearance by watermen—not only because the shell-
fish are harvested commercially, but because the mass of shell and sub-
37
strate in an oyster bed represents a hazard to small-boat navigation.
Oyster sensitivity to salinity is well established. They are affected
by temperature, food availability, water circulation, bottom character,
predation, disease, competition and commensalism, turbidity, sediments,
pollution, and human harvesting. Van Sickle and others (1976) investigated
these and other factors, determining that salinity is the most important
environmental factor for oysters. Freshwater influxes from storms or
floods which rapidly lower salinity levels can wipe out oysters and other
species inhabiting oyster beds (Munden, 1975; Wells, 1961; Butler, 1952).
High salinity can also damage the shellfish. Hoese (I960) found that oy-
sters in Texas were decimated by high salinity during a prolonged drought
with survivors later killed by low salinity when heavy rains ended the
drought. Oyster diseases and predators are also related to salinity (Haven,
etiL> 1978:260-269).
Sholar (1980) proposed using the Atlantic oyster as an indicator of
salinity changes in Pamlico Sound. Use of oysters as an environmental in-
dictor in this way does have several precedents. Butler (1952) found that
oyster bed mortalities in Mississippi Sound were a reliable indicator of
the importance of floodwaters from various sources to the biota of the
Sound. Lee (1979) notes that upstream penetration of natural oyster reefs
serves as an indicator of salinity in tributaries of Mobile Bay and East
Mississippi Sound. Van Sickle and others (1976) found that even after
other factors affecting oyster populations were accounted for, oyster dis-
tributions and salinity changes were highly correlated.
38
Salinity Ranges for Oysters
While there is little disagreement concerning the importance of sa-
linity for oysters, the tolerance and preference ranges are subject to
debate. The consensus is that at low salinity levels (either prolonged
or frequent) oyster larvae are unlikely to "set", or attach themselves
to substrate. Existing oysters are less likely to reproduce. At high
salinity levels predation and disease increase mortality. Following is
a summary of a sampling of the appraisals of the relationship between
oyster populations and the magnitude, range, and timing of salinity fluct-
uations:
1) North Carolina oysters are found at salinity levels from
2.5-33 ppt but cannot withstand salinity less than 9 ppt for
long periods (Chestnutt, 1951:149).
2) The Atlantic Oyster "will die after long exposure to fresh-
water, although it can withstand limited periods of such ex-
posure and can thrive in relatively high salinity water" (U.S.
Department of Interior, 1970:17).
3) Salinity in primary oyster-producing areas of Pamlico Sound
is typically 10-20 ppt. Freshwater associated with hurricanes
can obliterate oyster populations (Munden, 1975).
4) Oysters are most viable at salinity levels of 5-30 ppt with
populations outside that range marginal. An unstable pattern
of salinity with diurnal, seasonal or annual fluctuations is
an important ecological factor as the effect of salinity changes
on oysters depends on the suddenness as well as the range of the
fluctuation (Fiore, 1976:76-85).
39
5) The preferred salinity range for oysters is 15-22.5 ppt,
when predation is not considered. Actual preference
ranges vary from 10-28 ppt in Chesapeake Bay to 5-15 ppt
in Louisiana estuaries. Salinity affects mortality and
reproduction, varying according to the range and sudden-
ness of salinity fluctuations. Oysters in a given loca-
ti on tend to be adapted to the typical fluctuations there
(Van Sickle, ^ al, 1976:4-5).
6) The upstream limit of oyster beds in the Chesapeake Bay
system is where spring salinity averages about 5 ppt with
the downstream limit at the 15 ppt spring isohaline. Oy-
ster set can occur at 5-35 ppt, but above 15 ppt predation
and disease are limiting factors (Haven, ejt 1978:50-54,
268-269).
Despite the lack of agreement regarding the preference ranges for
oysters, it is clear that a significant reduction in salinity could
displace oysters, and that creation of an unstable pattern of salinity
could also result in spatial displacement.
Spatial Displacement of Oysters
The oyster has long been an important resource in the Pamlico area.
It was a key foodstuff for Indian inhabitants and is a commercial resource
today. Estimated dockside value of oysters harvested in Pamlico Sound
and its tributaries was $330,835 in 1979, which was considered an off-
year for oysters (N.C. Division of Marine Fisheries, 1980). There is
40
evidence of spatial displacement of this resource within the study area,
but commercial landings statistics are not sufficient to analyze the dis-
placement phenomenon. Records are incomplete, and such data as do exist
are reported chiefly by county. Dockside landings in each county are not
necessarily representative of oyster population locations as ports such as
New Bern and Washington served as unloading points for oysters from many
North Carolina locations. Exogenous economic factors also have an impact
on landings statistics. For example, demand for Pamlico Sound oysters
has historically been closely linked to oyster harvests in Chesapeake Bay
(Chestnutt, 1951).
Field observations showed few, if any, remaining viable oyster beds
in the Pamlico and Pungo Rivers and upstream of the mouth of the Neuse
River. Some bays and creeks of western Pamlico Sound, once considered
prime oyster grounds, also were found to support few beds. Interview
data presented by Sholar (1980) and based on interviews with veteran oy-
stermen of the region show that this has not always been the case:
1) The heads of most bays and creeks in western Pamlico Sound,
including Swanquarter, Rose, and Juniper Bays, had abundant
oysters as recently as 30 years ago. Reportedly a man walking
the shore with a dip net could pick up 30 tubs of oysters a day.
