Mangrove forests are hot spots in the global carbon cycle, yet the fate for a majority of mangrove net primary production remains unaccounted for. The relative proportions of alkalinity and dissolved CO 2 [CO 2 *] within the dissolved inorganic carbon (DIC) exported from mangroves is unknown, and therefore, the effect of mangrove DIC exports on coastal acidification remains unconstrained. Here we measured dissolved inorganic carbon parameters over complete tidal and diel cycles in six pristine mangrove tidal creeks covering a 26°latitudinal gradient in Australia and calculated the exchange of DIC, alkalinity, and [CO 2 *] between mangroves and the coastal ocean. We found a mean DIC export of 59 mmol m À2 d À1 across the six systems, ranging from import of 97 mmol m À2 d À1 to an export of 85 mmol m À2 d À1 . If the Australian transect is representative of global mangroves, upscaling our estimates would result in global DIC exports of 3.6 ± 1.1 Tmol C yr À1 , which accounts for approximately one third of the previously unaccounted for mangrove carbon sink. Alkalinity exchange ranged between an import of 1.2 mmol m À2 d À1 and an export of 117 mmol m À2 d À1 with an estimated global export of 4.2 ± 1.3 Tmol yr À1 . A net import of free CO 2 was estimated (À11.4 ± 14.8 mmol m À2 d À1 ) and was equivalent to approximately one third of the air-water CO 2 flux (33.1 ± 6.3 mmol m À2 d À1 ). Overall, the effect of DIC and alkalinity exports created a measurable localized increase in coastal ocean pH. Therefore, mangroves may partially counteract coastal acidification in adjacent tropical waters.

Mangroves provide vital ecosystem services at the dynamic interface between land and oceans. Recent reports of mangrove mortality suggest that mangroves may be adversely affected by climate change. Here, we review historical mortality events from natural causes (all mortality other than deforestation, land use change and pollution) and provide a global assessment of mortality drivers. Since the 1960's approximately 36,000 ha of mangrove mortality has been reported (0.2% of total mangrove cover in 2011) in 47 peer reviewed articles. Due to the uneven global distribution of research effort, it is likely that mangrove mortality events go unreported in many countries. It is therefore difficult to assess temporal changes in mortality due to the small number of reports and increasing effort in observations in recent years. From the published literature, approximately 70% of reported mangrove loss from natural causes has occurred as a result of low frequency, high intensity weather events, such as tropical cyclones and climatic extremes. Globally, tropical cyclones have caused the greatest area of mangrove mortality, equivalent to 45% of the reported global mangrove mortality area from events over six decades. However, recent large-scale mortality events associated with climatic extremes in Australia account for 22% of all reported historical forest loss. These recent mortality events suggest the increasing importance of extreme climatic events, and highlight that mangroves may be important sentinels of global climate change. Increasing frequency, intensity and destructiveness of cyclones as well as climatic extremes, including low and high sea level events and heat waves, have the potential to directly influence mangrove mortality and recovery, particularly in mid latitudes.

Mangrove forests produce significant amounts of organic carbon and maintain large carbon stocks in tidally inundated, anoxic soils. This work analyzes new and published data from 17 regions spanning a latitudinal gradient from 22°N to 38°S to assess some of the global drivers (temperature, tidal range, latitude, and rainfall) of mangrove carbon stocks. Mangrove forests from the tropics have larger carbon stocks (895 ± 90 t C ha−1) than the subtropics and temperate regions (547 ± 66 t C ha−1). A multiple regression model showed that 86% of the observed variability is associated with annual rainfall, which is the best predictor of mangrove ecosystem carbon stocks. Therefore, a predicted increase in rainfall along the tropical Indo‐Pacific may increase mangrove forest carbon stocks. However, there are other potentially important factors that may regulate organic matter diagenesis, such as nutrient availability and pore water salinity. Our predictive model shows that if mangrove deforestation is halted, global mangrove forest carbon stocks could increase by almost 10% by 2115 as a result of increased rainfall in the tropics.

Nitrous oxide (N2O) is an important greenhouse gas, but large uncertainties remain in global budgets. Mangroves are thought to be a source of N2O to the atmosphere in spite of the limited available data. Here we report high resolution time series observations in pristine Australian mangroves along a broad latitudinal gradient to assess the potential role of mangroves in global N2O budgets. Surprisingly, five out of six creeks were under-saturated in dissolved N2O, demonstrating mangrove creek waters were a sink for atmospheric N2O. Air-water flux estimates showed an uptake of 1.52 ± 0.17 μmol m−2 d−1, while an independent mass balance revealed an average sink of 1.05 ± 0.59 μmol m−2 d−1. If these results can be upscaled to the global mangrove area, the N2O sink (~2.0 × 108 mol yr−1) would offset ~6% of the estimated global riverine N2O source. Our observations contrast previous estimates based on soil fluxes or mangrove waters influenced by upstream freshwater inputs. We suggest that the lack of available nitrogen in pristine mangroves favours N2O consumption. Widespread and growing coastal eutrophication may change mangrove waters from a sink to a source of N2O to the atmosphere, representing a positive feedback to climate change.

Summary Growing evidence indicates that tree‐stem methane (CH4) emissions may be an important and unaccounted‐for component of local, regional and global carbon (C) budgets. Studies to date have focused on upland and freshwater swamp‐forests; however, no data on tree‐stem fluxes from estuarine species currently exist. Here we provide the first‐ever mangrove tree‐stem CH4 flux measurements from  >50 trees (n = 230 measurements), in both standing dead and living forest, from a region suffering a recent large‐scale climate‐driven dieback event (Gulf of Carpentaria, Australia). Average CH4 emissions from standing dead mangrove tree‐stems was 249.2 ± 41.0 μmol m−2 d−1 and was eight‐fold higher than from living mangrove tree‐stems (37.5 ± 5.8 μmol m−2 d−1). The average CH4 flux from tree‐stem bases (c. 10 cm aboveground) was 1071.1 ± 210.4 and 96.8 ± 27.7 μmol m−2 d−1 from dead and living stands respectively. Sediment CH4 fluxes and redox potentials did not differ significantly between living and dead stands. Our results suggest both dead and living tree‐stems act as CH4 conduits to the atmosphere, bypassing potential sedimentary oxidation processes. Although large uncertainties exist when upscaling data from small‐scale temporal measurements, we estimated that dead mangrove tree‐stem emissions may account for c. 26% of the net ecosystem CH4 flux.