Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 4 3 2007 10.5194/bg-4-311-2007 http://www.biogeosciences.net/4/311/2007/ http://www.biogeosciences.net/4/311/2007/bg-4-311-2007.html http://www.biogeosciences.net/4/311/2007/bg-4-311-2007.pdf 311 322 2007-06-12 Importance of intertidal sediment processes and porewater exchange on the water column biogeochemistry in a pristine mangrove creek (Ras Dege, Tanzania) S. Bouillon steven.bouillon@vub.ac.be J. J. Middelburg F. Dehairs A. V. Borges G. Abril M. R. Flindt S. Ulomi E. Kristensen Vrije Universiteit Brussel, Dept. of Analytical and Environmental Chemistry, Pleinlaan 2, 1050 Brussels, Belgium Netherlands Institute of Ecology, Centre for Estuarine and Marine Ecology (NIOO-KNAW), Yerseke, The Netherlands Université de Liège, Chemical Oceanography Unit, Institut de Physique (B5), 4000 Liège, Belgium Environnements et Paléoenvironnements Océaniques (EPOC), CNRS-UMR 5805, Université Bordeaux 1, Avenue des Facultés, 33405, Talence, France Institute of Biology, University of Southern Denmark, 5230 Odense, Denmark Faculty of Aquatic Sciences and Technology, University of Dar es Salaam, Tanzania We sampled a tidal creek (Ras Dege, Tanzania) during a 24-h cycle to document the variations in a suite of creek water column characteristics and to determine the relative influence of tidal and biological driving forces. Since the creek has no upstream freshwater inputs, highest salinity was observed at low tide, due to evaporation effects and porewater seepage. Total suspended matter (TSM) and particulate organic carbon (POC) showed distinct maxima at periods of highest water flow, indicating that erosion of surface sediments and/or resuspension of bottom sediments were an important source of particulate material. Dissolved organic carbon (DOC), in contrast, varied in phase with water height and was highest at low tide. Stable isotope data of POC and DOC displayed large variations in both pools, and similarly followed the variations in water height. Although the variation of δ¹³C_DOC (−23.8 to −13.8‰) was higher than that of δ¹³C_POC (−26.2 to −20.5‰), due to the different end-member pool sizes, the δ¹³C signatures of both pools differed only slightly at low tide, but up to 9‰ at high tide. Thus, at low tide both DOC and POC originated from mangrove production. At high tide, however, the DOC pool had signatures consistent with a high contribution of seagrass-derived material, whereas the POC pool was dominated by marine phytoplankton. Daily variations in CH₄, and partial pressure of CO₂ (pCO₂) were similarly governed by tidal influence and were up to 7- and 10-fold higher at low tide, which stresses the importance of exchange of porewater and diffusive fluxes to the water column. When assuming that the high dissolved inorganic carbon (DIC) levels in the upper parts of the creek (i.e. at low tide) are due to inputs from mineralization, δ¹³C data on DIC indicate that the organic matter source for mineralization had a signature of −22.4‰. Hence, imported POC and DOC from the marine environment contributes strongly to overall mineralization within the mangrove system. Our data demonstrate how biogeochemical processes in the intertidal zone appear to be prominent drivers of element concentrations and isotope signatures in the water column, and how pathways of dissolved and particulate matter transport are fundamentally different. 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