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Abstract: Spatial gradients in biomass and community composition have important consequences for ecosystem structure and function. In this study, small-scale inshore–offshore (1–10 km) and vertical (1–50 m) patterns of microphytoplankton biomass and community composition are described, and the environmental controls of microphytoplankton biomass are evaluated in a coastal ecosystem of the Southern California Bight (SCB). During a two-year period, persistent inshore–offshore gradients in phytoplankton biomass and occasional inshore–offshore gradients in community composition, coincident with regional precipitation, were found, although the strength of the gradients varied between sampling periods. The chlorophyll a maximum was generally present between 15 and 45 m, the cell abundance maximum occurred in surface waters, and there was little evidence of vertical gradients in community composition. Variability in chlorophyll a concentrations was linked to variability in environmental parameters only after some rain and upwelling events. This study demonstrates that inshore–offshore patterns in phytoplankton biomass previously documented at large spatial scales (100–700 km) in the SCB can also persist at smaller scales (1–10 km), although the mechanisms for the gradients are likely to be different at the different spatial scales. The results provide a baseline data set that can be used to focus monitoring and management efforts in the SCB. In particular, this work shows that a limited number of sampling stations are sufficient for phytoplankton monitoring in Santa Monica Bay. [Copyright &y& Elsevier]