The response of biogenic reef communities to freshet perturbations across estuarine scale environmental gradients
Understanding how coastal foundations species (and associated fauna) respond to natural and anthropogenic stressors and disturbance is essential for describing the resilience of estuarine ecosystems. Indeed, temperate estuarine systems that contain saltmarsh, seagrass, and oyster habitats are characteristically dynamic across time and space, and may be heavily influenced by changing climactic variables such as precipitation, saltwater intrusion, and sea level rise. In North Carolina, a 3000 mm rainfall event caused significant salinity decreases in two river estuaries during 2014: by as much as 50% at mesohaline, upriver locations; 32% at polyhaline, midriver sites; and 20% at downriver, euhaline sites. To test whether these salinity changes had an effect on oyster reef communities across preexisting salinity and tidal gradients, we utilized a long-term oyster reef community survey, sampling 6 intertidal and 5 subtidal reefs (enumerating oysters and associated fauna) in the months preceding and following this freshet. Additionally, salinity and temperature loggers were deployed at each site in order to detect multi-scale (hourly, daily, monthly) changes in these variables and significant changes that occurred due to the salinity perturbation.
Mesohaline, upriver sites were influenced most severely by the freshet, and experienced reductions in oyster density (down by 9%-100%), associated faunal density (down by 36%-85%) and species richness (down by 10%-69%). Conversely, polyhaline, mid-river reefs generally experienced large increases in oyster densities (up by >100%). Despite the change in oyster densities on polyhaline reefs, no changes in reef associated faunal densities or diversity were observed. Two euhaline, intertidal reefs saw differential responses in oyster densities (up by 25% and down by 41%), yet associated faunal densities decreased (down by 50%), while diversity decreased (down by 27%). This contrasting response at euhaline reefs may be explained by larval oyster settlement coinciding with the freshet, as a net decrease in oyster biomass on these reefs was observed during this study. The sole subtidal euhaline site saw increases in all measured responses following the freshet, suggesting oyster communities in salty, subtidal environments can resist and thrive under low order salinity reductions due to freshwater inputs. These data highlight that conservation and restoration efforts for biogenic reef communities may be complicated by the differential resilience of these communities to climactic (or other) disturbance across graded estuarine landscapes.