Nitrate repletion during spring bloom intensifies phytoplankton iron demand in Yangtze River tributary, China

Most aquatic systems show characteristic seasonal fluctuations in the total nutrient pool supporting primary productivity. The nutrient dynamics essentially exacerbate critical demand for the counterpart micronutrients towards achieving ecosystem equilibrium. Herein, the phytoplankton demand for iron (Fe) uptake under high concentration of nitrate-nitrogen during spring in Xiangxi Bay, China, was studied. Our result confirmed that significant Fe concentrations (P = 0.01) in both autumn (0.62 ± 0.02 mgL^-1) and winter (0.06 ± 0.03 mgL^-1) relative to spring (0.004 ± 0.01 mgL^-1) are linked to the low NO₃^-N paradigms during autumn and winter. As NO₃^-N showed a sharp increase in spring, a dramatic reduction in the Fe pool was observed in the entire tributary, driving the system to a critical Fe limited condition. Bioassay study involving Fe additions both alone and in combinations led to maximum growth stimulation with biomass as chla (16.44 ± 0.82 μgL^-1) and phytoplankton cell density (6.75 × 10⁶ cellsL^-1) which differed significantly (P = 0.03) with the control. Further, the study demonstrated that Fe additions triggered biomass productions which increased linearly with cell densities. The P alone addition caused biomass production (15.26 ± 2.51 μgL^-1) greater than both NO₃^-N (9.15 ± 0.66 μgL^-1) and NH₄⁺N (13.65 ± 1.68 μgL^-1) separate additions but reported a low aggregate cell density (3.18 × 10⁶ cellsL^-1). This indicates that nutrient and taxonomic characteristics e.g., high cell pigment contents rather than just the cell bio-volume also determine biomass. The Bacilliarophyta, Chlorophyta, and Cryptophyta with the total extinction of Cyanophyta characterized the bloom in spring. The anthropogenic NO₃^-N input into XXB would have driven to higher NO₃^-N than NH₄⁺N situation, and incapacitated the Cyanophyta that preferentially utilize NH₄⁺N. Our study provides a useful report for incorporation into the monitoring programs for prudent management of phytoplankton bloom and pollution across the eutrophic systems.