Acúmulo e exportação de carbono, nitrogênio, fósforo e metais em canais de maré dos manguezais de Marapanim, Costa Norte Brasileira

Abstract

In this study, we assessed the potential of intertidal mudflat sediments along mangrove forest to accumulate and export carbon, nutrients (N and P) and metals (Fe and Mn), in addition to examine the influence of the Amazonian seasonal rainfall patterns on the physicochemical conditions, diagenetic processes and exchange of nutrients and metals along the sediment-water interface (SWI) in intertidal mudflats fringed by pristine mangroves. The study was carried out in the Marapanim River estuarine system (northern Brazil), which is fringed by extensive areas of well-developed mangroves, part of the largest continuous and best preserved mangrove forest in the world, located approximately 200 km from west of the mouth of the Amazon River. The results of this work are presented in two articles. The first deals with the potential of stocks and accumulation of COT, NT and PT in intertidal mudflat sediments, in addition we investigate the potential sources of sedimentary organic matter (OM). The second assesses the influence of seasonal rainfall pattern on physicochemical properties, the diagenetic processes and the diffusive fluxes of nutrients at the SWI in the Marapanim mangrove creeks. During wet season the salinity values decreased as a consequence of the increase in rainfall, with a dilution-mixing zone in the top sediment (<15 cm). In the dilution-mixing zone, Fe (III) and Mn (IV) reduction are the dominant anaerobic respiratory processes in the sediments. The redox zonation of sediments oscillated in response to the rainfall patterns, with the highest concentrations of Fe2+ and Mn2+ in deeper sediment layers during the dry season. Under suboxic conditions, the mudflat sediments act as a source of Fe2+, Mn2+, NH4 +, and PO4 3- to the water column, and these fluxes were driven by rainfall. The results indicated that mangrove-fringed mudflats are quite effective in retaining carbon, nutrients and iron in sediment solid phases than exporting to the coastal waters, while can potentially be a significant contributor to the oceanic Mn cycle. The potential of these mangrove creeks to accumulate these elements is directly related to grain size, sources and susceptibility of OM degradation. In addition, temporal variability in pyrite formation revealed that the solid phase retention mechanisms are also susceptible to seasonal effects, with lower concentrations of chromium-reducible sulfur (CRS, mainly pyrite fraction) during the dry season. Therefore, we show that these seasonal variabilities implied substantial changes of physicochemical properties and the diagenetic processes, affecting the release of metals and nutrients from the SWI and their accumulation in the sediment.