Organic matter release by coral reef associated benthic algae in the Northern Red Sea

Recent research indicates that coral reef associated benthic algae may control important metabolic processes in reef ecosystems via organic matter release. Yet little information is available about quantity and chemical composition of these algae-derived exudates. Therefore first comprehensive studies on algal organic matter release were conducted at a fringing reef ecosystem in the Northern Red Sea. Dissolved organic carbon (DOC), particulate organic carbon (POC) and nitrogen (PN) release by dominant reef associated benthic algae (Caulerpa serrulata, Peyssonnelia capensis, turf algae assemblages) were quantified during 4 seasonally resolved expeditions. Additionally, 4 seasonal blooming (Ulva lactuca, Enteromorpha flexuosa, Liagora turneri, Hydroclathrus clathratus) and 2 patchy growing algae species (Lobophora variegata, Saragassum dentifolium) were included in these investigations. To complement the dataset organic matter release by Caulerpa was studied under different light conditions, simulating water depths of 1, 5, 10 and 20 m. Environmental parameters (temperature, light availability, inorganic nutrient concentrations) were monitored simultaneously to assess potential effects on algal organic matter release. All 9 investigated genera of benthic algae exuded DOC and POC in amounts of 12.2 ± 2.1 and 4.2 ± 0.3 mg organic C m−² algae surface area h− 1, respectively. Resident algae, primarily turf algae assemblages, displayed highest and seasonal blooming algae lowest organic matter release rates. Results therefore indicate that organic matter release rates are rather influenced by functional properties (growth form, life strategy) of algae than by taxonomic affiliation. Quantities of organic matter release showed seasonal and depth-mediated variations and were positively correlated with temperature and light availability within photosynthetically active radiation intensities of 0 to 300 µmol quanta m− 2 s− 1, suggesting photoinhibition as limiting factor for productivity and subsequent organic matter release. Stable isotope signatures of algae-derived organic carbon were within a common range and likewise subjected to seasonal variations (δ13C summer: − 11.2‰ ± 0.2‰; δ13 winter: − 16.7‰ ± 0.4‰). These data provide first comprehensive information about a) the potential contribution of different benthic reef algae to cycles of matter and b) environmental key factors influencing organic matter release by benthic algae in the investigated fringing reef ecosystem.