Variações de curto prazo do metabolismo e da pCO2 na lagoa rodrigo de freitas: Elevado dinamismo em um ecossistema tropical urbano

Abstract

The balance between photosynthesis and respiration (metabolism) play an important role on the partial pressure of carbon dioxide (pCO2) in productive urban waters. In this way, tropical coastal lagoons may show intense metabolic processes, influenced by common natural and anthropogenic fluctuations in inputs from the watershed or in the marine water exchange. Here, we assessed short-term changes in the aquatic metabolism and pCO2 in waters of an urban tropical lagoon (Rodrigo de Freitas lagoon) over seven consecutive days. The Rodrigo de Freitas Lagoon is located in a high densely populated area in the city of Rio de Janeiro - Brazil, receiving domestic sewage discharges that cause recurrent episodes of organism mortalities. The measurements of CO2 and O2 concentrations were performed at surface and bottom waters to estimate air-water CO2 fluxes, aquatic metabolism and the respiration index (RI). Our results revealed a negative relationship between both gases, confirming the role of the aquatic metabolism on the predominance of CO2 supersaturation and O2 undersaturation observed here, which is described for most inland waters. Additionally, we observed events of RI near the critical limit to aerobic respiration (around 0.7), especially in bottom waters with persistence of CO2 supersaturation over time. In contrast, surface waters were more variable for metabolic variables. Comparing the periods before and during the rain coupled to gate openings to the sea (Rainy+Gate) with that after, surface waters showed shifts from sources to sink of CO2 to the atmosphere (from 3.1 to - 1.4 mmol m-2 d-1). The decoupling between the daily aquatic metabolism and saturation of CO2 or O2 in waters of Rodrigo de Freitas Lagoon suggested that metabolic gases may be also influenced by intense changes in the metabolism over previous days. In conclusion, rainfall and the openings of the connection to the sea likely contributed to these drastic changes in the aquatic metabolism. The biological responses after natural and anthropogenic processes might be enhanced in those tropical urban ecosystems showing typical high temperatures and substantial nutrient inputs.