Mangrove mortality in a changing climate: an overview

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

Pristine mangrove creek waters are a sink of nitrous oxide

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

Are mangroves drivers or buffers of coastal acidification? Insights from alkalinity and dissolved inorganic carbon export estimates across a latitudinal transect

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 CO2 [CO2*] 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 [CO2*] between mangroves and the coastal ocean. We found a mean DIC export of 59 mmol m−2 d−1 across the six systems, ranging from import of 97 mmol m−2 d−1 to an export of 85 mmol m−2 d−1. 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−1, 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−2 d−1and an export of 117 mmol m−2 d−1 with an estimated global export of 4.2 ± 1.3 Tmol yr−1. A net import of free CO2 was estimated (−11.4 ± 14.8 mmol m−2 d−1) and was equivalent to approximately one third of the air-water CO2 flux (33.1 ± 6.3 mmol m−2 d−1). 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

Do invasive corals alter coral reef processes? An empirical approach evaluating reef fish trophic interactions

Understanding how invasive species affect key ecological interactions and ecosystem processes is imperative for the management of invasions. We evaluated the effects of invasive corals (Tubastraea spp.) on fish trophic interactions in an Atlantic coral reef. Remote underwater video cameras were used to examine fish foraging activity (bite rates and food preferences) on invasive cover levels. Using a model selection approach, we found that fish feeding rates declined with increased invasive cover. For Roving Herbivores (RH) and Sessile Invertivores (SI), an abrupt reduction of fish feeding rates corresponded with higher invasive cover, while feeding rates of Territorial Herbivores (TH) and Mobile Invertivores (MI) decreased linearly with cover increase. Additionally, some fish trophic groups, such as RH, SI and Omnivores (OM), had lower densities in reef sections with high invasive cover. These findings demonstrate that invasive corals negatively impact fish-benthic interactions, and could potentially alter existing trophic relationships in reef ecosystems

Data from: Mangrove outwelling is a significant source of oceanic exchangeable organic carbon

Exchangeable dissolved organic carbon (EDOC) makes up a significant proportion of the oceanic dissolved organic carbon (DOC) pool, yet EDOC sources to the coastal ocean are poorly constrained. We measured the exchange of EDOC and concentrations of EDOC and DOC in mangrove waters over a 26° latitudinal gradient. A clear latitudinal trend was observed, with the highest EDOC concentrations in the tropics. EDOC exports to the coastal ocean were 4.7 ± 1.9 mmol m−2 d−1, equivalent to 11% of DOC exports (42.1 ± 6.7 mmol m−2 d−1). Pore-water and groundwater exchange were minor sources of EDOC. EDOC concentrations were equal to 13% ± 4% of DOC concentrations. Based on previous global DOC export estimates, and our EDOC : DOC ratios, mangroves outwell 3.1 Tg C yr−1 as EDOC, equivalent to ∼ 60% of the global EDOC flux from the ocean to the atmosphere. However, seasonality of mangrove EDOC cycling requires further research

Manganese and iron release from mangrove porewaters: a significant component of oceanic budgets?

Mangrove porewater can be highly enriched in dissolved manganese (Mn), iron (Fe), and other trace metals. As a result, porewater exchange may release dissolved metals to surface waters. This study assessed dissolved Mn exchange with the coastal ocean in four mangroves ecosystems, and whether porewater exchange represents a major driver of the oceanic exchange along a latitudinal gradient in Australia (from 28° S to 12° S). Dissolved Fe was also determined but concentrations were below detection in most surface water samples, preventing any flux estimates. Average concentrations of Mn in porewater were approximately an order of magnitude greater than surface waters at all sites, resulting in average porewater-derived Mn fluxes of 441 kmol km− 2 year− 1 at the four sites. Time series surface water observations indicate that average Mn concentrations decrease at lower latitudes. The average dissolved Mn export rate from the four mangrove systems to the coastal ocean was 88 kmol km− 2 year−1. Porewater-derived Mn inputs were greater than surface water exports, which may be explained by dissolved Mn precipitation, oxidation or flocculation at the sediment water interface. While the removal of Mn at the sediment-water interface brings about uncertainties in the estimated porewater fluxes, it has no impact on estimated surface water exports to the coastal ocean. If our surface water export estimates are representative of the global mangrove area (140,000 km2), mangroves may deliver 12 Gmol year−1 of dissolved Mn to the coastal ocean. These fluxes are greater than the estimated flux from global riverine (5.4 Gmol year−1) and atmospheric (11 Gmol year−1) sources, demonstrating that mangroves may be a major player in the oceanic cycle of Mn

Reconstructing extreme climatic and geochemical conditions during the largest natural mangrove dieback on record

A massive mangrove dieback event occurred in 2015-2016 along ~ 1000km of pristine coastline in the Gulf of Carpentaria, Australia. Here, we use sediment and wood chronologies to gain insights into geochemical and climatic changes related to this dieback. The unique combination of low rainfall and low sea level observed during the dieback event had been unprecedented in the preceding 3 decades. A combination of iron (Fe) chronologies in wood and sediment, wood density and estimates of mangrove water use efficiency all imply lower water availability within the dead mangrove forest. Wood and sediment chronologies suggest a rapid, large mobilization of sedimentary Fe, which is consistent with redox transitions promoted by changes in soil moisture content. Elemental analysis of wood cross sections revealed a 30-to 90-fold increase in Fe concentrations in dead mangroves just prior to their mortality. Mangrove wood uptake of Fe during the dieback is consistent with large apparent losses of Fe from sediments, which potentially caused an outwelling of Fe to the ocean. Although Fe toxicity may also have played a role in the dieback, this possibility requires further study. We suggest that differences in wood and sedimentary Fe between living and dead forest areas reflect sediment redox transitions that are, in turn, associated with regional variability in groundwater flows. Overall, our observations provide multiple lines of evidence that the forest dieback was driven by low water availability coinciding with a strong El Niño-Southern Oscillation (ENSO) event and was associated with climate change

Response to Gallagher (2022)—the Australian Tidal Restoration for Blue Carbon method 2022—conservative, robust, and practical

The Blue Carbon Accounting Model (BlueCAM) is a tool for tidal restoration projects established under the Tidal Restoration for Blue Carbon method (2022) of the Australian voluntary carbon market. The commentary of Gallagher discussed that BlueCAM did not subtract allochthonous carbon, which is carbon in wetland soils from external sources, either terrestrial or marine sources, from estimated net abatement. This approach was used because all organic carbon preserved in a restored wetland soil, irrespective of its source, is deposited because the wetland was restored, and thus all carbon preserved above the baseline is “additional.” As restoration projects develop, further characterization of different soil carbon fractions as suggested by Gallagher may improve BlueCAM. BlueCAM is transparent, conservative, and importantly is feasible for implementation, as well as being consistent with the Intergovernmental Panel for Climate Change guidelines for National Greenhouse Gas Inventories and the Australian legislative offsets integrity standards