The formation of aggregates in coral reef waters under elevated concentrations of dissolved inorganic and organic carbon: A mesocosm approach

Abstract

The transformation of dissolved organic carbon (DOC) to particulate organic carbon is the major mechanism through which large sinking organic particles are formed in aquatic systems. Global stressors, such as high concentrations of dissolved inorganic carbon (DIC) due to ocean acidification, as well as local stressors, such as high DOC concentrations due to coastal eutrophication, can significantly affect the formation and settling of aggregates and thereby the marine biogeochemical carbon cycle. Increasing aggregate formation rates can contribute to the mortality of benthic organisms in coral reef ecosystems, but relevant knowledge is scarce. Therefore, the present study addresses this issue and studies the individual and combined effects of high DIC (900 μatm) and DOC (150μM glucose) on organic matter dynamics as well as the formation of organic aggregates in a manipulative study over 42days using 24 mesocosms dominated by either benthic calcifying algae or by hard corals. Organic aggregates in terms of transparent exopolymer particle (TEP) concentrations and total aggregated volume were measured. Results showed lower TEP concentrations and aggregated volume under high DIC concentrations. By contrast, under DOC enrichment higher rates of aggregate formation and microbial oxygen uptake were observed. Surprisingly, the highest aggregate formation rates were observed under the combined DIC and DOC enrichment. Furthermore, benthic organisms influenced the availability of DOC resulting in higher aggregate formation in coral compared to calcifying algae mesocosms. These experiments simulate future ocean conditions in coastal ecosystems where elevated DOC concentrations could aggravate the effect of high DIC on aggregate formation. In coral reef ecosystems, this may have important consequences on benthic organisms.