Salinity disturbance affects faunal community composition and organic matter on a restored Crassostrea virginica oyster reef

Abstract

Ecological restoration has become a widely recognized tool to ameliorate the effects of habitat loss. Half Moon Reef, once a highly-productive 2 km2Crassostrea virginica oyster reef in Matagorda Bay, Texas, was harvested to depletion in the early 20th century. In 2014, a 0.23 km2 reef complex was created using limestone and concrete substrates to restore oyster populations—one of the largest contiguous oyster reef restoration efforts in the U.S. In the three years post-construction, two large freshwater inflow events provided a unique opportunity to determine the effects of prolonged salinity reductions (<10) on oyster reef faunal community composition and organic matter quality. Oyster size generally increased over the 3-year study period, enhancing habitat provision for reef fauna. Reef-resident and reef-associated species metrics (abundance, biomass, diversity, and species richness) were positively correlated with salinity. Following a large salinity reduction (from ∼26 to ∼9) one year post-construction, the reef-resident faunal community shifted from dominance by small gastropods to larger and more resilient crustaceans. A second, lower magnitude salinity reduction two years post-construction did not elicit the same biotic response. Reef-associated fauna showed no patterns related to distance from the reef (13 m vs. 150 m), indicating limited influence of the restored oyster reef on fauna outside of the physical reef structure. Salinity decreases were associated with 13C-depletion of suspended particulate organic matter, whereas no change in isotopic composition was observed for surface sediment organic matter. Carbon/chlorophyll a and carbon/nitrogen ratios of suspended particulate organic matter indicate the quality of organic matter was higher following low salinity events, likely due to increased autochthonous production facilitated by pulses of freshwater inflow. Oysters assimilated nearly equal proportions of surface sediment organic matter and suspended particulate organic matter. It is important to understand the effects of salinity variability for the effective management of coastal restoration projects because freshwater inflow has a major influence on coastal ecosystems.