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Mangroves are one of the most carbon‐dense forests on the Earth and have been highlighted as key ecosystems for climate change mitigation and adaptation. Hundreds of studies have investigated how mangroves fix, transform, store, and export carbon. Here, we review and synthesize the previously known and emerging carbon pathways in mangroves, including gains (woody biomass accumulation, deadwood accumulation, soil carbon sequestration, root and litterfall production), transformations (food web transfer through herbivory, decomposition), and losses (respiration as CO2 and CH4, litterfall export, particulate and dissolved carbon export). We then review the technologies available to measure carbon fluxes in mangroves, their potential, and their limitations. We also synthesize and compare mangrove net ecosystem productivity (NEP) with terrestrial forests. Finally, we update global estimates of carbon fluxes with the most current values of fluxes and global mangrove area. We found that the contributions of recently investigated fluxes, such as soil respiration as CH4, are minor (<1 Tg C year−1), while the contributions of deadwood accumulation, herbivory, and lateral export are significant (>35 Tg C year−1). Dissolved inorganic carbon exports are an order of magnitude higher than the other processes investigated and were highly variable, highlighting the need for further studies. Gross primary productivity (GPP) and ecosystem respiration (ER) per area of mangroves were within the same order of magnitude as terrestrial forests. However, ER/GPP was lower in mangroves, explaining their higher carbon sequestration. We estimate the global mean mangrove NEP of 109.1 Tg C year−1 (7.4 Mg C ha−1 year−1) or through a budget balance, accounting for lateral losses, a global mean of 66.6 Tg C year−1 (4.5 Mg C ha−1 year−1). Overall, mangroves are highly productive, and despite losses due to respiration and tidal exchange, they are significant carbon sinks.

The effect of land use on the biogeochemistry of small tropical rivers and their estuaries was studied using the Kallada River and Ashtamudi estuary located in the State of Kerala, India, as a model system. Water, suspended matter and sediments collected during the monsoon and intermonsoon periods in 2002 and 2003 were analyzed for dissolved nutrients (nitrate, nitrite, phosphate, silicate) and for phytoplankton abundance and composition, amino acid contents and stable carbon (C)) and nitrogen (N) isotope ratios. Seasonal and spatial variations of dissolved nutrients and suspended matter along the course of the river point to distinct differences in the C and N sources that are controlled by hydrology, geology and land use. Unusually low concentrations of dissolved silicate and suspended matter suggest low erosion rates of the Precambrian basement rocks and the firm lateritic soils in non-agricultural areas. Most dissolved nutrients and suspended particulate organic matter originated from fertilized agricultural soils. The biogeochemistry of sedimentary organic matter indicates that most of the Kallada River load is deposited in the upper Ashtamudi estuary, while the middle and lower parts have a stronger marine influence. The spatio-temporal variation of dissolved and particulate river fluxes clearly indicates an effect of land use and land cover on the biogeochemistry of the Kallada River. While the phosphate yield was high (6 x 10³ mol km⁻² year⁻¹ or 185 kg km⁻² year⁻¹), the N yield was relatively low (10 x 10³ mol km⁻² year⁻¹ or 141 kg km⁻² year⁻¹), which is unlike the situation in many other densely populated regions of tropical Asia.

The biogeochemistry of the Dumai River estuary in eastern Sumatra, Indonesia, was studied in order to obtain information on the sources, transformation, and fate of organic matter. Between October and December 2003, water, total suspended matter (TSM), and sediments were sampled along a salinity gradient during four campaigns, and plants and soils were collected from the catchment. Water samples were analyzed for dissolved inorganic nutrients and dissolved organic carbon (DOC). The concentrations of organic carbon (Corg) and total nitrogen (N) and the stable carbon (δ13Corg) and nitrogen (δ15N) isotope distributions were determined in TSM, sediments, plants, and soils. The pH as well as the concentrations of dissolved inorganic nutrients and TSM were very low in the river and increased toward the sea. A maximum DOC concentration of $5,050 \\mu mol L^{-1}$ was measured in the river, and concentrations decreased toward the sea. Low-gradient relief and a dense vegetation cover, and hence little weathering and erosion, appear to be responsible for low river loads of dissolved nutrients and TSM in this black-water river. Leaching from extensive peat soils in its catchment may account for the high DOC content of the Dumai River. Peat swamps drained by numerous small rivers are estimated to cover $3.3 times 10^{4} km^2$ in eastern Sumatra, suggesting that leaching of DOC may be a significant source of carbon to the adjacent coastal seas. A comparison with \"normal\" rivers shows that black-water rivers can export similar amounts of DOC from catchments that are orders of magnitude smaller. Thus, export from small black-water rivers may be quantitatively more significant for the global DOC input into the ocean than previously thought.

