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

Author(s)

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]

Affiliation

^[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

• Document type: Journal article
• Source: INTERNATIONAL REVIEW OF HYDROBIOLOGY; v. 106, n. 1 NOV 2020.
• Web of Science Citations: 0

Abstract

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 Opportunities: Regular Research Grants