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Carbon sequestration in mangroves is often estimated from their sediment storage capacity. However, lateral transport (outwelling) of dissolved carbon and alkalinity from intertidal wetlands can also drive long-term storage in the ocean. Here, we quantified outwelling rates across the groundwater-creek-shelf continuum of the world's largest continuous mangrove belt in the Amazon region. Dissolved inorganic carbon (DIC) fluxes exceeded dissolved organic carbon over all spatial scales. Since 90% AE 13% of mangrove-derived DIC was bicarbonate, there is potential for long-term carbon storage offshore. Outwelling seems to exceed sediment burial as a carbon sequestration pathway in this and other mangrove-dominated coastlines.

Marine heatwaves (MHWs) are a major concern worldwide due to their increasing impacts in recent years, and these extreme events may trigger deoxygenation of coastal waters affected by sewage and eutrophication. Here we investigate the combined effects of MHWs and nutrient enrichment on the water quality and biodiversity of the Bay of Santa Catarina Island (Brazil). We used historical (1994–2020) sea surface temperature data from satellites and in situ physical, chemical and biological parameters to assess temporal trends. Oxygen levels have been decreasing whilst phosphorus levels have been increasing in the bay. During the austral summer of 2020 a regional sea surface heatwave was detected by satellite, lasting for 9 days and coinciding with our research cruise. During this period, seawater temperatures reached 29.8°C and anoxia was detected for the first time in the bay. A decrease in macrobenthic and phytoplankton community richness correlated with decreases in oxygen both through time and towards more urbanized areas. Overall, poor wastewater treatment is a key stressor that combined with MHWs to degrade coastal waters. Mitigation strategies are needed to minimize the impact of MHWs, including improved sewage treatment, restoration and conservation of wetlands and the use of nature-based technologies to promote coastal ecosystem recovery.

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