Responses of dominant marsh macrophytes to inundation and disturbance and assessing marsh ecosystem services

Abstract

Within two hydrodynamically different inundation regimes in North Carolina, the growth response and interactions of two dominant marsh macrophytes were evaluated over a range of inundation periods to understand better how coastal marshes will respond to rising sea levels. Marsh vegetation is likely key to accretion processes. Greater aboveground biomass baffles floodwaters, potentially increasing sedimentation, and greater belowground biomass contributes directly to the elevation of marsh substrate. Multi-level planters were used to evaluate experimentally the response of Spartina alterniflora Loisel and Juncus roemerianus Scheele to an array of inundation periods. Groundcover changes were evaluated before and after a pulsed disturbance to simulate conditions where these species are naturally delineated on the marsh platform. Peak production of S. alterniflora and J. roemerianus occurred at inundation periods of 0.5 to 14% and 0.4 to 28%, respectively, in marsh planters. The growth response patterns of S. alterniflora and J. roemerianus to inundation period were similar, although J. roemerianus appeared to experience greater stress in an astronomically-dominated than in a meteorologically-dominated inundation regime. Spartina alterniflora was found to be more resilient to disturbance than J. roemerianus in planters and on the marsh platform. Sixteen months following a one-time cutting disturbance, a significant decline in J. roemerianus and encroachment of S. alterniflora in experimental platform plots was observed within both regimes. Estuarine inundation appears to function as both a subsidy and stressor to marsh macrophytes. I propose a conceptual model to describe this relationship. The scaling of the losses and gains in marsh ecosystem services is reviewed within the context of coastal marsh habitat injury, restoration and creation as achieved by the Natural Resource Damage Assessment and compensatory restoration processes. An analysis into the current methods used to assess ecosystem services found that U.S. statutes provide for the compensation of loss of public trust resources, yet current metrics for ecosystem services serve as incomplete proxies of ecosystem function and condition. Coastal ecosystems have and will transgress in response to rising sea levels. Data from this investigation suggest that marsh accretion is challenged by increased inundation rates, especially where sediment supplies are limiting, thus, highlighting the need for accommodating transgression within the coastal zone.