2) In the Pungo River, oystermen worked off Sandy and Woodstock
Points and in Slade Creek about 25 years ago. A private oy-
ster garden was maintained in Slade Creek at the time, and
other reports place oyster beds as far upstream as Durants
Point near Bel haven within the past 30 years. Oysters were
reported all the way up to Wilkerson Creek in the upper Pungo
41
River until the Alligator-Pungo Canal connecting the
creek with the freshwater Alligator River was con-
structed in the 1920s.
3) Commercial oyster harvesting took place as far up-
stream as Bayview in the Pamlico River 25 to 30 years
ago.
4) In the Neuse River, commercial harvesting occurred up
to Cherry Point at the major bend in the river until
oysters began dying off about 30 years ago. Within
seven years, oysters had disappeared from Cherry Point
to near the river's mouth (Sholar, 1980).
The spatial displacement of oysters in the area within the past 30
years has been noted by other researchers. Doucette and Phillips (1979)
recorded the statement of a Hyde County fisherman who said Rose Bay be-
gan to "freshen up" about 1971, changing the oyster fishery from one able
to support 10-15 commercial boats through the winter to "essentially a
zero-boat level". A study of the macrobenthos of the Pamlico River termed
"unexplainable" the absence of oysters in the river, since evidence of old
beds was found, and since adjacent areas produced oysters (Tenore, 1970:57)
Oyster beds have been wiped out or reduced in bays and creeks in por-
tions of western Pamlico Sound and have been spatially displaced approxi-
mately ten miles in the Pamlico, Neuse, and Pungo Rivers. The estimated
current and circa 1950 upstream limits of oyster beds are shown in Figure 5
LEECMVILLE
• WASHINGTON
'^csr
HAVELOCK
SOliRCtS; EAST CAROLIIIA UNIVERSITY ARCHEOLOGICAL LABORATORY PHELPS, BiU.
rv)
SHOLA«R. l9C0; FIELD OBSERVATIONS BY AUTHOR.
FIGURE 5: HISTORIC OYSTER RANGES
43
Prehistoric Oyster Displacement
A number of Indian middens dating from 1000 BC to 1650 AD have been
located within the study area. Analysis of these sites indicates that a
previous spatial displacement of oysters has occurred and that the dis-
placement was related to salinity.
The Archaeological Laboratory at East Carolina University has located
and investigated a number of middens, or Indian refuse heaps, in areas ad-
joining the Pamlico and Pungo Rivers. The sites that were found to contain
oyster shell are indicated in Figure 5 with the approximate latest date of
occupation indicated. Oyster shell middens are located considerably up-
stream of where oyster beds were located even 30 years ago—to Rodman Point
near Washington in the Pamlico River and well upstream in Pungo Creek near
Belhaven. Numerous middens exist in some areas, such as Bath Creek, where
no oysters have been reported for years.
It is a safe assumption that the middens are located near where the
oysters were gathered. Indians did not carry refuse far from the point of
consumption to dispose of it. Settlement location decisions were based
primarily on the location of food sources with oysters an important dietary
component (Phelps, 1981).
There is no evidence of significant environmental degradation during
the era in which the oyster populations utilized by the Indians were dis-
placed. The displacement has been attributed to a decline in salinity
throughout the Pamlico-Albemarle system associated with the closing of
a major inlet in the Outer Banks (Phelps, 1981:24-52). Archeological and
geological evidence indicate that an inlet during the period of upstream
44
oyster utilization existed north of Kitty Hawk and that the entire Albemarle-
Pamlico system was of generally higher salinity than has been the case for
most of recorded history (Sampair, 1976; Phelps, 1981).
Summary
During the past few decades, the same period for which a declining
trend in salinity has been noted, there has been a marked change in the
spatial distribution of oysters within the study area. It has been shown
that oysters are highly sensitive to salinity, and that lower salinity
levels or an unstable salinity regime can eliminate oyster beds at a
given location. Archeological evidence indicates that a prehistoric change
in oyster distribution was linked to changing salinity associated with the
closing of an inlet. In recent years, oysters have all but disappeared in
some bays and creeks, while an approximate ten mile downstream migration
of oyster beds has been observed in the major rivers.
A number of factors, independent of or interactive with salinity,
could be related to spatial displacement of oysters in the region. No at-
tempt is made to isolate salinity trends as the sole or major variable.
If observed trends in salinity data reflect actual conditions in the es-
tuary, an environmental response should accompany the salinity trends.
Spatial displacement of oysters is evidence of just such a response.
CHAPTER FOUR
SPATIAL VARIATION OF SALINITY AND ANTHROPIC DRAINAGE
Salinity and Anthropic Drainage
Several research efforts have investigated the impact of man-made
drainage ditches and canals on runoff and on salinity in receiving estu-
aries. Such drainage eliminates or diminishes the natural function of
wetlands as runoff regulators. During wet periods runoff reaches the
estuaries faster and in greater volume than under natural conditions,
causing rapid, often drastic reductions in salinity. These events stress
shrimp, oysters, and other aquatic species through reduced salinity levels
and the unstable pattern of salinity. Drainage and associated land cover
changes can also increase total runoff. Surface runoff is determined by
SRO = P - (E + I) (8)
where P is precipitation, E is évapotranspiration, and I is infiltration
of moisture into the soil. Though infiltration in the immediate vicinity
of drainageways is increased due to water table drawdown, this is more
than offset by a decrease in évapotranspiration. Drainage networks quickly
remove water, making it unavailable for plant utilization. The decrease
in évapotranspiration increases the proportion of precipitation available
for runoff (Seuna, 1980; Skaggs, ^ a]_, 1980). Larger runoff volumes de-
crease salinity in receiving waters. Total runoff also rises in some cases
due to water table drawdown by canals (Wang and Overman, 1981). Lower sa-
Unity levels resulting from higher runoff volume and unstable salinity re-
gimes during wet periods places stress on salinity-dependent organisms
(Newbolt and Herbich, 1970).
