Simulated mussel mortality thresholds as a function of mussel biomass and nutrient loading

A freshwater "mussel mortality threshold" was explored as a function of porewater ammonium (NH₄⁺) concentration, mussel biomass, and total nitrogen (N) utilizing a numerical model calibrated with data from mesocosms with and without mussels. A mortality threshold of 2 mg-N L^-1 porewater NH₄⁺ was selected based on a study that estimated 100% mortality of juvenile Lampsilis mussels exposed to 1.9 mg-N L^-1 NH₄⁺ in equilibrium with 0.18 mg-N L^-1 NH₃. At the highest simulated mussel biomass (560 g m^-2) and the lowest simulated influent water "food" concentration (0.1 mg-N L^-1), the porewater NH₄⁺ concentration after a 2,160 h timespan without mussels was 0.5 mg-N L^-1 compared to 2.25 mg-N L^-1 with mussels. Continuing these simulations while varying mussel biomass and N content yielded a mortality threshold contour that was essentially linear which contradicted the non-linear and non-monotonic relationship suggested by Strayer (2014). Our model suggests that mussels spatially focus nutrients from the overlying water to the sediments as evidenced by elevated porewater NH₄⁺ in mesocosms with mussels. However, our previous work and the model utilized here show elevated concentrations of nitrite and nitrate in overlying waters as an indirect consequence of mussel activity. Even when the simulated overlying water food availability was quite low, the mortality threshold was reached at a mussel biomass of about 480 g m^-2. At a food concentration of 10 mg-N L^-1, the mortality threshold was reached at a biomass of about 250 g m^-2. Our model suggests the mortality threshold for juvenile Lampsilis species could be exceeded at low mussel biomass if exposed for even a short time to the highly elevated total N loadings endemic to the agricultural Midwest.