18
R.E Dame et al.
as an important link in a feedback loop with grazers inducing a flux of particulate nutrients from the water column to the sediment. That flux, in turn,
feeds mineralization and a reverse flux of inorganic nutrients from the
sediment to the water column. Several in situ studies, using flumes or benthic
tunnels on mussel beds (Dame and Dankers 1988; Prins and Smaa11990, 1994;
Asmus and Asmus 1991, 1993; Dame et al. 1991a,b; Asmus et al. 1995) and
natural oyster reefs (Dame et al. 1989; Dame and Libes 1993; Zurburg et al.
1994a,b), have confirmed that bivalve communities can be intense sites of
mineralization and sources of inorganic nutrients. Generally, direct excretion
by the bivalves is considered to be a less important nutrient source than geochemical processes. In addition to this positive feedback on inorganic nutrient pools through regeneration processes, the reduced storage of nutrients in
phytoplankton biomass as a consequence of grazing forms another positive
feedback that influences inorganic nutrient availability (Prins et al. 1995b,
1997). This process is important as it may stimulate phytoplankton primary
production in the case of nutrient limitation. It may also influence phytoplankton development as differences in nutrient regeneration rates can
change nutrient stoichiometry.
Evidence supporting the hypothesis that bivalve grazing exerts a significant control on plankton biomass comes from studies on experimental ecosystems, i. e., mesocosms (Table 1.2). In the MERL mesocosms at the University of Rhode Island, grazing by the clam Mercenaria mercenaria did not
reduce phytoplankton biomass, but resulted in higher algal growth rates
(Doering et al. 1986,1987). However, other experiments did show reduction in
phytoplankton biomass by bivalve grazing (Riemann et al. 1988; Olsson et al.
1992; Gramm et al. 1993; Prins et al. 1995b). Laboratory experiments have
shown that bivalves can filter microzooplankton species like ciliates (Le Gall
et al.1997) as well as larger zooplankton like copepods (Kimmerer et al.1994).
Mesocosm studies show that bivalve grazing can reduce biomass of microzooplankton (Horsted et al. 1988; Prins et al. 1995a,b, 2000). Some enclosure
studies showed no effects of mussels on copepods (Horsted et al. 1988),
probably as a consequence of the short duration of the studies (2-3 weeks)
relative to the generation time of the copepods. However, recent mesocosm
experiments showed a strong inhibition of copepod development (Prins et al.
2000). Newell (1988) argued that mesozooplankton could have increased in
abundance in Chesapeake Bay as a result of the decline in oyster stock. This
increase in copepods may be responsible for the increase in jellyfish as well as
a change in phytoplankton composition, because copepods have a more sizeselective feeding mode than oysters. Declines in copepod biomass in the San
Francisco Bay were ascribed to predation by the introduced clam Potamocorbula amurensis on nauplii (Kimmerer et al. 1994). Consequently, bivalve
grazing may affect the structure of the pelagic food web, and control significant ecological processes within the estuary.
R.E Dame et al.
as an important link in a feedback loop with grazers inducing a flux of particulate nutrients from the water column to the sediment. That flux, in turn,
feeds mineralization and a reverse flux of inorganic nutrients from the
sediment to the water column. Several in situ studies, using flumes or benthic
tunnels on mussel beds (Dame and Dankers 1988; Prins and Smaa11990, 1994;
Asmus and Asmus 1991, 1993; Dame et al. 1991a,b; Asmus et al. 1995) and
natural oyster reefs (Dame et al. 1989; Dame and Libes 1993; Zurburg et al.
1994a,b), have confirmed that bivalve communities can be intense sites of
mineralization and sources of inorganic nutrients. Generally, direct excretion
by the bivalves is considered to be a less important nutrient source than geochemical processes. In addition to this positive feedback on inorganic nutrient pools through regeneration processes, the reduced storage of nutrients in
phytoplankton biomass as a consequence of grazing forms another positive
feedback that influences inorganic nutrient availability (Prins et al. 1995b,
1997). This process is important as it may stimulate phytoplankton primary
production in the case of nutrient limitation. It may also influence phytoplankton development as differences in nutrient regeneration rates can
change nutrient stoichiometry.
Evidence supporting the hypothesis that bivalve grazing exerts a significant control on plankton biomass comes from studies on experimental ecosystems, i. e., mesocosms (Table 1.2). In the MERL mesocosms at the University of Rhode Island, grazing by the clam Mercenaria mercenaria did not
reduce phytoplankton biomass, but resulted in higher algal growth rates
(Doering et al. 1986,1987). However, other experiments did show reduction in
phytoplankton biomass by bivalve grazing (Riemann et al. 1988; Olsson et al.
1992; Gramm et al. 1993; Prins et al. 1995b). Laboratory experiments have
shown that bivalves can filter microzooplankton species like ciliates (Le Gall
et al.1997) as well as larger zooplankton like copepods (Kimmerer et al.1994).
Mesocosm studies show that bivalve grazing can reduce biomass of microzooplankton (Horsted et al. 1988; Prins et al. 1995a,b, 2000). Some enclosure
studies showed no effects of mussels on copepods (Horsted et al. 1988),
probably as a consequence of the short duration of the studies (2-3 weeks)
relative to the generation time of the copepods. However, recent mesocosm
experiments showed a strong inhibition of copepod development (Prins et al.
2000). Newell (1988) argued that mesozooplankton could have increased in
abundance in Chesapeake Bay as a result of the decline in oyster stock. This
increase in copepods may be responsible for the increase in jellyfish as well as
a change in phytoplankton composition, because copepods have a more sizeselective feeding mode than oysters. Declines in copepod biomass in the San
Francisco Bay were ascribed to predation by the introduced clam Potamocorbula amurensis on nauplii (Kimmerer et al. 1994). Consequently, bivalve
grazing may affect the structure of the pelagic food web, and control significant ecological processes within the estuary.