Are increases in nutrients associated with increases in algal biomass (chlorophyll a) in agricultural streams?
NAWQA developed statistical models to predict algal biomass (chlorophyll a) on the basis of specific nutrient concentrations at 143 sites in 5 different agricultural regions across the Nation. The statistical model relating total phosphorus to algal biomass explained only 12 percent of the variability. The relation between total nitrogen concentrations and chlorophyll a was even weaker. Weak relations in some regions can be explained by nutrient concentrations so far above what plants require that additional increases in nutrients have little effect on algal biomass. Modeled relations improved—explaining up to 52 percent of the variability in algal biomass in some regions —when stream characteristics, such as water temperature and canopy cover, were included in the analysis. The wide range in biological response to nutrient concentrations supports the need for a regional approach to nutrient criteria, but factors related to stream habitat and flow characteristics should be part of any
