In estuaries, primary producers take up nutrients such as nitrogen and phosphorus (D’Elia 1986). Therefore the growth of phytoplankton is correlated with a depletion of nutrients in estuaries. Freshwater inflows carry nutrients into estuaries, replenishing the stock for phytoplankton use (D’Elia 1986). The sources of many of the nutrients are from human activities including sewage inputs and agricultural runoff.

Annual variations in delivery of nutrient to the estuary by freshwater inflows are primarily due to annual rain and snowfall (TWDB 2012). Human activities have altered the concentrations and timing of nutrients like nitrogen and phosphorus. The changes in nutrient concentration have implications for the species dynamic of the estuary (D’Elia 1986). The species composition of algae may change to favor species, such as toxic algae, having negative affects on human and aquatic species, such as oysters, health (USEPA 2013).

The EPA has developed nutrient pollution outreach and education materials for the public that can be found online.