Iron operates as an important factor promoting year-round diazotrophic cyanobacteria blooms in eutrophic reservoirs in the tropics

We hypothesize that iron availability plays an important role in driving phytoplankton structure and dynamics in tropical reservoir. This role has often been neglected in routine monitoring of water quality since the main focus has been addressed on macronutrients such phosphorus and nitrogen. To elucidate the potential regulation exerted by iron availability on phytoplankton, a limnological investigation was carried out in two neighboring eutrophic reservoirs in tropical China. Traditional physical and chemical variables were measured along with total and dissolved iron concentrations to explore how they could influence phytoplankton structure. Statistical analyses showed that dissolved iron (DFe) as well as dissolved nitrogen (DIN) and soluble reactive phosphorus (SRP) significantly explained the variance of phytoplankton assemblages in both reservoirs. We could also find that diazotrophic cyanobacteria blooms generally developed when DFe concentrations were above 0.025 mg/L. Moreover, the DFe availability in the water columns was modulated by the effects exerted by the stratification/circulation patterns on redox conditions and pH values. Anaerobic conditions favored the release of iron and phosphorus from sediments, and likely favored an enhanced nitrogen assimilation and a decrease of DIN in the epilimnion. Our results highlighted that iron functions as an important factor promoting diazotrophic cyanobacteria and driving seasonal phytoplankton succession in tropical eutrophic reservoirs, not only with a direct effect on phytoplankton growth but also indirect effect by modifying the availability of nitrogen and phosphorus.