Natural events of anoxia and low respiration index in oligotrophic lakes of the Atlantic Tropical Forest

[摘要] Hypoxia is a well-recognized condition reducing biodiversity andincreasing greenhouse gas emissions in aquatic ecosystems, especiallyunder warmer temperatures of tropical waters. Anoxia is a natural eventcommonly intensified by human-induced organic inputs in inland waters. Here,we assessed the partial pressure of O₂ (pO₂) and CO₂(pCO₂), and the ratio between them (represented by the respirationindex, RI) in two oligotrophic lakes of the Atlantic Tropical Forest,encompassing dry and rainy seasons over 19 months. We formulated thehypothesis that thermal stratification events could be coupled to naturalhypoxia in deep waters of both lakes. Our results indicated a persistence ofCO₂ emissions from these tropical lakes to the atmosphere, on average ± standard error(SE) of 17.4 mg C m⁻² h⁻¹ probably subsided byterrestrial C inputs from the forest. Additionally, the thermalstratification during the end of the dry season and the rainy summer wascoupled to anoxic events and very low RI in deep waters, and tosignificantly higher pO₂ and RI at the surface (about 20 000 μatmand 1.0, respectively). In contrast, the water mixing during dry seasons atthe beginning of the winter was related to a strong destratification inpO₂, pCO₂ and RI in surface and deep waters, without reaching anyanoxic conditions throughout the water column. These findings confirm ourhypothesis, suggesting that lakes of the Atlantic Tropical Forest could bedynamic, but especially sensitive to organic inputs. Natural anoxic eventsindicate that tropical oligotrophic lakes might be highly influenced byhuman land uses, which increase organic discharges into the watershed.

[学科分类] 地球化学与岩石