46
The operation of relationships between anthropic drainage networks
and estuarine salinity within the study area was analyzed by determining
the areal association of salinity variation and drainage activities.
Other factors related to salinity changes must be noted, however, before
the drainage-salinity interaction can be isolated for study.
Spatial Variation in Salinity Trend
A spatial variation in the temporal trend of salinity was noted pre-
viously (Table 1, p. 27). Subareas one and ten, located in the upper
Pamlico and Neuse River estuaries (Figure 1, p. 8), showed no significant
trend. Subarea four, located in the upper Pungo estuary, had too few
data observations for a reliable trend estimate to be calculated. Two
distinct groupings can be observed in the seven remaining subareas, all
of which showed statistically significant negative salinity trends. One
group of four subareas had correlation coefficients for salinity with time
of -.20 to -.24. These subareas (numbers two, three, seven, and nine)
are located, repsectively, in: 1) the middle reaches of the Pamlico River,
2) the middle reaches of the Neuse River, 3) the area near the confluence
of the Pamlico and Pungo Rivers, and 4) in Pamlico Sound between the mouths
of the Neuse and Pamlico Rivers. The second group has stronger negative
correlations ranging from -.40 to -.53. Included are subareas five and
six located along the northern shore of the study area from the mouth of
the Pamlico River to the eastern boundary near Bluff Point, and subarea
eight near and just south of the mouth of the Neuse River.
47
Factors Related to Salinity Change
Factors related to salinity change may be grouped in two broad
categories: 1) those associated with overall salinity trends in the
study area, and 2) those related to spatial differences in salinity
trend within the study area.
General salinity changes over several decades have resulted from
fluctuations in oceanic exchange due to inlet openings and closings and
long term climatic cycles. Geologic processes such as a change in re-
lative sea level and sedimentation within estuaries may alter salinity
regimes over a period of centuries but are not considered significant
over shorter periods, such as the 34-year period considered here
(Gagliano, et 1970B).
Most oceanic exchange in western Pamlico Sound is via Ocracoke
Inlet which has remained open throughout recorded history (Marshall,
1951:46). Storm erosion sometimes results in new inlet openings. None
of the storm openings since World War II have resulted in inlets that
connected Pamlico Sound and the Atlantic Ocean for long periods.
Hatteras and Oregon Inlets, the only other sound-ocean connections in
the Pamlico area, have both been continuously open since 1846 (Stick,
1958:9). There have been changes in the inlets connecting Core Sound,
just south of Pamlico Sound, to the Atlantic. Effects on Pamlico Sound
are minimal, however (Roelofs and Bumpus, 1953:185-188), Inlet changes
from 1948-1981 have served to increase salinity for brief periods in
the vicinity of the inlets. Such short term localized effects have no
relationship with a declining salinity trend.
48
Climatic change and long term climatic cycles which alter fresh-
water input can also result in changes in salinity regimes. Such a
climatic change, or a trend in a cycle, would manifest itself in the dis-
charge of streams in the region. A declining salinity trend would re-
suit from an increasing trend in river discharge—in other words, the
slope b of a regression line describing the time series of river dis-
charge would be positive and significantly different from zero. The
null and research hypotheses may be expressed as
Ho: b = 0 (9)
H^: h ^ 0 (10)
To test this hypothesis monthly discharge records of the Neuse River
at Kinston and the Tar River at Tarboro for the 1948-1981 period were
regressed as dependent variables with time as the independent variable
in separate calculations. For both rivers, F-tests showed both slope
coefficients and correlation coefficients to be insignificantly differ-
ent from 0 at the 0.05 level. Correlation coefficients were -.0919 for
the Neuse and -.1084 for the Tar River. The null hypothesis of no signi-
ficant trend in discharge was accepted. There is no data indicative of
long term climatic trends which are related to salinity trends.
Hydrographic characteristics of the subareas were compared using
data from 1980 National Oceanic and Atmospheric Administration nautical
charts. Characteristics of depth, bottom material, major wind exposures,
dredged channels, shoaling, and circulation were recorded for each sub-
area. No areal association of these factors with salinity trends was
apparent from map comparisons. It was not felt that the time and expense
49
of a quantitative morphological comparison was justified, especially con-
sidering the dynamic nature of estuarine hydrography.
Areal Association of Salinity and Drainage
There are three general factors related to an understanding of the
relationships between salinity and man-made drainage practices; 1) the
general relationship of drainage with local hydrology and by extension
with estuarine salinity, 2) the location and intensity of drainage in the
study area, and 3) the areal association of salinity trend variation and
drainage developments.
Relationships of Drainage and Local Hydrology
In their natural state, wetlands such as swamps, marshes, and poco-
sins, serve as natural runoff filters and regulators. Drainage of wet-
lands for agriculture and forestry or routing drainage canals through the
wetlands diminishes the effectiveness of these functions and alters the
hydrology of an area.
Drainage developments in eastern North Carolina in recent years are
similar to past drainage activity in the bogs of Finland. Finnish re-
searchers have investigated the hydrological impact of such drainage.
Seuna (1980) observed adjacent watersheds of approximately equal size,
calibrated them against each other for 20 years, and established their
hydrologic differences after 40 percent of one basin was drained for
forestry. It was found that runoff in the altered basin rose substanti-
ally soon after the drainage, summer peak discharges increased, and both
summer and winter maximum flows were greater. Hyvaren and Vehilainen
(1980) found that drainage had increased spring and summer flows of rivers
50
in central and northern Finland. They were also able to separate ef-
fects of climatic fluctuation from effects of drainage, concluding that
the magnitude of spring floods was greater due to drainage. Ahti (1980)
determined that drainage was associated with increases in summer runoff
volumes and magnitudes of runoff peaks. Where there was no substantial
tree stand, it was found that runoff peaks were inversely proportional
to ditch spacing, i.e., denser drainage networks produced greater peaks.
