Eutrophication effects on CH₄ and CO₂ fluxes in a highly urbanized tropical reservoir (Southeast, Brazil)

Shallow urban polluted reservoirs at tropical regions can be hotspots for CO₂ and CH₄ emissions. In this study, we investigated the relationships between eutrophication and GHG emissions in a highly urbanized tropical reservoir in São Paulo Metropolitan Area (Brazil). CO₂ and CH₄ fluxes and limnological variables (water and sediment) were collected at three sampling stations classified as hypereutrophic and eutrophic. Analysis of variance (ANOVA) and the principal component analysis (PCA) determined the most significant parameters to CO₂ and CH₄ fluxes. ANOVA showed significant differences of CO₂ and CH₄ fluxes between sampling stations with different trophic state. The hypereutrophic station showed higher mean fluxes for both CO₂ and CH₄ (5.43 ± 1.04 and 0.325 ± 0.167 g m^-2 d^-1, respectively) than the eutrophic stations (3.36 ± 0.54 and 0.060 ± 0.005 g m^-2 d^-1). The PCA showed a strong relationship between nutrients in the water column (surface and bottom) and GHG fluxes. We concluded that GHG fluxes were higher whenever the trophic state increases as observed previously in temperate and tropical reservoirs. High concentrations of nutrients in the water column in the studied area support the high production of autotrophic biomass that, when sedimented, ends up serving as organic matter for CH₄ producers. These outcomes reinforce the necessity of water quality improvement and eutrophication mitigation in highly urbanized reservoirs in tropical regions.