A biogeochemical model for metabolism and nutrient cycling in a Southeastern Piedmont impoundment

While non-point nutrient loads are important determinants of biological productivity in Southeastern Piedmont impoundments, productivity can be attenuated by concomitant sediment loads that reduce the biological availability of these nutrients. A biogeochemical model is proposed that explicitly accounts for the effects of sediment–nutrient interactions on multiple components of phytoplankton metabolism dynamics, including algal photosynthesis and respiration, pH, carbonate speciation, dissolved oxygen, and biochemical oxygen demand. Sediment–nutrient interactions relate nutrient uptake and release to pH, sediment oxygen demand, sediment organic matter, and iron. pH is a state variable in our model, affects sediment–nutrient adsorption, and constrains model parameters. The model replicates water quality observations in a small Southeastern Piedmont impoundment and suggests that pH-dependent sediment–nutrient adsorption dominates both orthophosphate and ammonium dynamics, with phosphate adsorption being controlled by ligand exchange to iron oxides, and ammonium adsorption being controlled by the cation exchange capacity. Sediment organic matter accumulation and decay also affects nutrient availability, and may explain the long-term increase of hypolimnetic dissolved oxygen deficit in Lake Lanier, a large Southeastern Piedmont impoundment.