The Finnish studies show that when bogs were drained, runoff reached
streams much faster than before alteration increasing peaks and, in some
cases, total runoff. The same holds true for man-made drainage networks
in eastern North Carolina and the study area. U.S. Geological Survey
reports in the 1970s stated that the intensive drainage on the Pamlico-
Albemarle peninsula would quicken peak runoff after precipitation, sending
water into the estuaries much faster than under unaltered conditions and
that runoff peaks would be higher (Daniel, 1978; Heath, 1975). Skaggs
and others (1980) found that peak runoff events occur sooner following
percipitation and are three to four times higher on developed than on
undeveloped land in tidewater North Carolina. Kirby-Smith and Barber
(1979) showed that water quality in South River was related to farm drain-
age systems discharging into the river. Surface water salinity was de-
creased with the decrease occurring in direct proportion to the frequency
and intensity of precipitation. Research in Rose Bay determined that
while total freshwater inflow was not altered by anthropic drainage, dis-
charge rates became less stable (Pate and Jones, 1980). Drainage networks
such as those in the Pamlico area often require that receiving streams be
51
channelized to accept the increased peak flows. Kuenzler and others
(1979) found that temporal patterns of discharge of channelized and non-
channelized streams in the North Carolina Coastal Plain differed with
more erratic discharge in channelized streams (except during low-flow
summer periods when small natural streams often have no flow).
The specific impact of an artificial drainage system on the hydro-
logical regime will vary according to a number of location factors, in-
eluding the size and topography of the drainage basin, land use within
the basin, climate, geology, and engineering aspects of the drainage
system itself. In addition to these complicating factors, it is especi-
ally difficult to determine the relationship of drainage to low flow of
streams. Still, the research review presented above clearly illustrates
that wetland drainage in the study area will result in less stable pat-
terns of freshwater input to estuaries, increasing magnitude of peak
runoff events, and increases in total runoff. Land cover changes asso-
ciated with drainage are also associated with hydrological change, but
no attempt will be made here to separate the effects of drainage itself
from those of land cover alterations.
Drainage in the Pamlico Area
Drainage has been utilized in the North Carolina Tidewater region
since Europeans first settled the area. Major projects, however, were
not feasible until the advent of heavy equipment after World War II
(Daniel, 1978). Most drainage in the region has been developed in the
past 35 years.
Dramatic increases in artificially-drained acreage are due in large
measure to large corporate landholders and to drainage districts. Corporate
52
farming operations such as Open Grounds Farms near the South River,
Mattamuskeet Farms near Swan Quarter, and First Colony Farms, north of
Mattamuskeet, have developed large tracts of lowlands for agriculture
through drainage systems. Timber companies such as Weyerhaeuser Corpor-
ation also drain considerable acreages in the study area. Most of these
developments have occurred in the past several decades (Carter, 1975),
Smaller landowners have become involved in large-area drainage projects
through North Carolina's drainage district program. Through this program
landowners may organize into legal districts to assess fees for drainage
improvement projects (Cramer, 1975:132-133).
The expected trend is toward more drainage for agriculture and
forestry. A survey of North Carolina agricultural extension chairmen
in coastal counties in 1978 showed that the chairmen believe more land will
be brought into production through drainage, though the rate was expected
to be slower than in the 1970s. As of 1978, seven percent of the acreage
of 20 coastal counties was considered naturally well drained, 15 percent
not drainable, 33 percent already drained for agriculture and forestry,
and 45 percent potentially drainable. The chairmen also felt use of prime
farmland for non-agricultural purposes would increase the demand to con-
vert wetland to farmland (Doucette, 1981:6-9).
Figure 6 depicts the extensive drainage networks within the study
area. Frequently-occurring high-density ditch-and-canal systems are an
obvious characteristic of the landscape. Drainageway statistics, however,
are difficult to obtain. The U.S. Soil Conservation Service estimated that
in 1978 about 1.87 million acres of land in coastal North Carolina had
been drained for agriculture and forestry. The North Carolina Office of
PAMLICO SOUND
• AGRICULTURE AND FORESTRY
0 HDSOJITO CONTROL Al® FINGER CANALS
* IUNlCIPALyii«JSTRI/iL OUTLETS
DRAINAGE AREAS
—
dense agriculture/forestry
= ^ .«DERATE AGRICXTTJRE/FORESTRY
I I I M I ri FtlSQUITO control/finger CANALS
FIGURE 6: ANTHROPIC
approximate location of SlfFQLK SCARP
DRAINAGE WITHIN THE cn
CO
STUDY AREA S0URŒS; U.S. ARMY CORPS OF ENGINEERS; U.S. GEOLOGICAL SURVEY; FIELD
OBSERVAT IaiS BY AUTTIOR.
54
Coastal Management estimated drained acreage at 1.70 million acres in
1980 (Doucette, 1981:6; Schmidt, 1981:5). In the 1970s, the Soil Con-
servation Service assisted North Carolina landowners with the installation
of approximately two million feet of subsurface tile and three million
feet of ditch drainage a year, nearly all within the Coastal Plain and
most in the outer Coastal Plain (Doucette and Phillips, 1979:16-17).
The Pamlico-Albemarle peninsula and the peninsula between the Neuse
and Pamlico Rivers are among the most densely-drained areas of the state.
