ECOSYSTEM INFLUENCES OF NATURAL AND CULTIVATED
POPULATIONS OF SUSPENSION-FEEDING BIVALVE MOLLUSCS: A REVIEW
R.I.E. Newell-2004
Journal of Shellfish Research, 23(1): 51-61
Abstract:
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 can 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
their 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 was 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, 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.
(Horn Point Laboratory, University of Maryland Center for Environmental
Science, Cambridge, Maryland 21613, USA, e-mail: newell@hpl.umces.edu)