Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 1 1 2004 10.5194/bg-1-71-2004 http://www.biogeosciences.net/1/71/2004/ http://www.biogeosciences.net/1/71/2004/bg-1-71-2004.html http://www.biogeosciences.net/1/71/2004/bg-1-71-2004.pdf 71 78 2004-10-28 Carbon sources supporting benthic mineralization in mangrove and adjacent seagrass sediments (Gazi Bay, Kenya) S. Bouillon T. Moens F. Dehairs Department of Analytical and Environmental Chemistry, Mangrove Management Group, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium Biology Department, Marine Biology Section, Ghent University, Krijgslaan 281/Sterre S8, B-9000 Gent, Belgium The origin of carbon substrates used by in situ sedimentary bacterial communities was investigated in an intertidal mangrove ecosystem and in adjacent seagrass beds in Gazi bay (Kenya) by &delta;¹³C analysis of bacteria-specific PLFA (phospholipid fatty acids) and bulk organic carbon. Export of mangrove-derived organic matter to the adjacent seagrass-covered bay was evident from sedimentary total organic carbon (TOC) and &delta;¹³C_TOC data. PLFA &delta;¹³C data indicate that the substrate used by bacterial communities varied strongly and that exported mangrove carbon was a significant source for bacteria in the adjacent seagrass beds. Within the intertidal mangrove forest, bacterial PLFA at the surface layer (0-1cm) typically showed more enriched &delta;¹³C values than deeper (up to 10cm) sediment layers, suggesting a contribution from microphytobenthos and/or inwelled seagrass material. <br /><br /> Under the simplifying assumption that seagrasses and mangroves are the dominant potential end-members, the estimated contribution of mangrove-derived carbon to benthic mineralization in the seagrass beds (16-74%) corresponds fairly well to the estimated contribution of mangrove C to the sedimentary organic matter pool (21-71%) across different seagrass sites. Based on the results of this study and a compilation of literature data, we suggest that trapping of allochtonous C is a common feature in seagrass beds and often represents a significant source of C for sediment bacteria - both in cases where seagrass C dominates the sediment TOC pool and in cases where external inputs are significant. Hence, it is likely that data on community respiration rates systematically overestimate the role of in situ mineralization as a fate of seagrass production.