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Abstract Phytoplankton form the base of the food web and are often the main primary producer in estuaries. Freshwater inflow variability will modulate phytoplankton biomass and production in estuaries through effects on nutrient delivery and cycling in estuaries, as well as on light availability and flushing rates. The goal of this chapter is to build upon the conceptual framework of inflow-nutrient-phytoplankton dynamics proposed by Longley (1994) by highlighting advances that have been made since then. Specific objectives are to: quantify coast-wide relationships between inflow, nutrients, and phytoplankton biomass (chlorophyll a ; hereafter “chlorophyll”) in Texas estuaries; quantify variability in phytoplankton biomass in relation to inflow variability; identify key drivers of this variability in individual systems where sufficient data exists (Galveston Bay, Matagorda Bay, Baffin Bay); and highlight deficiencies and future data needs. The overarching conclusion is that freshwater inflow plays a major role in external nutrient loadings to Texas estuaries, but its influence on phytoplankton dynamics is complicated because of effects on not only nutrient availability but also flushing rates and the light environment within estuaries. One recommendation based on the synthesis here is that there is a strong need for more system-specific studies to elucidate mechanistic linkages between inflow, nutrients, and phytoplankton, because this information will be vital for developing management plans in areas where inflow rates are changing and/or where nutrient pressures may be a growing concern.