Local oyster ecology prevents sustainable development of oyster reef breakwaters

Oyster reef breakwaters are designed to develop into sustainable, vertically accreting reefs, maximizing engineering and ecological benefits over extended timelines. In practice, the performance of oyster reef breakwaters are highly variable across projects, and the relevant ecological drivers are often unclear. Breakwaters in coastal Alabama have consistently failed to develop into sustainable oyster reefs, so we ran an experiment to assess patterns of oyster recruitment and post-settlement survival across the tidal gradient at multiple restoration sites. We observed very low rates of wild spat recruitment, and post-settlement mortality in both years appeared to be driven by predation at the lower-intertidal, and excess temperatures and exposure at the upper intertidal. The relative importance of predation appeared greater at higher salinity. At most sites, oyster survival and growth was maximized at a thin intertidal elevation band (~0.3 m NAVD88, ~50% inundation period). Many Alabama oyster reef breakwaters have crest heights below the optimal tidal elevation, and thus may not be maximizing effective oyster substrate, but current oyster densities are minimal even on taller breakwaters. Designing effective oyster reef breakwaters requires balancing the engineering and ecological benefits of oyster reef breakwaters in the context of local oyster ecology, which may preclude sustainable reef development in some systems.