Spatiotemporal Variability in Sedimentation Drives Habitat Loss on Restored Subtidal Oyster Reefs

Abstract

Restored oyster reefs enhance commercial harvests and ecosystem services in coastal environments. Spatial and temporal changes in habitat availability can affect reef persistence and restoration benefits, and understanding how construction and location of a restored habitat influence its persistence over time is key to optimizing restoration efforts. The short-term persistence of six subtidal restored oyster reefs in Pamlico Sound, NC, USA was characterized by sidescan sonar and bathymetric mapping conducted immediately after restoration in August 2016 and again 21 months later in May 2018. A U-net convolutional neural network architecture was trained to classify reef pixels within the sonar imagery using image-based and image-texture features calculated from gray-level co-occurrence matrices. Oyster reef restoration used shell and limestone marl to construct flat substrates, with only a few 10’s of centimeters of local relief, spread over areas of ~3000–12,000 m2. All six reefs provided habitat for the settlement and growth of oyster populations, but this role changed as reefs underwent varying degrees of sediment burial between surveys. Reefs constructed in relatively low-energy environments lost ~18–35% of their substrate area, primarily by deposition of sediment along their margins. Reefs having greater sediment supply and greater exposure to predominant winds and currents were most susceptible to burial and became heavily fragmented with ~50–65% of the restored habitat lost. Sediment dynamics appear to exert a controlling influence on the success of these reefs, and oyster restoration sites in high-energy environments may have limited long-term economic and ecosystem benefits.