Inorganic nitrogen and phytoplankton dynamics in a subtropical reservoir during low-flow winter periods: Implications for nitrogen management

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.