Different responses of phytoplankton taxa to water N and P inputs in a freshwater wetland: A mesocosm study.

The intensification of human activities has led to a large amount of nitrogen (N) and phosphorus (P) inputs into water, resulting in an increase in nutrient load and an imbalance of N and P stoichiometric ratio in wetlands. However, whether and how water eutrophication influences phytoplankton diversity, community composition, and biomass remain largely unclear. As part of a two-year (2022-2023) field experiment, this study was conducted to examine the effects of N and P inputs on phytoplankton community in a freshwater wetland in the North China Plain. The results showed that N and P inputs did not change the Shannon-Wiener or Evenness indices of phytoplankton community, but increased phytoplankton biomass by 30 % and 62 %, respectively. In addition, N input enhanced the biomass of non-dominant taxa (e.g., Cryptomonas, Chrysophyta, Dinoflagellates, and Euglena) by two-fold, likely due to the inhibited growth of submerged macrophytes and thus the increased water temperature. In contrast, P input increased the biomass of dominant taxa (e.g., Cyanobacteria, Bacillariophyta, and Chlorophyta) by 116 %, which was primarily attributed to the elevated water pH and reduced light intensity. Moreover, the enhanced phytoplankton biomass under water eutrophication positively contributed to the water-air interface methane (CH) emissions in this study, suggesting an important role of phytoplankton in regulating wetland C cycling. Our findings indicate the differentially regulatory mechanisms of N and P inputs on the structure and biomass of phytoplankton community, and can provide more insights for protecting and managing wetland ecosystems under water eutrophication.