This study addresses the composition of biogenic matter and the metabolic activity of coastal waters of the eastern Brazilian shelf, bordered by small river-mangrove systems. Oceanic Brazil Current waters induce oligotrophic and near to homogeneous conditions of chemical constituents along the inner shelf. The impact of small river-mangrove systems upon coastal waters is minor and of local nature. Bottom topography, coral reef habitats, and local upwelling also induce minor local spatial variability of dissolved inorganic and organic nutrients and O2 and CO2 saturation levels in the coastal waters. Metabolic activity during the daylight period, inferred from O2 and CO2 saturation levels, varied from slightly autotrophic to heterotrophic.

Settling particles collected at 1550 m water depth off the Sao Francisco River, Brazil, between January and May 1995 showed peak fluxes of amino acids, hexosamines, and carbohydrates, which formed the onset of a three-week period of high organic matter (OM) flux, coinciding with the high discharge period of the river. Two phases of OM deposition exist: (1) the fluvial input of nutrients triggering a bloom of non-biomineralizing plankton, and (2) suspended sediment mainly derived from shelf erosion increasing the fluxes of refractory OM. This indicates the importance of seasonally varying hydrodynamic conditions and nutrient input from the continent for the production and sedimentation of OM to the continental margin of eastern Brazil.

The Asian monsoon system governs seasonality and fundamental environmental characteristics in the study area from which two distinct peculiarities are most notable: upwelling and convective mixing in the Arabian Sea and low surface salinity and stratification in the Bay of Bengal due to high riverine input and monsoonal precipitation. The respective oceanography sets the framework for nutrient availability and productivity. Upwelling ensures high nitrate concentration with temporal/spatial Si limitation; freshwater-induced stratification leads to reduced nitrogen input from the subsurface but Si enrichment in surface waters. Ultimately, both environments support high abundance of diatoms, which play a central role in the export of organic matter. It is speculated that, additional to eddy pumping, nitrogen fixation is a source of N in stratified waters and contributes to the low-d super(15)N signal in sinking particles formed under riverine impact. Organic carbon fluxes are best correlated to opal but not to carbonate, which is explained by low foraminiferal carbonate fluxes within the river-impacted systems. This observation points to the necessity of differentiating between carbonate sources for carbon flux modeling. As evident from a compilation of previously published and new data on labile organic matter composition (amino acids and carbohydrates), organic matter fluxes are mainly driven by direct input from marine production, except the site off Pakistan where sedimentary input of (marine) organic matter is dominant during the NE monsoon. The explanation of apparently different organic carbon export efficiency calls for further investigations of, for example, food web structure and water column processes.

Tropical regions have been reported to play a key role in climate dynamics. To date, however, there are uncertainties in the timing and the amplitude of the response of tropical ecosystems to millennial-scale climate change. We present evidence of an asynchrony between terrestrial and marine signals of climate change during Heinrich events preserved in marine sediment cores from the Brazilian continental margin. The inferred time lag of about 1000 to 2000 years is much larger than the ecological response to recent climate change and appears to be related to the nature of hydrological changes.

