Salinity disturbance affects community structure and organic matter on a restored Crassostrea virginica oyster reef in Matagorda Bay, Texas

Oyster reefs are one of the most degraded marine habitats, with estimated 85-91% global habitat loss compared to historic levels (Beck et al. 2011, Lotze et al. 2006). However, the restoration of oyster reefs is becoming a widely recognized tool to ameliorate the effects of habitat loss. Half Moon Reef, once a highly-productive 2 km2 Crassostrea virginica oyster reef located in Matagorda Bay, Texas, was harvested to depletion in the early 20th century. In 2014, The Nature Conservancy restored 0.23 km2 of reef—one of the largest oyster reef restorations in the country. In the three years following reef restoration, two salinity disturbances (prolonged salinities <10) provided a unique opportunity to determine the effects of large salinity variations on oyster reef community structure and quality of organic matter. Oyster growth generally increased over the 3-year study period, enhancing habitat provisioning for reef fauna. Reef-resident species metrics showed strong positive correlations with salinity. Following a low salinity event (25 to 9) one year post-restoration, the reef-resident fauna shifted from a community dominated by pioneer organisms to one comprising larger and more resilient crustaceans and gastropods. A second low salinity event two years post-restoration did not show a similar response, indicating the presence of larger oysters facilitated species that may otherwise not exist in high disturbance environments. Fauna from adjacent areas showed no patterns with distance from the reef, indicating restoration did not influence faunal communities away from the physical reef structure. As salinity decreased, suspended particular organic matter became more 13C-depleted whereas surface sediment organic matter did not show significant change. Carbon/chlorophyll a and carbon/nitrogen ratios of suspended particulate organic matter indicated the quality of organic matter was higher following low salinity events, implying pulses of freshwater inflow increased autochthonous production. Surface sediment organic matter and suspended particulate organic matter contributed nearly equally to assimilation by oysters. Results were integrated into a conceptual diagram to visualize the effects of salinity on oyster reef communities, providing a tool that natural resource managers can use for a broader perspective on the effects of salinity variations on oyster reef communities.