Dan Yu, Tengyue Fang, Jixin Chen , Nengwang Chen, Jian Ma

Journal of Environmental Management

https://doi.org/10.1016/j.jenvman.2025.125429

Published: 18 April 2025

Abstract

The dynamics of nutrient cycling in inland waters, particularly during algal blooms, play a critical role in shaping aquatic ecosystems. However, the interaction between phytoplankton and inorganic nitrogen during low-flow periods remains poorly understood. This study presents high-frequency monitoring of ammonium (NH4-N), nitrate (NO3-N), nitrite (NO2-N) and phytoplankton communities in a subtropical river-type reservoir during the low-flow winter period, characterized by blooms dominated by cryptophytes and green algae. Our results revealed that NH4-N concentrations exhibited a diurnal pattern of decreasing during the day and increasing during the night, which was negatively correlated with total algal biomass at the intraday fluctuation scale (coefficient = −0.378, p = 0.028), indicating strong algal uptake of ammonium during daytime. NO3-N and NO2-N concentrations, however, did not show clear diurnal co-varied patterns with algae. On the day-to-day scale, the external nitrogen inputs resulting from rainfall contributed to the changes, particularly after extended dry periods. We observed low NH4-N concentrations and total algal biomass during the end of algal bloom. However, 3–4 days later ammonium thrived, followed by another algal bloom. Algal bloom occurrences caused large diurnal fluctuations in reservoir NH4-N concentrations (daily differences >5 μmol L−1), resulting the maximum nighttime NH4-N flux reaching up to five times the minimum daytime flux. Our study highlights the advantages of high-frequency synchronous monitoring of nutrient-algae dynamics to understand their interactions, and the importance of ammonium control on preventing algal blooms during low-flow winter periods.