There are also extensive areas of drainage south of the Neuse River. Farm
and forestry drainage is designed to control the water table in shallow
water table soils so that the root zone is not saturated during the grow-
ing season, and/or to increase the rate of runoff so surface detention of
water is minimized. These problems are most critical east of the Suffolk
Scarp, a relict marine shoreline shown in Figure 6. Land east of the
scarp is low and flat with elevations averaging less than ten feet above
sea level. Natural drainage is poor, but when drained, the wet mineral
soils of the area are among the most productive in the nation for corn,
soybeans, and loblolly pine (Doucette, 1981:4).
Most drainage systems in the study area consist of a series of field
ditches which drain into collector ditches. Collector ditches receive the
on-farm outlets and may empty into streams or into part of an area-wide
drainage system. Larger canals may have outlets into streams or estuaries.
Subsurface tile or pipe is sometimes utilized instead of field ditches, and
pump-and-dike systems are sometimes necessary east of the Suffolk Scarp
to prevent drained water from backing up onto the land.
55
Investigators noted the beginning of extensive land use changes with-
in the study area in the 1960s. Studies of this era focussed chiefly
on "reclamation" of the lowlands and on changing spatial patterns of agri-
culture rather than on environmental impact of drainage practices (Anderson,
1960; Wilkinson, 1967). As public and scientific awareness of potentially
adverse environmental impacts grew, researchers responded. Hawley (1974)
catalogued wetlands and wetland destruction in eastern North Carolina,
noting that "the alteration and destruction of wetlands in North Carolina
is proceeding rapidly." The phenomenon gained national attention when
Luther Carter (1975) noted in Science that "a process of irreversible
change has been set in motion" in coastal North Carolina. As land and
water modifications continued and negative impacts became increasingly
clear, issues of drainage and impact began to get widespread attention
from government and news media. As a consequence both state and federal
legislation was enacted to protect coastal zone resources which might be
damaged by drainage. These include: 1) Section 404 of the Federal Water
Pollution Control Act, which requires a permit from the U.S. Army Corps
of Engineers for the discharge of dredge and fill material into U.S. waters
or adjacent wetlands; 2) Executive Order 11990 of May, 1977, which re-
quires all federal water resource development projects to avoid the de-
struction or modification of wetlands; 3) The North Carolina Dredge and
Fill Act, which as enforced by the N.C. Division of Marine Fisheries has
not allowed new drainage outlets into primary nursery areas since 1974;
and 4) The North Carolina Coastal Area Management Act of 1974, which de-
signâtes wetlands as areas of environmental concern and requires permits
for developments in coastal area.
56
Delimitation of Impact Areas
Areas of high-intensity drainage impact illustrated in Figure 6
were determined from field observations, interpretation of U.S. Geological
Survey Ih' series topographic maps, and interpretation of 1978 color
aerial photography by the U.S. Army Corps of Engineers. Outlets for
anthropic drainage were classified as to whether they were outlets for
agriculture and forestry, mosquito control and finger canal, or sewage
outfall systems. Where this was not obvious from map and photo anlaysis,
sites were field checked. Areas of anthropic drainage were classified
as mosquito control and finger canal type drainage, moderate agricultural
and forestry drainage, and dense agriculture-forestry drainage. The
latter distinction is based upon a visual inspection of the number of
drainage channels identifiable from maps and photos relative to the land
area artifically drained. Areas classified as "dense" have an estimated
ten or more miles of artificial drainage channel per square mile, while
areas classified as "moderate" have fewer than ten miles of channel per
square mile. The mean artificial drainage density for the entire North
Carolina coastal area has been estimated at 1.5 miles per square mile, and
the most densely-drained areas of the Pamlico-Albemarle peninsula at 20
miles per square mile (Schmidt, 1981:5). Each outlet shown does not re-
present an individual canal or outlet. In many cases outlets are in close
proximity, making it impossible to identify individual outlets on a small
scale map.
Figure 6 probably underestimates current drainage density because
of the following: 1) drainage activities are ongoing with new land being
57
developed almost constantly; 2) many drainage ditches, especially in dense
forest areas, do not appear on maps and photographs; and 3) recent air photo
coverage of the Neuse River area is not available.
Areal Association of Salinity Variation and Drainage
Kirby-Smith and Barber (1979) found that drainage and land cover
changes associated with the Open Grounds Farm, which drains into subarea
eight of the study area, were related to changes in salinity, turbidity,
and dissolved nutrient levels in the South River. This, they claim, "is
a model of what has happened on a larger scale in North Carolina estuaries
over the last 200 years." The temporal coincidence of salinity trends and
drainage activities supports these findings.
Areal association is defined as the similarity between two or more
spatial distributions--in this case, the spatial distributions of varia-
tions in salinity trends and anthropic drainage. The fact that spatial
distributions are similar does not always indicate a functional relation-
ship. Nevertheless, an areal association is usually indicative of some
type of functional relationship when there is a sound rationale to believe
a relationship between the variables exists.
