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Environmental factors and thresholds for nitrogen fixation by phytoplankton in tropical reservoirs
Author(s) -
Moutinho Fellipe H. M.,
Marafão Gabriela A.,
Calijuri Maria do Carmo,
Moreira Marcelo Z.,
Marcarelli Amy M.,
Cunha Davi G. F.
Publication year - 2021
Publication title -
international review of hydrobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.524
H-Index - 52
eISSN - 1522-2632
pISSN - 1434-2944
DOI - 10.1002/iroh.202002057
Subject(s) - eutrophication , phytoplankton , nitrogen fixation , cyanobacteria , chlorophyll a , dominance (genetics) , environmental chemistry , temperate climate , phosphorus , chlorophyll , nitrogen , biology , environmental science , ecology , nutrient , botany , chemistry , organic chemistry , biochemistry , genetics , bacteria , gene
In theory, the phytoplankton community of freshwater ecosystems with low concentrations of dissolved inorganic nitrogen (DIN) can obtain this element by atmospheric nitrogen (N 2 ) fixation. This process could explain the dominance of cyanobacteria in tropical reservoirs, yet is rarely quantified in these systems. Assessing the factors related to N 2 fixation can assist in the management of cyanobacterial blooms. Our study characterized environmental factors related to N 2 fixation in Brazilian tropical reservoirs with contrasting trophic states, and defined quantitative thresholds for water chemistry and physical characteristics that stimulated N 2 fixation. We used field assays with 15 N for estimating N 2 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 N 2 fixation rates. In general, the N 2 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 N 2 throughout the year in tropical reservoirs. Our results suggested that phosphorus is the main nutrient controlling the rates of N 2 fixation when N 2 ‐fixing cyanobacteria were present. Phosphorus abatement is, thus, crucial for managing the trophic state and controlling N 2 ‐fixing cyanobacteria in these ecosystems.