Seasonal changes of dissolved organic carbon photo-oxidation rates in a tropical humic lagoon: the role of rainfall as a major regulator

Abstract

In this paper, we report the seasonal variation of photo-oxidation rates in a tropical humic lagoon and its relation to annual rainfall regime. Photo-oxidation rates ranged from 8.96 to 415.06 µmol C·L–1·day–1, being higher in the beginning to middle of the rainy season and declining throughout the year. Although dissolved organic carbon (DOC) concentration, water color, and sunlight incidence were generally higher in the rainy season, photo-oxidation rates were not significantly related to any of these parameters. Photo-oxidation seems to be influenced mainly by changes in DOC photoreactivity, which was up to threefold higher early in the rainy season, when inputs of fresh allochthonous DOC take place. In the following months, in addition to being continuously degraded by sunlight, DOC is also removed from the water column by processes such as microbial degradation and sedimentation, leading to a decline in DOC concentration and photoreactivity throughout the year until the next rainy season. Thus, the dynamics of DOC inputs caused by the rainfall regime in Comprida Lagoon lead to a yearly pulse of DOC photoreactivity and photo-oxidation rates. We believe this pulse model also fits other aquatic ecosystems subject to similar seasonal inputs of allochthonous DOC, although rainfall would not necessarily be the driving factor.