A description of these spatial distributions was presented and com-
pared qualitatively using map comparisons. Results are presented in
Table 2. With subarea four excluded, all seven subareas which exhibited
a negative salinity trend also receive drainage from agriculture and
forestry field systems. Subareas five, six and eight with the strongest
negative salinity trends, all receive drainage from very large, dense
systems. Though relatively few outlets exist in subarea eight, this
Table 2: Salinity Trend, Drainage and Oyster Displacement in Pamlico Subareas
Subarea Salinity Oyster Displacement N of Drainage Outlets Character of Drainage
Trend 1 2 3
1 0.02400 No Oysters 1 1 0 Small field system; some finger canals
2 -.20480 Displaced out of 6 0 1 Some drainage from small field systems;
Subarea since 1950 one industrial outfall
3 -.22812 Displaced out of 3 11 0 Field systems, some extensive; several
Subarea since 1950 mosquito control systems
4 Displaced out of 11 0 1 Massive field systems into upper Pungo;
Subarea since 1900 large field systems into creeks; town
outfal1
5 -.39707 Displaced from upper 22 0 0 Massive field systems from large areas
Bays and Creeks
6 -.53021 Displaced from upper 17 0 0 Massive field systems from large areas
Bays and Creeks
7 -.20237 No Serious Displace- 2 12 0 Small field systems; large system of
ment Reported mosquito-control ditches
8 -.45349 Displaced from West 6 0 5 Few outlets, but from massive Open Grounds
Part of Subarea drainage system
since 1950
9 -.23961 Displaced out of 5 1 0 Small field systems
Subarea since 1950
10 0.14610 No Oysters 0 0 2 Municipal outfalls
KEY: Salinity trend represented by Pearson's product-moment correlation coefficient for salinity with time.
Type 1 drainage outlets are farm and forestry outlets; type 2 are marsh canals for mosquito control or
extensive finger canal systems; and type 3 are muni ci pal-industrial outfalls. Key to subareas: 1, upper
Pamlico River; 2, middle reaches of Pamlico River; 3, confluence of Pamlico and Pungo Rivers; 4, upper
Pungo River; 5, area of Swanquarter, Juniper and Rose Bays; 6, northeast portion of study area, off Hyde
shore; 7, area between mouths of the Neuse and Pamlico Rivers, off Pamlico Co. shore; 8, area near and
just south of mouth of Neuse River; 9, middle reaches of Neuse River; and 10, upper Meuse River.
59
region does receive drainage from a large, dense corporate farming opera-
tion. Subareas two, three, seven, and nine also receive drainage from
field systems, but the systems are smaller and less intense. Environmental
responses should be apparent in areas of salinity decline, and all such
subareas except subarea seven have been cited as areas of oyster displace-
ment. If current trends continue, biological changes can be expected to
intensify as the estuarine systems reach their assimilative capacities
(Kirby-Smith and Barber, 1979:1).
Data available for salinity trends, anthropic drainage impact, and
spatial displacement of oysters differs in both form and quality. Data
must be reduced to the lowest common level of measurement—in this case,
nominal level arbitrary group assignments--to quantitatively test for
systematic relationships.
Each of 46 of the original uniform sample grid cells was classified
according to categories representing salinity trends, drainage impact, and
oyster displacement. A value of 0 was assigned if the cell was in an area
with no significant observed salinity trend, a value of 1 assigned in areas
where there was a coefficient of r = -.20 to r = -.24 for salinity with
time, and a value of two where the salinity-time coefficient was -.40 to
-.53. The entire study area can be grouped into these three categories.
For oyster displacement, a value of 0 was assigned if the area never had
oyster populations since 1948 or if no displacement was reported. A value
of 1 was assigned where bed displacement was reported. Drainage impacts
were rated on a 1-4 integer scale as follows: extensive influence of areas
classified as densely drained (see Fig. 7) = 4, influence by extensive areas
of moderate drainage = 3, influence by areas of mosquito-control drainage or
60
from small areas of moderate agriculture and forestry drainage = 2, and
relatively minor drainage impact = 1.
Contingency tables based on these classifications are presented in
Table 3. Since the standard chi-square test of significance for nominal-
level values requires more observations than are available here, modified
tests based on chi-square were utilized. The Phi statistic makes a cor-
recti on for the fact that the value of chi-square is directly proportional
to the number of cases N by adjusting the chi-square value:
=
N (11)
Phi is suitable for 2X2 contingency tables, and when calculated for larger
tables such as in Table 3, has no upper limit. Cramer's V statistic is
a modified version of Phi suitable for larger tables, adjusting Phi for
either the number of rows or columns, whichever is smaller:
— (12)
min (r-1, c-1)
Cramer's V ranges from 0 to 1 with a large value of V signifying a high de-
gree of association. Like the chi-square test of significance, V does not
reveal the manner in which the variables are associated.
Cramer's V values are 1.00 for the salinity-drainage relationship,
0.47 for the salinity-displacement relationship, and 0.67 for the drainage-
displacement relationship. This signifies an extremely high degree of associ-
ation between salinity trends and impact of anthropic drainage. Associations
with regard to oyster displacement are less obvious, perhaps due to lack of
anything but the grossest available information on oyster displacement, and
to the presence of areas with a displacement value of 0 that never supported
61
Table 3
Contingency tables and tests of significance for areal associations
of salinity trends, anthropic drainage impacts, and spatial displace-
ment of oysters. Numbers in the cells represent the number of sample
areas in each categorical pair.
Drainage Impact
s minimal light moderate heavy
a 1 2 3 4
1
i 0 7 0 0 0
n none
i
t 1 0 12 7 0
y negative
t
r 2 0 0 0 20
e stronger
n negative
d
Table 3a: Salinity Trend and Drainage Impact
Cramer's V = 1.00
Source: Computations by author
62
Table 3
(continued)
Salinity Trend
none negative stronger negative
D 0 1 2
i 0
s no 7 9 6
P
1
a
c
e
m 1
e yes 0 10 14
n
t
Table 3b: Oyster Displacement and Salinity Trend
Cramer's V = 0.47
Drainage Impact
minimal light moderate heavy
1 2 3 4
D 0 7 9 0 6
i no
s
P
1
a
c
e
m
e 1 0 3 7 14
n yes
t
Table 3c: Oyster Displacement and Drainage Impact
Cramer's V = 0.67
Source: Computations by author
63
oyster populations to begin with. There are also midsound areas with 0
displacement values where it is unlikely that displacement would be noted
and reliably reported.
