The effect of clam grazing on phytoplankton spring blooms in the low-salinity zone of the San Francisco Estuary: A modelling approach

Benthic grazing by estuarine bivalves can be an important top-down process impacting pelagic food webs. In the low-salinity zone of the San Francisco Estuary mass balance calculations and models have reported that clams (especially the invasive Potamocorbula amurensis) suppress phytoplankton blooms. However, spring blooms frequently occur. We aimed to understand this clam paradox using a biogeochemical modelling approach to evaluate the effects of clam grazing and excretion on phytoplankton production and nutrient uptake. The conceptual framework combines both the reduction of phytoplankton biomass by grazing and the role of ammonium (from clam excretion, wastewater plant discharge and sediment efflux) in minimizing chlorophyll accumulation, since phytoplankton cannot access nitrate (the major form of available nitrogen) for growth, due to ammonium suppression of nitrate uptake. We constructed the CLAMFLOW model by adding a clam module (with pathways of nitrogen for clam grazing, clam excretion and sediment efflux) to an existing phytoplankton-nitrogen-flow model. Whatever the parameter that was varied in the model (clam abundance, grazing, excretion, sediment efflux or flow) it decreased the peak nitrate uptake by the phytoplankton and shifted the time to reach peak uptake so delaying the likelihood of bloom initiation.