Environmental factors and thresholds for nitrogen fixation by phytoplankton in tropical reservoirs

In theory, the phytoplankton community of freshwater ecosystems with low concentrations of dissolved inorganic nitrogen (DIN) can obtain this element by atmospheric nitrogen (N₂) fixation. This process could explain the dominance of cyanobacteria in tropical reservoirs, yet is rarely quantified in these systems. Assessing the factors related to N₂ fixation can assist in the management of cyanobacterial blooms. Our study characterized environmental factors related to N₂ fixation in Brazilian tropical reservoirs with contrasting trophic states, and defined quantitative thresholds for water chemistry and physical characteristics that stimulated N₂ fixation. We used field assays with ¹⁵N for estimating N₂ fixation rates by phytoplankton. The highest rates normalized by chlorophyll-a (maximum of 143 × 10⁻⁴ μg-N μg-Chl-a⁻¹ h⁻¹) coincided with eutrophic conditions and presence of diazotrophs. Receiver operating characteristic (ROC) analysis provided significant thresholds for water temperature (≥22°C), soluble reactive phosphorus (SRP) (≥3.0 μg-P L⁻¹), total phosphorus (TP) (≥20.5 μg-P L⁻¹), DIN:SRP (≤487) and DIN:TP (≤82) molar ratios, chlorophyll-a (≥12 μg L⁻¹), and total suspended solids (≥4 mg L⁻¹). Censored regressions confirmed that temperature, chlorophyll-a, and phosphorus were important predictors of N₂ fixation rates. In general, the N₂ fixation rates determined in this study were lower than those found for temperate reservoirs. However, the temperature threshold of 22°C or above, identified in our analysis, suggests that phytoplankton have the potential to fix N₂ throughout the year in tropical reservoirs. Our results suggested that phosphorus is the main nutrient controlling the rates of N₂ fixation when N₂-fixing cyanobacteria were present. Phosphorus abatement is, thus, crucial for managing the trophic state and controlling N₂-fixing cyanobacteria in these ecosystems.