Aquaculture: 2004 Publications

Newell, R.I.E.  Ecosystem influences of natural and cultivated populations of suspension-feeding bivalve molluscs: A review.  Journal of Shellfish Research 23(1): 51-61, 2004.
© National Shellfisheries Association

Notes: Suspension-feeding bivalves serve to couple pelagic and benthic processes because they filter suspended particles from the water column and the undigested remains ejected as mucus-bound feces and pseudofeces, sink to the sediment surface. This biodeposition call be extremely important in regulating water column processes where bivalves are abundant in coastal waters and in seasons when water temperatures are warm enough to promote active feeding. Bivalves under these conditions can exert ''top-down'' grazer control on phytoplankton and in the process reduce turbidity thereby increasing the amount of light reaching the sediment surface. This has the effect of reducing the dominance of phytoplankton production and extending the depth to which ecologically important benthic plants, such as seagrasses and benthic microalgae, can grow. Nitrogen and phosphorus, excreted by the bivalves and regenerated from then biodeposits, are recycled back to the water column and support further phytoplankton production. In some situations, however, bivalves can also exert ''bottom-up'' nutrient control on phytoplankton production by changing nutrient regeneration processes within the sediment. Some of the N and P that vas originally incorporated in phytoplankton, but was not digested by the bivalves, can become buried in the accumulating sediments. Where biodeposits are incorporated in aerobic surficial sediments that overlay deeper anaerobic sediments, microbially mediated, coupled nitrification-denitrification can permanently remove N from the sediments as N-2 gas. Consequently, natural and aquaculture-reared stocks of bivalves are potentially a useful supplement to watershed management activities intended to reduce phytoplankton production by curbing anthropogenic N and P inputs to eutrophied aquatic systems. Environmental conditions at bivalve aquaculture sites should be carefully monitored, however, because biodeposition at very high bivalve densities may be so intense that the resulting microbial respiration reduces the oxygen content of the surrounding sediments. Reduction in sediment oxygen content can inhibit coupled nitrification-denitrification, cause P to become unbound and released to the water column and the resulting buildup of H2S can be toxic to the benthos.