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Abstract. Hypersaline tidal flats (HTFs) are coastal ecosystems with freshwater deficits often occurring in arid or semi-arid regions near mangrove supratidal zones with no major fluvial contributions. Here, we estimate that organic carbon (OC), total nitrogen (TN) and total phosphorus (TP) were buried at rates averaging 21 (±6), 1.7 (±0.3) and 1.4 (±0.3) gm-2yr-1, respectively, during the previous century in three contrasting HTF systems, one in Brazil (eutrophic) and two in Australia (oligotrophic). Although these rates are lower than those from nearby mangrove, saltmarsh and seagrass systems, the importance of HTFs as sinks for OC, TN and TP may be significant given their extensive coverage. Despite the measured short-term variability between net air–saltpan CO2 influx and emission estimates found during the dry and wet season in the Brazilian HTF, the only site with seasonal CO2 flux measurements, the OC sedimentary profiles over several decades suggest efficient OC burial at all sites. Indeed, the stable isotopes of OC and TN (δ13C and δ15N) along with C:N ratios show that microphytobenthos are the major source of the buried OC in these HTFs. Our findings highlight a previously unquantified carbon as well as a nutrient sink and suggest that coastal HTF ecosystems could be included in the emerging blue carbon framework.

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In Brazil and in many other tropical countries, large urban cities and populations are still growing on the coast and coverage in terms of sewage treatments is far from desirable. Cultural eutrophication is not solely a threat for the coastal ocean; it is now acting as one of its major biogeochemical and ecological driver. Along the littoral of the state of Rio de Janeiro, semi-enclosed marine bays and lagoons show clear spatial and temporal pattern of increasing concentrations of chlorophyll a (Chl a), organic carbon, and nutrients in their waters and sediments in urbanized regions. Acting as a buffer, the nearshore ecosystems have turned highly eutrophic and their autotrophic metabolism has been enhanced creating strong carbon dioxide (CO2) sinks. We compile here data of CO2fluxes recently gathered in four coastal marine ecosystems in the state of Rio de Janeiro: the Guanabara Bay and the Araruama, Saquarema and Jacarepagua lagoons. We observed intense CO2 sources in restricted areas at the vicinity of sewage loads, where microbial degradation of organic matter predominates, and large CO2 sinks in confined and nearshore brackish, marine and hypersaline waters, where phytoplankton blooms occur. We also report a correlation across the four ecosystems between the partial pressure of CO2 in waters and the Chl a concentration. Chl a satellite data all along the Brazilian coast suggest that the CO2 sink induced by eutrophication probably occurs in many coastal ecosystems including bays, lagoon and shelf waters, and could contribute to an additional blue carbon. Part of the additional organic carbon is stored in sediments, and part is exported offshore. However, this additional blue carbon has dramatic environment impacts as it would evolve toward the formation of marine dead zones, and could contribute to a production of methane (CH4) a more powerful greenhouse gas. We emphasize an urgent need for multidisciplinary research to promote simultaneously the storage of atmospheric carbon, and the preservation of biodiversity and socio-economic goods in the eutrophic tropical coastal ocean. Keywords: tropical coastal ecosystems, cultural eutrophication, phytoplankton blooms, marine dead zones, blue carbon

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