BIOLOGICAL EFFECTS OF MOLLUSCS: BIOGEOCHEMICAL CYCLING AND HABITAT PROVISION

Benthic suspension feeders, such as many species of bivalve molluscs, influence the nutrient and organic coupling of benthic and pelagic systems (Dame, 1996) through their ability to filter a wide size range of particles and deposit organic wastes that sink to the bottom (biodeposition). Suspension-feeding bivalves perform this function in a range of habitats and physiographic conditions (e.g., estuaries, lagoons, coastal oceanic systems) where they filter out and deposit significant amounts of suspended material, as well as excrete dissolved nutrients. In estuarine systems, the influence of benthic suspension-feeding bivalves on benthic-pelagic coupling, turbidity, nutrient remineralization, primary production, deposition, and habitat complexity has been well documented (reviewed in Dame and Olenin, 2005). Kaiser (2001) reviews the effects of molluscan cultivation on the ecology of systems, identifying a similar set of mechanisms of influence, and concludes that such processes have a generally positive influence on the overall water quality of a system. Suspension-feeding bivalves also drive many other biogeochemical processes and cycles, which are well described for intertidal oysters by Dame (2005).

Nutrient Dynamics

Molluscs influence nutrient dynamics through direct excretion and indirectly through microbially mediated remineralization of their organic deposits in the sediments (McKindsey et al., 2006a). Therefore, nutrient regeneration is related to the abundance and location (shallow versus deep water) of bivalves in a system. The extent to which this affects overall nutrient budgets and thus primary production is related to the system flushing rate and residence time (Dame, 1996; Newell et al., 2005). The subsequent proportions of elements in the system will influence the levels of recycling and possibly result in one or more being limited (Dame, 1996).

The majority of studies of bivalve effects on nutrient recycling have focused on nitrogen because this is the most common nutrient-limiting biological production in marine and estuarine systems (Parsons et al., 1983; Howarth, 1988; National Research Council, 2000). Benthic bivalves are important contributors of nitrogen (usually in the form of ammonium, NH4+) to both subtidal and intertidal systems. Nixon et al. (1976) conclude that nitrogen flux across oyster reefs is highly variable and is heavily influenced by tidal flow. Dame (1986) reviews a body of work relating to nutrient fluxes involving Crassostrea gigas in northern France and concludes that 15–40% of nitrogen in the system was derived from the oysters. In addition, measured values were always higher than the estimated values, likely due to remineralization occurring in adjacent



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