1. Tropical peatlands, which provide important functions such as biodiversity provisioning and carbon (C) storage, are currently threatened by land-use conversions. Thus, conservation and restoration efforts are needed to maintain their functions. Conservation concepts aiming to separate human from ecosystems are no longer conceivable. Therefore, understanding peatland resilience to human disturbance, that is the ability of peatland ecosystems to maintain their structure and function despite perturbations and to return to their predisturbance states, can assist with integrating human needs into conservation strategies and improving restoration effectiveness. 2. Understanding ecosystem resilience is often impeded by a lack of long-term data, which can be obtained from palaeoecological studies. Located close to the archaeological remains of the Malayu Empire, the Sungai Buluh peatland in Sumatra, Indonesia provides an opportunity to study the resilience of a tropical peatland to past human disturbance. We subjected a 250-cm-long peat core to palynological, charcoal and C content analyses to delineate the anthropogenic impact on the peatland and the ecosystem's response. 3. The results revealed that extensive human activities in Sungai Buluh such as logging, grazing/cut-and-carry, and wild-harvesting started soon after humans occupied the vicinity of the peatland c. 1,000 cal yr BP. Even without fire use and cultivation, these activities were able to alter vegetation composition and decrease the peatland's C sequestration capacity. 4. Following site abandonment after the demise of the Malayu Empire at c. 600 cal yr BP, the palaeoecological record suggests that the Sungai Buluh peatland recovered in terms of both floristic composition and C sink function, with the latter recovering faster (c. 60 years) than the former (c. 170 years). 5. Synthesis. The palaeoecological record from Sungai Buluh provides the first evidence of tropical peatland recovery following human disturbance, which can help improve present peatland conservation/restoration strategies. The design of peatland wise-use strategies can mimic the \"resilience-friendly\" human activities identified in this study. Consideration should also be given to selecting rapidly regenerating taxa for cost-and-effort-efficient restoration strategies. Additionally, the 170-year recovery time of the Sungai Buluh peatland suggests that the 60 year timeframe currently allocated in most tropical peatland restoration projects may be insufficient.

Changes in tropical zonal atmospheric (Walker) circulation induce shifts in rainfall patterns along with devastating floods and severe droughts that dramatically impact the lives of millions of people. Historical records and observations of the Walker circulation over the 20th century disagree on the sign of change and therefore, longer climate records are necessary to better project tropical circulation changes in response to global warming. Here we examine proxies for thermocline depth and rainfall in the eastern tropical Indian Ocean during the globally colder Last Glacial Maximum (19–23 thousand years ago) and for the past 3000 years. We show that increased thermocline depth and rainfall indicate a stronger-than-today Walker circulation during the Last Glacial Maximum, which is supported by an ensemble of climate simulations. Our findings underscore the sensitivity of tropical circulation to temperature change and provide evidence for a further weakening of the Walker circulation in response to greenhouse warming. Debate exists on the sign of change in tropical atmospheric circulation during global warming. Here the authors show a weaker Walker cell over the Indian Ocean during the warmer late Holocene compared to the globally colder Last Glacial Maximum, implying a further slowdown of the Walker cell in response to warming.

Coral reefs are declining globally, but region-specific drivers of degradation remain poorly quantified, hindering local conservation policymaking. By collecting two decades of field data from 102 sites across 22 coral reefs in the northern South China Sea and employing panel regressions and structural equation modeling, we identify key stressors that are responsible for 40% (17–50%) of the declines in live coral cover. Local anthropogenic stressors—overfishing, nutrient pollution from agriculture and coastal urbanization—collectively explain 73% of live coral coverage variance, outweighing climate-associated thermal stress. We then propose an Integrated Coast-Reef Management framework that couples land-sea interventions—prioritizing sustainable fisheries, watershed nutrient management, and controls of crown-of-thorns starfish outbreaks. Spatial simulations indicate that this synergistical strategy could elevate live coral coverage by two to four times under global warming scenarios, avoiding reef calcification collapse. Our findings contribute to coral conservation paradigms by highlighting tailored strategies at the local level beyond globalized approaches, which offer scalable solutions for regions facing similar pressures. Coral reefs are declining globally, but regional degradation drivers are poorly quantified. This study reveals that coral loss in the northern South China Sea is predominantly linked to human coastal activities, suggesting that tailored local management may effectively reduce reef collapse.