Organic matter release by Red Sea coral reef organisms-potential effects on microbial activity and in situ O-2 availability

This study presents a comprehensive dataset (223 reef organisms that were separately incubated during 44 independent experiments during 4 seasonal expeditions) of dissolved and particulate organic matter (DOM and POM) release by dominant benthic organisms from the Northern Red Sea. Reef organisms studied were scleractinian and fire corals, the upside-down jellyfish and reef-associated algae. Subsequently, the effect of this organic matter (OM) release on microbial activity was determined. These studies were complemented by high resolution, in situ O-2 concentration measurements within reef environments that were dominated by corals or algae. Dissolved organic carbon (DOC) release was 14.5 +/- 2.3 mg m(-2) surface area h(-1) for all 9 investigated reef algae, which was significantly higher than DOC release by scleractinian corals during all seasons except winter. POM release (particulate organic carbon and nitrogen, POC and PON, respectively) was observed for all investigated reef organisms. Benthic reef algae released 5.1 +/- 0.5 mg POC m(-2) h(-1) and 0.35 +/- 0.03 mg PON m(-2) h(-1), which are significantly higher than POM release rates by scleractinian corals in spring and autumn. Algae-derived OM, presumably the DOC fraction, stimulated microbial activity in the adjacent water more significantly than OM released by the investigated scleractinian and fire corals. Consequently, the daily mean and minimum in situ O-2 concentrations in the water directly above the reef (<= 10 cm) were significantly higher in coral dominated than in algae dominated sites, confirming the in situ relevance of results of previous laboratory studies. Findings also suggest that benthic reef algae decrease O-2 availability in waters close to reef environments via the release of labile OM and its subsequent fast microbial degradation.