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(Reference retrieved automatically from Web of Science through information on FAPESP grant and its corresponding number as mentioned in the publication by the authors.)

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

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Moutinho, Fellipe H. M. [1] ; Marafao, Gabriela A. [1] ; Calijuri, Maria do Carmo [1] ; Moreira, Marcelo Z. [2] ; Marcarelli, Amy M. [3] ; Cunha, Davi G. F. [1]
Total Authors: 6
[1] Univ Sao Paulo SHS EESC USP, Escola Engn Sao Carlos, Dept Hidraul & Saneamento, Ave Trabalhador Sao Carlense, BR-13566590 Sao Carlos, SP - Brazil
[2] Univ Sao Paulo CENA USP, Ctr Energia Nucl Agr, Piracicaba, SP - Brazil
[3] Michigan Technol Univ, Dept Biol Sci, Houghton, MI 49931 - USA
Total Affiliations: 3
Document type: Journal article
Web of Science Citations: 0

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 N-15 for estimating N-2 fixation rates by phytoplankton. The highest rates normalized by chlorophyll-a (maximum of 143 x 10(-4) mu g-N mu g-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 degrees C), soluble reactive phosphorus (SRP) (>= 3.0 mu g-P L-1), total phosphorus (TP) (>= 20.5 mu g-P L-1), DIN:SRP (<= 487) and DIN:TP (<= 82) molar ratios, chlorophyll-a (>= 12 mu g 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 degrees 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. (AU)

FAPESP's process: 14/02088-5 - Nitrogen biological fixation and uptake by phytoplankton in subtropical reservoirs with different trophic status
Grantee:Davi Gasparini Fernandes Cunha
Support type: Regular Research Grants