Assessing the effects of native freshwater mussels on aquatic nitrogen dynamics

Management of the nitrogen cycle has been identified as one of fourteen grand challenges for engineering and is an especially important issue for agricultural watersheds of the Upper Midwest. In large river systems, it has become increasingly important to understand the interactions between abiotic and biotic processes that influence nitrogen cycling. Native freshwater mussels are one of the most influential organisms in aquatic ecosystems due to their ability to transfer nutrients from the overlying water to the sediments and stimulate production across multiple trophic levels. The goal of this study was to utilize flow-through laboratory mesocosms, highly time resolved water chemistry data, and a mass balance model to assess the effects of native freshwater mussels on aquatic nitrogen dynamics. The effects of mussels on concentrations of nitrate, ammonium, organic nitrogen, nitrite, total nitrogen, and phytoplankton in the overlying water of the mesocosms were analyzed using untreated Iowa River water.

Concentration changes for nitrate, ammonium, and phytoplankton were determined to be significantly different (ANCOVA, p 0.05) between mesocosms containing mussels and mesocosms without mussels (control). Results from this study indicated that mussels increased ammonium via mussel excretion, indirectly increased nitrate via nitrification of the excreted ammonium, and decreased phytoplankton via mussel filtration. Results also indicated that mussels increased nitrite and total nitrogen concentrations and demonstrated minimal impacts on organic nitrogen. The majority of nitrogen mass delivered to the overlying water by mussels was in the form of ammonium and nitrate (nitrate mass was added via nitrification of the excreted ammonium).

The deterministic mass balance model developed to better understand the effects of mussels on nitrogen dynamics was calibrated with literature values and highly time resolved data and grab samples obtained from laboratory mesocosm experiments. Sensitivity analyses identified hydraulic retention time, temperature, denitrification rate, and mussel ammonium excretion rate as the most influential variables in mesocosms containing mussels. The sensitivity analyses also demonstrated the difficulty in modeling the dynamic nature of the mesocosms and emphasized the need to constrain the model variables with observed experimental measurements. Application of the model predicted that increases in phytoplankton concentrations significantly influenced the effect of mussels on nitrogen dynamics in the overlying water of the mesocosms.

The results of this study will aid the scalability of mussel effects to larger systems and will help to predict how changes in environmental conditions influence the interactions of biotic and abiotic processes. These findings will help determine to what extent the effects of mussels should be included in strategies for nitrogen management.