5th Coral Reef Ecology Symposium May 22nd, 2015 Bremen Quantifying the roles of primary production and the sponge loop in maintaining biological activity in a northern Red Sea coral reef van Hoytema, N. 1, Rix, L. 1, van Oevelen, D. 2, Cardini, U. 1, Bednarz, V.N. 1, Naumann, M.S. 1, Al-‐ Horani, F.A. 3, Wild, C. 1,4 1 Coral Reef Ecology, Leibniz Center for Tropical Marine Ecology, Bremen, Germany 2 Ecosystem studies, Royal Netherlands Institute for Sea Research, Yerseke, the Netherlands 3 Faculty of Marine Sciences, The University of Jordan, Aqaba, Jordan 4 University of Bremen, Germany corresponding author: n.vanhoytema@gmail.com Coral reefs in the northern Red Sea experience strong seasonal oligotrophic conditions due to water column stratification during summer. These conditions may limit reef primary production, and subsequently, overall biological activity. Cavity sponges in the Pacific and Caribbean potentially take up dissolved organic carbon (DOC) at rates per reef surface area comparable to total primary production. Part of this DOC may then be made available to higher trophic levels by high particulate organic carbon (POC) release by the sponges. This "sponge loop" may be a key component of the characteristically high biological activity in those coral reefs and may far exceed pelagic prokaryote DOC uptake (i.e. the microbial loop). Coral reefs in the northern Red Sea harbor high densities of cavity sponges, indicating that these may play a major role in local reef carbon cycling. The cavity sponge DOC transformation capacity in Red Sea reefs remains, however, to be determined. Here, we present novel findings from extensive fieldwork in a Jordanian fringing reef. Seasonal photosynthetic carbon fixation, as well as POC and DOC uptake and release fluxes of the benthic, pelagic, and cavity-‐sponge communities have been combined into inverse trophic models to compare the magnitude of the different ecosystem processes. Seasonal reef metabolic balance and overall biological activity will be discussed with particular focus on the contribution to primary production and DOC cycling by dominant functional groups. In addition, sponge vs. pelagic prokaryote DOC uptake will be compared, to determine the seasonal importance of sponges for DOC uptake, and overall carbon cycling in this northern Red Sea coral reef. http://coresymposium.zmt-‐bremen.com coresymposium@gmail.com