Origin, concentration, availability and fate of dissolved organic carbon in coastal lagoons of the Rio de Janeiro State/Origem, concentração, disponibilidade e destino do carbono orgânico dissolvido em lagoas costeiras do Estado do Rio de Janeiro

The coastal lagoons in the northern Rio de Janeiro State (Brazil) present a wide gradient of dissolved organic carbon (DOC) and water color, with the highest DOC concentrations reported in the literature for aquatic ecosystems. The authors reviewed data from 2 decades of studies on the carbon cycle in these coastal lagoons and discussed the fluctuations in the concentration and quality of DOC, factors affecting DOC microbial and photochemical degradation, CO2 emission, as well as the role of humic and non-humic carbon to the energy flow through the trophic chains. They show that DOC quality, not its quantity, determines the rates of photochemical and microbial degradation both seasonally and spatially, with the exception of DOC photo-oxidation among lagoons, which is partially explained by DOC concentration at regional scale. In humic lagoons, there is a fairly predictable pattern of seasonal variation in DOC concentration associated to rainfall-induced inputs of allochthonous C. Depth-integrated photo-oxidation rates were less representative in highly humic lagoons, due to strong light attenuation in the water column.