| Popis: |
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. |
