THE ROLE OF DISSOLVED ORGANIC CARBON (DOC) PHOTO-OXIDATION IN AQUATIC ECOSSYSTEMS

Abstract

Dissolved organic carbon (DOC) is major and dynamic carbon pool in the biosphere, with great influence on the global carbon cycle. Besides being consumed by aquatic bacteria, DOC is also degraded by sunlight. This process of photodegradation, or photo-oxidation, transforms DOC, changing its bioavailability, or even oxidizes DOC directly into CO2. In this paper, we review the main biogeochemical motivations for the research on this process in aquatic ecosystems. First, we discuss general aspects of DOC, its degradation by bacteria and light and the relationship between these two processes. We mention the role of photo-bleaching in regulating the light penetration in the water column and its effects on aquatic organisms. The potential of sunlight for the photo-degradation of toxic compounds is also shown. We shortly mention some methods for the study of photodegradation. We also point out the main regulating factors of photo-degradation and how they may affect the seasonal variation of photo-oxidation rates. In a global analysis, we show that photo-oxidation rates are significantly higher in tropical ecosystems, what is partially explained by higher DOC concentrations. Photo-oxidation rates is positively related to DOC, although the relation is not quite strong (r2 = 0.42), due to factors such as seasonal variation of DOC photo-reactivity. We propose that the high solar incidence throughout the year and seasonal inputs of photo-reactivity interact to produce high photo-oxidation rates in tropical ecosystems.