Biodiversity and habitat characteristics as indicators of community change following oyster reef restoration

Abstract

As coastal habitats, oyster reefs are often on the frontlines of both direct and indirect sources of anthropogenic ecological degradation. This has profound impacts on both the structure and function of these foundational habitats, which offer a litany of ecosystem services to the surrounding environment and its inhabitants as well as the nearby coastal communities. Restoration of these ecologically significant habitat-forming species offers not only a way to study the resilience and recovery of communities following human disturbance and over time, but also provides a mechanism for restoring lost ecosystem function and services following these disturbances. To this end, two oyster restoration approaches were implemented in 2018 in the Rachel Carson Reserve in Beaufort, North Carolina: a traditional shell bag approach, and at the time, a novel biodegradable substrate, Oyster CatcherTM. These restored sites underwent initial post-restoration monitoring (habitat: 2018-2020 and biodiversity: 2018-2019), but had not been reassessed since that time. My project analyzed changes habitat characteristics and community composition of the two restoration approaches and nearby natural reefs five years post-restoration relative to these earlier monitoring points. My results shed light on the roles that substrate and time can play on the effectiveness of oyster reef restoration efforts, as well as the applicability of restoration as a tool for studying ecological succession. For habitat characteristics, I compared previous monitoring data to samples collected in 2023 to determine how oyster densities and elevation varied by restoration treatment. While oyster densities steadily increased from 2018-2020 in the previous monitoring effort, my results showed a significant decrease in oyster densities from 2020-2023. Moreover, in 2020, the restored oyster reefs were within the “Optimal Growth Zone”, while in 2023, varying water levels resulted in all live oysters on the reefs residing above this ideal elevation range. In all cases, however, Oyster Catcher reefs outperformed shell bag reefs in terms of oyster densities, offering some insight into the potential application of novel breakwater substrates to the issue of oyster reef restoration efforts. For community composition, I examined both free-living and parasite communities and compared samples gathered in 2023 to previous data to understand how these metrics have changed with time since restoration. While free-living fish communities peaked in biodiversity one-year post restoration, the species documented are typically more transient species; by contrast, the species more strongly affiliated with the 2023 samples (the oyster toadfish, Opsanus tau, for example) are broadly recognized as true reef resident species. Free-living crab communities showed a different trend, with biodiversity increasing with time post-restoration. In both cases, treatment was not a significant driver of community composition, although mud crab abundances appeared to be higher at restored reefs than nearby loose cultch reefs. For parasite communities, samples collected from fish and snail hosts broadly reflected trends described in previous literature, with increasingly prevalence and biodiversity with time. However, crab parasite data revealed a peak in prevalence one-year post-restoration, with significantly lower prevalence across most taxa in 2023. These results illustrate the utility of using surrogate taxa, such as parasites, to augment biodiversity surveys and provide an additional metric by which community composition and function can be assessed. Above all, the habitat and biodiversity results collectively stress the importance of monitoring oyster reef restoration projects over time, as early post-restoration trends do not consistently reflect the long-term condition of these efforts.