Benthic Algal Biomass in the Etowah Basin and Implications to Establishing Nutrient Criteria in Streams

Abstract

We determined algal biomass in 30 tributaries to the Etowah River in northern Georgia. Mean chlorophyll a ranged from extremely low to 150 mg/m². Several physical and chemical variables were measured as part of this study including dissolved nutrients, total suspended solids, substrate particle size and stream slope. In a multivariate model of algal biomass, three variables, average particle size (+), dissolved oxygen (+), and soluble reactive phosphorus (SRP) (-), entered into the model, which explained 78% of the variation in algal biomass. We subsequently determined that annual concentrations of SRP were strongly related to measures of sedimentation and fine particles at a site. Thus, higher concentrations of SRP were associated with increases in small particles and any positive effect of increased SRP on algal biomass was out-weighed by the negative effect of sedimentation among sites. In a model that included only dissolved nutrients and TSS as potential independent variables, dissolved inorganic nitrogen was positively related and TSS was negatively related to algal biomass. Our data illustrate that sedimentation strongly impacts availability of algae in these watersheds. In addition, these effects can obscure predictive relationships between nutrients and algal biomass, such that alternative nutrient criteria (e.g., biotic indicators) may be more robust where watersheds receive excess sediment.