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The formation and spatial distribution of phytoplankton blooms in estuaries are controlled by (1) local mechanisms, which determine the production-loss balance for a water column at a particular spatial location (ie control if a bloom is possible), and (2) transport-related mechanisms, which govern biomass distribution (ie control if and where a bloom actually occurs). In this study, the first of a 2-paper series, we use a depth-averaged numerical model as a theoretical tool to describe how interacting local conditions (water column height, light availability, benthic grazing) influence the local balance between phytoplankton sources and sinks. We also explore trends in the spatial variability of the production-loss balance across the topographic gradients between deep channels and lateral shoals which are characteristic of shallow estuaries. For example, under conditions of high turbidity and slow benthic grazing the …