The statistics do not reveal the nature of the associations, yet, it
is clear that zones of the greatest drainage impact are areally associated
with the zones that have the strongest indication of salinity declines. As
expected, there is also some areal association between spatial displacement
of oysters and the drainage and salinity factors.
The limitations of chi-square and related statistics such as Cramer's
V used with nominal-level data are realized. These tests can test for inde-
pendence of factors. The factors above are not independent, and our know-
ledge of the system allows us to generalize about the nature of the de-
pendency and linkages. The analysis supports the hypothesis that systematic
relationships and spatial linkages do exist.
Summary
Factors such as inlet changes and climatic variation cannot satisfac-
torily explain the observed temporal trends in salinity in the study area.
The increase in anthropic drainage developments within the study area paral-
lels salinity trends in time. Since man-made drainage alterations are re-
lated to freshwater input, and thus salinity, in estuaries, the areal as
well as the temporal association of salinity trends and drainage was examined.
The negative temporal trend in salinity was found to have strong areal
association with the level of anthropic drainage. Environmental response,
as indicated by oyster displacement, was also associated with salinity and
drainage. Cramer's V, a statistical test of independence, supports this
assessment. Tests of contingency tables based on categorical variables
64
describing salinity trends, drainage impacts and oyster displacement found
the variables, especially salinity and drainage, to be non-independent.
Declining trends in salinity are related to impacts of anthropic drainage.
CHAPTER FIVE
CONCLUSIONS
Pamlico Sound and surrounding areas are part of a large, critical
estuarine region that has been subjected to considerable human modifi-
cation in recent years. These modifications have altered, at least tern-
porarily, the character of the estuary itself. Due to the complex spatial
linkages between anthropic, physical, and biological components of the
region, all three components are affected. This research had two major
objectives as follows: 1) the assembly and analysis of time-series data
regarding salinity, environmental response to salinity trends, and drain-
age activities; and 2) the examination of the areal association of these
factors in order to determine spatial relationships.
Temporal Trends
Within the western portion of Pamlico Sound, including the estuarine
portions of the Neuse, Pamlico, and Pungo Rivers, water analyses revealed
a general erratic decline in salinity since 1948. At present there is no
data indicating a relationship between the negative trend and changes in
sea level, changes in inlets between Pamlico Sound and the Atlantic Ocean,
or long term climatic variation. Since the data did not exhibit intrinsic
determinism or cyclic components, it was hypothesized that freshwater in-
put, the remaining major variable related to salinity at a given location,
must be associated with the salinity trend.
Freshwater inputs via streamflow and overland runoff can be altered
in volume and timing by man-made drainage projects which diminish the
natural role of wetlands as runoff regulators. Drainage development is
66
not a recent phenomenon in the study area, but most development has oc-
curred since World War II with increasing amounts of artificially-drained
land added each year. While specific time-series data on acres drained
were not available, the exten.t and density of anthropic drainage in 1981
was delimited.
Salinity is a critical ecological factor in estuaries. Significant
changes in salinity have been accompanied by biological environmental
responses. Historical data on species responses are nonexistent with
the exception of the Atlantic Oyster. Oyster bed locations are noted by
area watermen, and the spatial distribution of oysters may be inferred
from this information. Beginning about 1950, evidence of oyster displace-
ment was noted by commercial fishermen. By 1980, oyster beds had been
displaced or reduced from some bays and creeks of western Pamlico Sound.
A downstream migration of approximately ten miles had occurred in the
Pungo, Pamlico, and Neuse River estuaries.
The striking temporal association of the foregoing trends shows that
the possible presence of a spatial relationship should be investigated.
Spatial Relationships
Analyses of data by subareas of the study area revealed a significant
areal association between salinity trends and artificial drainage networks.
Subareas which showed the sharpest statistical evidence of a negative sa-
linity trend were also found to be under the influence of intensive arti-
ficial drainage networks. Such areas include the portion of Pamlico Sound
from the mouth of Pamlico River northeast to the boundary of the study area
near Bluff Point, and the area near and just south of the mouth of the Neuse
67
River. Other subareas showing statistically significant signs of sali-
nity decline were also areally associated with anthropic drainage acti-
vities. These areas include the middle reaches of the Neuse and Pamlico
River estuaries, the mouth of Pamlico River, the lower half of the Pungo
River estuary, and the portion of Pamlico Sound lying between the mouths
of the Neuse and Pamlico Rivers. The upper Neuse and Pamlico River es-
tuaries, which are not heavily influenced by artificial drainage, do not
exhibit any statistically significant trend in salinity. Data were not
sufficient to analyze the trends in the upper Pungo River, but since the
impact of drainage in the area is heavy, it may be assumed that a negative
salinity trend also exists for that subarea. All but one subarea (between
the mouths of the Neuse and Pamlico Rivers) having a negative salinity
trend also experienced a spatial displacement of oysters in the 1950-1980
period.
Contingency table analysis based on systematic samples of the study
area substantiates the visual examination of the spatial distribution of
salinity trend, drainage activities, and oyster displacement. Cramer's V,
a statistic which indicates the level of association of nominal-level vari-
ables, shows the areal association of negative salinity trend and intensive
anthropic drainage.
Conclusions
Anthropic drainage developments in the Pamlico area diminish or elimi-
nate the runoff-regulating function of wetlands. Peak runoff events are
increased in magnitude, total runoff increases, and peaks occur more sud-
denly after precipitation events. Lower salinity levels and a less stable
68
salinity regime in receiving estuaries are commonly the result.
