Concentrations of dissolved and particulate nutrients, chlorophyll, and microorganisms (0.01 to 200 μm) were simultaneously measured during a 1 d survey of 12 stations in Florida Bay, USA, to characterize the microbial plankton community with respect to resource limitation. Three distinct types of trophic conditions, reflected in seston elemental stoichiometry and community structure, were identified within the bay. The first type, characteristic of the isolated eastern region, had low nutrient concentrations, imbalanced stoichiometry, and small microbial biomass with a large proportion of bacteria. The microbial community in this region was characterized by weak relationships between microzooplankton and phytoplankton and the predominance of mixotrophic taxa and the autotrophic ciliate Mesodinium rubrum. The second type, found in the north-central region influenced by Taylor Slough inflow, had elevated nutrient concentrations, elemental stoichiometry skewed toward N, and high turbidity. Under these conditions, the picocyanobacterium Synechococcus formed a dense bloom and coincided with an abundant, multi-step microbial food web. Finally, at the boundary with the Gulf of Mexico, low concentrations of nutrients were balanced at approximately the Redfield ratio and supported nanophytoplankton that were tightly correlated with microzooplankton. These data are consistent with the notion of P limitation in Florida Bay but also demonstrate that Si, light, and N may be co-limiting to phytoplankton in the eastern, north-central, and western boundary regions, respectively. Our findings suggest that multiple resource gradients, in conjunction with microbial food web processes, are important factors determining the plankton community structure in Florida Bay and should be considered in studies on ecological disturbances.
