Community structure has greater effect on water column ammonium cycling than nutrients and temperature in shallow lake mesocosms

Harmful algal blooms (HABs) impact lakes worldwide and are caused by excess nitrogen (N) and phosphorus (P) loading from watersheds. Climate warming and nutrient loading effects on N cycling were examined in shallow lake mesocosms in Denmark. N loading to some mesocosms ceased in June 2018 and resumed in June 2019. Ammonium (NH4+) uptake, regeneration, and nitrification and nitrate uptake rates were evaluated. High nutrient, ambient temperature mesocosms exhibited the highest NH4+ cycling rates. Before resumption of N loading in high nutrient mesocosms, NH4+ regeneration supported 46 % of potential microbial NH4+ demand, versus 24 % with N loading. Nutrient additions generally had a larger effect on rates than temperature changes; however, community structure (phytoplankton versus macrophytes) was the best predictor of NH4+ dynamics. In eutrophic, shallow lakes, where internal NH4+ regeneration can sustain HABs, as observed in some mesocosms, management strategies should aim to reduce external N and P loads and, if deemed necessary, implement biomanipulation methods to obtain macrophyte-dominated, clear-water states