Benthic Suspension Feeders in Shallow Coastal Water
17
from field observations of reduced algal biomass in areas of extensive mussel
culture (Cadee and Hegeman 1974) and depletion of phytoplankton biomass
in waters passing bivalve beds (Wright et al. 1982; Carlson et al. 1984). More
quantitative estimates of the potential effects of bivalve grazing on a system
level, viz. South San Francisco Bay, came from mathematical models that
included bivalve filtration rates and phytoplankton growth rates (Cloern
1982; Officer et al.I982).
Studies over the last two decades indicate that bivalve grazing can be a
major factor controlling phytoplankton biomass in numerous estuarine and
coastal systems (see Smaal and Prins 1993; Dame 1996 for reviews). An
elaborate computer model simulation has shown that bivalve filtration can
effectively control phytoplankton biomass irrespective of nutrient loading
(Herman and Scholten 1990). In the latter case, the open nature of these shallow coastal systems is essential, otherwise the accumulation of nutrients will
inevitably result in eutrophication symptoms despite high grazing pressure
(Herman 1993).
Predictions of the system level effects of bivalve grazing are based on the
up-scaling of bivalve filtration rates from laboratory observations to the scale
of an entire estuary. A few experimental studies have attempted to measure
filtration rates under in situ conditions. Filtration rates calculated from
chlorophyll depletion by mussel beds generally showed agreement between in
situ values and filtration rate estimates from laboratory incubations with food
in natural seawater (Prins et al. 1996; however, see Asmus et al. 1998). Other
studies indicated that laboratory estimates of filtration rates using algal diets
could significantly overestimate the natural filtration activity of bivalves
(Doering and Oviatt 1986).
In addition to estimates of water column turnover rates by filtration,
physical factors have to be included for the assessment of the grazing impact
of bivalves on the pelagic system. The upper limit to the biomass of benthic
suspension feeders in an estuarine system depends, to a large extent, on the
residence time of the water as a measure of food exchange with the sea, and
benthic biomass is ultimately limited by system productivity (Heip et al.
1995). Similarly, the strength of benthic grazing impact on the pelagic community depends on the water renewal rate (Smaal and Prins 1993). This point
was elaborated by Dame (1996), who argued that systems with a short residence time need a high bivalve biomass to water volume ratio, resulting in a
short clearance time, in order to control phytoplankton biomass via bivalve
grazing. A prerequisite for benthic grazing control is a well-mixed water column. In poorly mixed systems, vertical stratification uncouples the water column from the benthos and creates phytoplankton bloom conditions (Koseff
et al. 1993; Lucas et al. 1998).
Dame et al. (1980,1984,1985) recognized that, in addition to the processes
leading to removal of material from the water column, bivalve grazing can act
17
from field observations of reduced algal biomass in areas of extensive mussel
culture (Cadee and Hegeman 1974) and depletion of phytoplankton biomass
in waters passing bivalve beds (Wright et al. 1982; Carlson et al. 1984). More
quantitative estimates of the potential effects of bivalve grazing on a system
level, viz. South San Francisco Bay, came from mathematical models that
included bivalve filtration rates and phytoplankton growth rates (Cloern
1982; Officer et al.I982).
Studies over the last two decades indicate that bivalve grazing can be a
major factor controlling phytoplankton biomass in numerous estuarine and
coastal systems (see Smaal and Prins 1993; Dame 1996 for reviews). An
elaborate computer model simulation has shown that bivalve filtration can
effectively control phytoplankton biomass irrespective of nutrient loading
(Herman and Scholten 1990). In the latter case, the open nature of these shallow coastal systems is essential, otherwise the accumulation of nutrients will
inevitably result in eutrophication symptoms despite high grazing pressure
(Herman 1993).
Predictions of the system level effects of bivalve grazing are based on the
up-scaling of bivalve filtration rates from laboratory observations to the scale
of an entire estuary. A few experimental studies have attempted to measure
filtration rates under in situ conditions. Filtration rates calculated from
chlorophyll depletion by mussel beds generally showed agreement between in
situ values and filtration rate estimates from laboratory incubations with food
in natural seawater (Prins et al. 1996; however, see Asmus et al. 1998). Other
studies indicated that laboratory estimates of filtration rates using algal diets
could significantly overestimate the natural filtration activity of bivalves
(Doering and Oviatt 1986).
In addition to estimates of water column turnover rates by filtration,
physical factors have to be included for the assessment of the grazing impact
of bivalves on the pelagic system. The upper limit to the biomass of benthic
suspension feeders in an estuarine system depends, to a large extent, on the
residence time of the water as a measure of food exchange with the sea, and
benthic biomass is ultimately limited by system productivity (Heip et al.
1995). Similarly, the strength of benthic grazing impact on the pelagic community depends on the water renewal rate (Smaal and Prins 1993). This point
was elaborated by Dame (1996), who argued that systems with a short residence time need a high bivalve biomass to water volume ratio, resulting in a
short clearance time, in order to control phytoplankton biomass via bivalve
grazing. A prerequisite for benthic grazing control is a well-mixed water column. In poorly mixed systems, vertical stratification uncouples the water column from the benthos and creates phytoplankton bloom conditions (Koseff
et al. 1993; Lucas et al. 1998).
Dame et al. (1980,1984,1985) recognized that, in addition to the processes
leading to removal of material from the water column, bivalve grazing can act