Factors affecting recruitment, growth and survival of the eastern oyster Crassostrea virginica across an intertidal elevation gradient in southern New England

Reef-building oysters are important coastal foundation species that provide ecosystem services. Overharvesting, destructive fishing practices, and environmental degradation have caused significant declines in global oyster populations, necessitating restoration efforts in many regions to rebuild oyster habitat and recover lost services. Understanding how biological and physical gradients regulate oyster recruitment, survivorship and growth will help guide future efforts to successfully restore oyster populations. We conducted a field experiment in Ipswich, Massachusetts, to quantify the effects of tidal elevation, predator exclusion, and reef vertical relief on oyster settlement, recruitment and survivorship. Oyster settlement was 3 times greater at the deepest intertidal elevation compared to the 2 shallower intertidal elevations. Reef vertical relief and predator exclusion did not affect oyster settlement or survivorship. Despite increased sedimentation and algal fouling with increasing depth, living oyster densities remained significantly elevated at the deepest intertidal elevation 8 mo after the experimental reefs were constructed. Oyster mortality during this period was highest (>70%) at our shallowest elevation treatment, likely the result of desiccation and food limitation. Meanwhile, mortality during the first winter post-recruitment was high across all elevations (all >64 %), with our deepest elevation treatment experiencing the highest mortality rate (90%). Our results suggest that there is a gradient in oyster settlement rates, which increase at lower elevation, and that the dominant drivers of mortality also likely vary with elevation. Our findings highlight the need for region-specific studies to quantify biological and physical gradients prior to large-scale restoration efforts.