Processes governing phytoplankton blooms in estuaries. I: The local production-loss balance

The formation and spatial distribution of phytoplankton blooms in estuaries are controlled by (1) local mechanisms, which determine the production-los s balance for a water column at a particular spatial location (i.e, control if a bloom is possible), and (2) transport-related mechanisms, which gover n biomass distribution (i.e. control if and where a bloom actually occurs). In this study, the first of a 2-paper series, we use a depth-averaged nume rical model as a theoretical tool to describe how interacting local conditi ons (water column height. light availability, benthic grazing) influence th e local balance between phytoplankton sources and sinks. We also explore tr ends in the spatial variability of the production-loss balance across the t opographic gradients between deep channels and lateral shoals which are cha racteristic of shallow estuaries. For example, under conditions of high tur bidity and slow benthic grazing the highest rates of phytoplankton populati on growth are found in the shallowest regions. On the other hand, with low turbidity and rapid benthic grazing the highest growth rates occur in the d eeper areas. We also explore the effects of semidiurnal tidal variation in water column height. as well as spring-neap variability. Local population g rowth in the shallowest regions is very sensitive to tidal-scale shallowing and deepening of the water column, especially in the presence of benthic g razing. A spring-neap signal in population growth rate is also prominent in the shallow areas. Population growth in deeper regions is less sensitive t o temporal variations in tidal elevation. These results show that both shal low and deep regions of estuaries can act as sources or sinks for phytoplan kton biomass, depending on the local conditions of mean water column height , tidal amplitude, Light-Limited growth rate, and consumption by grazers.