Effects of bacterial dynamics on organic matter decomposition and nutrient release from sediments: A modeling study

Nutrient inputs to lakes, reservoirs, estuaries, and continental shelf waters are often dominated by nutrient release from sediments. Rates of nutrient release from the decomposition of sedimentary organic matter are determined by bacterial demands for food and energy. This dependence of decomposition kinetics on bacterial dynamics, however, is not explicitly considered in current models of early diagenesis. Here, we present a new model of early diagenesis that is based on soil decomposition models and that explicitly includes bacterial biomass as a state variable. Using the new model, we perform steady-state sensitivity analyses and integrate two time-dependent scenarios to determine the major controls on the abundance of bacteria in sediments and on sediment nutrient release. First, we test the sensitivity of bacterial biomass and nutrient release to substrate quantity and quality, and to bacterial growth parameters, growth efficiency and mortality. The model predicts that bacterial abundance in the sediments and nutrient release rates increase significantly with higher substrate inputs and higher quality of organic substrate. High growth efficiencies, on the other hand, reduce nutrient release rates and lead to more nutrient immobilization into bacterial biomass. Efficiency driven increases in bacterial abundances must be counteracted by higher mortality rates, if bacterial pool sizes are to remain within the range of values reported in the literature. Second, we simulate eutrophication and lake recovery over a 20-year period and the effect of a short-term peak in substrate input from an algal bloom. The model predicts that nutrient immobilization in the bacterial pool during eutrophication reduces nutrient release, while the decline of bacterial biomass in response to reduced substrate loadings prolongs lake recovery. The model further suggests that bacterial dynamics dampen the response of nutrient release to short-term peak inputs of organic matter. When bacterial biomass is allowed to vary over time, nutrient release rates are suppressed at their peak but remain elevated for a longer time. Models that do not consider the role of bacterial growth dynamics for organic matter decomposition are not able to simulate the potentially important effects of nutrient immobilization and variable decomposition rates on nutrient release.