Preliminary results from a shallow water benthic grazing study

The nutrient-rich, shallow waters of San Francisco Bay support high rates of primary production, limited not by nutrients but by light availability and benthic grazing (Alpine and others 1992; Cloern 1982). Phytoplankton blooms are an important food source for upper trophic levels. Consequently animal populations, such as fish, may suffer under conditions of high benthic bivalve grazing. It has been hypothesized that several species of fish are suffering as a result of severe decreases in available phytoplankton since the introduction of Potamocorbula amurensis into San Francisco Bay (Feyrer 2003).

The extent of reduction in phytoplankton biomass by benthic bivalves is dependent on both physical and biological factors in addition to their spatial and temporal variability. Physical factors identified as important include: (1) vertical mixing rates, which are a function of wind velocity, currents, and bottom roughness; (2) suspended sediment concentrations; and (3) phytoplankton settling rates. The biological factors controlling the extent of phytoplankton grazing include animal density and organism size, pumping rate, food type and concentration, metabolic demands, assimilation efficiency, and behaviour (Wildish and Kristmanson 1997).

Several laboratory studies involving model and live clams have shown that benthic grazers can deplete phytoplankton in the water column (for example, Cole and others 1992). Initially, these studies assumed that the water column remained well mixed above benthic suspension feeders; therefore, parameters measured in the bulk water column were believed to be representative of available particle concentration. For this reason many relationships describing the influence of the bulk flow and bulk seston concentration on benthic grazers physiological processes exist (for example, Levinton 1991). 

Laboratory measurements using live animals have shown that filtration rates vary with free stream velocity (for example, Levinton 1991). Increases in current speed lead to an increase in filtration rate; however, several studies have shown that filtration may cease at some critical current speed. It has been suggested that resuspension, occurring as a result of high current speeds, may be a factor that negatively affects uptake (Cloern 1987; Levinton 1991). Several mechanisms have been invoked to explain the effects of low speed on growth rates of active suspension feeders. These mechanisms include the formation of a concentration boundary layer and the limiting horizontal flux of seston. It is now accepted that a combination of these factors dictates the growth success of benthic grazers in a particular area.

Several field studies have shown that concentration boundary layers can form over benthic ecosystems (for example, Frechette and others 1989, Dolmer 2000); however, many of these studies have failed to measure the hydrodynamics needed to calculate benthic grazing rates. Furthermore, calculating benthic grazing rates with vertical measurements at a single point is problematic due to lack of knowledge of the horizontal gradients in seston (Thompson and others, forthcoming).

Despite great improvements in our knowledge on the effects of benthic grazers on seston concentrations in water columns, the effects of different hydrodynamic conditions on grazing rates has not been formulated. This makes it difficult to assess the system-wide effect of the benthic ecosystem on phytoplankton concentrations. Furthermore, it affects our ability to predict the potential success of a benthic species, such as the invasive clams Corbicula fluminea and Potamocorbula amurensis. This paper presents the preliminary results of a control volume approach to elucidate the effect of different hydrodynamic conditions on the grazing rates of Corbicula fluminea.