Continual drainage developments in the area for the past few decades
have, by altering freshwater inputs as described above, created an erratic,
but significant, declining trend in salinity. Salinity-dependent organisms
in the estuary are responding to the salinity alteration through changes
in their spatial distribution.
Spatial linkages between man's activities, physical properties of
the estuary, and estuarine biota are indicated by the available data. Due
to this spatial relationship, the changes in man's activities have resulted,
at least temporarily, in changes in the physical and biological character
of the Pamlico estuary.
This macro-scaled, spatially-oriented aoproach has implicated drainage
activities as a factor in the long term salinity decline in portions of
Pamlico Sound. However, a full understanding of the relationship between
anthropic, physical and biological components of the system and their vari-
ation in space and time calls for further research. Specifically, the
following avenues of research are called for:
1) The analysis presented here is based on a limited data base,
but data collection is ongoing, and future studies may incor-
porate this data. Also, more past salinity data could be un-
covered. Statistical analyses applied here could be repeated
and expanded with the improved data sets.
2) An analysis by grouping observations according to seasonal
moisture criteria was considered in this study. Though this
was not thought to be useful at the current stage of study,
similar approaches could prove fruitful.
69
3) Commercial landings statistics and other sources indicate
the possible presence of salinity-related responses in
several species. If a data base can be found, observa-
tions of changes in the spatial distribution of these
species would be useful in supplementing evidence of bio-
logical responses based on oyster bed locations.
4) Rough measures of drainage intensity are now possible using
available maps, photographs, and observations. Increasingly
sophisticated mapping and remote sensing techniques are
producing more accurate representations of existing drain-
age systems in eastern North Carolina. New drainage pro-
jects are heavily regulated. With this new information,
more reliable estimates of drainage intensity should be
possible with more powerful variables describing drainage
available for evaluation with respect to salinity changes.
5) This study did not attempt to separate the relationship
of associated land cover changes with estuarine salinity
from those of drainage itself. Urbanization has not been
a major factor in the study area in recent years, but could
also have future hydrologic impacts.
6) Studies now underway in the study area examining the mech-
anics of relationships between land use, hydrology, estuar-
ine water quality, and organisms will prove useful in the
future.
7) Impact of drainage on local hydrology is not constant. The
functioning of the drainage system becomes impaired by sedi-
70
mentation, ditch wall failure, vegetation invasion, and
other processes, resulting in drainage system deteriora-
ti on and altering the perturbations of the watershed.
8) Effects of man-made drainage on the hydrologic regime
may vary significantly according to the character and
design of the drainageways, topography, geology, native
vegetation, and other factors. These factors could help
explain the spatial variation in salinity trends.
9) The modelling of linkages and interactions between man
and the physical and biological components of estuarine
systems is necessary for full understanding and predic-
tive ability. Such interactions have been demonstrated
here, but more detailed understanding of these inter-
actions, as well as smaller scale interactions among
species and physical components is desirable.
10) This study considered only salinity as a water quality
parameter in estuaries, but other impacts are also proba-
ble, such as sediments and nutrients.
11) Methodological studies of the nature of serial and
spatial autocorrelation in hydrologic variables and
analytical methods for dealing with it are not uncom-
mon in estuarine research and should be pursued. Auto-
correlation was noted and accounted for in analysis of
salinity data. Tests for autocorrelation in Tar and
Neuse River discharge data proved negative.
71
12) Finally, several management and policy questions raised
by the relationships identified here bear further study.
The questions of whether impacts are serious enough to
warrent efforts at minimization may be answered by re-
search efforts mentioned above, and will be a matter of
public debate.
Future Prospects
Declining salinity trends can be expected to continue in the near
future. Land development, including drainage, continues in eastern North
Carolina with few signs at present of a slowdown in land use changes.
Long term prospects are uncertain. Several variables are involved:
1) the location, rate, magnitude, and character of land development;
2) threshold levels and feedback mechanisms of the hydrologic and biologi-
cal components of the system; and 3) the equilibrium potential of the
hydrologic system with regard to watershed adjustment to altered inputs.
This study has provided further evidence of the complex interactions
in estuarine zones, where human modifications can affect ecosystem com-
ponents in other locations according to the spatial linkages. The Pamlico
Sound system, complex even when man's activity is not considered, is under
going a number of changes related to a single form of land use development
Resource management strategies in the coastal zone should be formulated
with these relationships in mind. If drainage impacts on estuaries are
to be minimized, several strategies should be considered. These include
the routing of man-made drainage to non-critical estuarine areas; leaving
intact buffer zones of unaltered wetlands between drainage outlets and
72
receiving estuaries; use of holding ponds to slow runoff from man-made
drainage systems; and preservation of wetlands.
73
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APPENDIX A
SALINITY DATA SOURCES
Published sources:
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North Carolina coastal plain streams and effects of channelization.
University of North Carolina Water Resources Research Institute
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81
APPENDIX A (continued)
Woods, W.J. 1967. Hydrographic studies in Pamlico Sound in Proceedings:
Symposium on the Hydrology of the Coastal Waters of North Carolina.
Raleigh: Water Resources Research Institute, p. 104-114.
Agencies and individuals which provided unpublished raw data:
Hester, J.M. Raw data from M.S. thesis. Department of Biology, North
Carolina State University, Raleigh, N.C.
Institute of Coastal and Marine Resources, East Carolina University,
Greenville, N.C.
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N.C.
North Carolina Division of Marine Fisheries. Oyster monitoring program,
Washington, N.C.
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Phillips, J.D., Greenville, N.C.
Roelofs, E.W., Chapel Hill, N.C.
Sources from which pre-1948 data were collected:
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Fisheries, Washington, D.C.).
Winslow, F. 1889. Report on the sounds and estuaries of North Carolina
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