Physical-biological coupling on oyster reefs: Hydrodynamics, sedimentation, and the production of oysters

Field experiments were conducted to test whether the morphology of biogenic oyster reef habitat influences hydrodynamics and, subsequently, whether hydrodynamics influence sedimentation and the production (recruitment, growth and survival) of the eastern oyster Crassostrea virginica (Gmelin). To test this hypothesis, and an another hypothesis, that hydrographic conditions control oyster production, 100 experimental oyster reefs were built at 3-, 4- and 6-m water depths in the Neuse River estuary, NC.; Flow over experimental reefs behaved similarly to that predicted by the principle of continuity: flow speed was greater over reefs than unstructured bottom, and it increased with elevation on reefs and with reef height. Short reefs in shallow water were rapidly buried beneath sediment during storms. Recruitment of oysters and sedimentation were greatest at the base of reefs where flow speed was lowest. Growth and condition of oysters increased with flow speed; growth was higher on the crests of taller reefs than the crests of shorter reefs, and was greater on the crests than bases of reefs. Oyster mortality was greater where flow speeds were lowest and where sedimentation and burial were greatest. Predation accounted for only 4-20% of total mortality and showed no distinct pattern among treatments. Growth of oysters in a controlled laboratory experiment also increased with flow speed, while food flux did not significantly influence growth.; Hydrographic conditions (T, S and DO) and water depth were found not to significantly influence production of oysters in one experiment. However, results of another experiment showed that interaction of bottom water hypoxia/anoxia, resulting from stratification and eutrophication, and harvest-related degradation of oysters reefs caused mass mortality of oysters and other oyster reef inhabitants on harvest-degraded reefs located in deep water. Interaction between habitat structure, hydrodynamics, sedimentation, water quality and habitat degradation implies a need for the integrative approach of ecosystem management to preserve and restore oyster populations and the temperate biogenic reef habitat they provide.