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Mangroves are highly complex ecosystems occupying a major part of tropical coastlines. High primary productivity, efficient biological nutrient recyling and a permanent exchange with terrestrial and marine ecosystems are their common features. Despite the high production and export rates of leaf litter, mangrove detritus has been reported to be of minor importance in sustaining marine food webs. The geographical distribution of mangrove-derived organic matter (OM) in marine sediments is found to be restricted to the vicinity of its source. Dissolved nutrient inputs from mangroves and rivers may fuel the production of marine OM. In this paper we assess the relevance of mangroves for the production and sedimentation of OM in the tropical coastal ocean based on data available from the literature and our own research results. We estimate the rates of carbon accumulation in mangrove sediments and of carbon export to the coastal seas. From the rates of litter fall and export we calculate carbon accumulating in mangrove sediments to be in the order of 23x10(12) g C per year and mangrove carbon introduced into the coastal ocean to be in the order of 46x10(12) g C per year. They account for about 11% of the total input of terrestrial carbon into the ocean and 15% of the total carbon accumulating in modern marine sediments.

We tested the hypothesis that CO 2 supersaturation along the aquatic conduit over Sweden can be explained by processes other than aquatic respiration. A first generalized-additive model (GAM) analysis evaluating the relationships between single water chemistry variables and pCO 2 in lakes and streams revealed that water chemistry variables typical for groundwater input, e.g., dissolved silicate (DSi) and Mg 2 1 had explanatory power similar to total organic carbon (TOC). Further GAM analyses on various lake size classes and stream orders corroborated the slightly higher explanatory power for DSi in lakes and Mg 2 1 for streams compared with TOC. Both DSi and TOC explained 22-46% of the pCO 2 variability in various lake classes (0.01-4100 km 2 ) and Mg 2 1 and TOC explained 11-41% of the pCO 2 variability in the various stream orders. This suggests that aquatic pCO 2 has a strong groundwater signature. Terrestrial respiration is a significant source of the observed supersaturation and we may assume that both terrestrial respiration and aquatic respiration contributed equally to pCO 2 efflux. pCO 2 and TOC concentrations decreased with lake size suggesting that the longer water residence time allow greater equilibration of CO 2 with the atmosphere and inlake mineralization of TOC. For streams, we observed a decreasing trend in pCO 2 with stream orders between 3 and 6. We calculated the total CO 2 efflux from all Swedish lakes and streams to be 2.58 Tg C yr À1 . Our analyses also demonstrated that 0.70 Tg C yr À1 are exported to the ocean by Swedish watersheds as HCO 3 À and CO 3 2À of which about 0.56 Tg C yr À1 is also a residual from terrestrial respiration and constitute a long-term sink for atmospheric CO 2 . Taking all dissolved inorganic carbon (DIC) fluxes along the aquatic conduit into account will lower the estimated net ecosystem C exchange (NEE) by 2.02 Tg C yr À1 , which corresponds to 10% of the NEE in Sweden.

Abstract. Hypoxia has become a world-wide phenomenon in the global coastal ocean and causes a deterioration of the structure and function of ecosystems. Based on the collective contributions of members of SCOR Working Group #128, the present study provides an overview of the major aspects of coastal hypoxia in different biogeochemical provinces, including estuaries, coastal waters, upwelling areas, fjords and semi-enclosed basins, with various external forcings, ecosysCorrespondence to: J. Zhang (jzhang@sklec.ecnu.edu.cn) tem responses, feedbacks and potential impact on the sustainability of the fishery and economics. The obvious external forcings include freshwater runoff and other factors contributing to stratification, organic matter and nutrient loadings, as well as exchange between coastal and open ocean water masses. Their different interactions set up mechanisms that drive the system towards hypoxia. Coastal systems also vary in their relative susceptibility to hypoxia depending on their physical and geographic settings. It is understood that coastal hypoxia has a profound impact on the sustainability of ecosystems, which can be seen, for example, by the change in the food-web structure and system function; other Published by Copernicus Publications on behalf of the European Geosciences Union. 1444 J. Zhang et al.: Natural and human-induced hypoxia and consequences for coastal areas influences include compression and loss of habitat, as well as changes in organism life cycles and reproduction. In most cases, the ecosystem responds to the low dissolved oxygen in non-linear ways with pronounced feedbacks to other compartments of the Earth System, including those that affect human society. Our knowledge and previous experiences illustrate that there is a need to develop new observational tools and models to support integrated research of biogeochemical dynamics and ecosystem behavior that will improve confidence in remediation management strategies for coastal hypoxia.

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