Driving Forces Behind Nutrient and Organic Matter Dynamics in a Mangrove Tidal Creek in North Brazil

In the mangrove of the Caeté Estuary (North Brazil) a detailed sampling was carried out in order to gain insight into the dynamics of inorganic nutrients and organic matter in one of the world's largest mangroves and to identify the driving forces behind these processes. Throughout 36 tidal cycles in the course of one year, concentrations of dissolved and particulate organic carbon and nitrogen (DOC, POC, DON, PON) and dissolved inorganic nutrients (N, P, Si compounds) were determined in a mangrove tidal creek. Annual average concentrations (in uM) were DOC 360, POC 240, DON 20, PON 29, dissolved inorganic nitrogen (DIN) 11, silicate 170 and phosphate 2.4. Ammonium and nitrite were about 80 and 3% of DIN, respectively. Nutrient dynamics in the creek were significantly influenced by porewater input from the upper forest sediment layer. This led to characteristic tidal signatures of nutrient and organic matter concentrations with maximum values during low tide. Annual phosphate and DOC oscillations were caused mainly by tidally dependent variations of porewater input. Their concentrations in the creek were directly proportional to the hydraulic gradient between creek and sediment water table. Due to autotrophic activity, dissolved oxygen, pH and DOC were higher and DIN was lower during daytime than at night. Annual oscillations of DIN and DON could also be attributed to varying phytoplanktonic activity. Silicate and phosphate showed only weak response to aquatic photosynthesis with slightly lower concentrations during the day. Assuming equilibrated fluxes, nitrogen fixation in the forest could be estimated to be 2.3 mmol N/m2/d. Based on these findings, tidal range and porewater concentrations were identified as driving forces behind coastal outwelling of nutrients and organic matter from mangroves. Outwelling probably occurs only from mangroves where the nutrient concentration in porewater exceeds the demands of the benthic community and trees, caused by positive sedimentation balances and high nitrogen fixation rates, and only in macrotidal regions where porewater can flow in considerable amounts to the tidal creeks and the ocean