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) and winter (0.06 ± 0.03 mgL) relative to spring (0.004 ± 0.01 mgL) are linked to the low NON paradigms during autumn and winter. As NON 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) and phytoplankton cell density (6.75 × 10 cellsL) 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) greater than both NON (9.15 ± 0.66 μgL) and NHN (13.65 ± 1.68 μgL) separate additions but reported a low aggregate cell density (3.18 × 10 cellsL). 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 NON input into XXB would have driven to higher NON than NHN situation, and incapacitated the Cyanophyta that preferentially utilize NHN. Our study provides a useful report for incorporation into the monitoring programs for prudent management of phytoplankton bloom and pollution across the eutrophic systems.