Mangroves are a critical habitat that provide a suite of ecosystem services and support livelihoods. Here we undertook a global analysis to model the density and abundance of 37 commercially important juvenile fish and juvenile and resident invertebrates that are known to extensively use mangroves, by fitting expert-identified drivers of density to fish and invertebrate density data from published field studies. The numerical model predicted high densities throughout parts of Southeast and South Asia, the northern coast of South America, the Red Sea, and the Caribbean and Central America. Application of our model globally estimates that mangroves support an annual abundance of over 700 billion juvenile fish and invertebrates. While abundance at the early life-history stage does not directly equate to potential economic or biomass gains, this estimate indicates the critical role of mangroves globally in supporting fish and fisheries, and further builds the case for their conservation and restoration.

Coastal megalopolises are transportation gateways of the global economy, but marine traffic around these developed urban areas exerts considerable pressure on the environment. A key starting point in understanding how this pressure may impact marine species and ecosystems is to map the spatial footprint of maritime vessels. Using six years of vessel tracking data from 2013-18, we mapped fine-scale spatial (0.002 degrees) and temporal (daily) distribution of marine traffic within the Guangdong-Hong Kong-Macao Greater Bay Area (GBA) - a coastal megalopolis with the largest population (similar to 86 million) and second-largest economy of bay areas globally. Data indicates that on average, 5981 ( +/- 1941 SD) unique vessels were active in the GBA daily. Space-use of both fishing and non-fishing vessels showed temporally consistent, persistent, and spatially-dense coverage, with 80 % of vessel traffic occurring in shallow water (0-20 m). Movement in such shallow water of the GBA likely results in increased rates of shoreline erosion, turbidity, noise pollution, and vessel collisions - posing a substantial threat to many regionally important marine species of conservation concern. Daily rates of disturbance in the GBA by non-fishing vessels were eight times higher than for fishing vessels, with some areas experiencing disturbance across more than 95 % of days in the study period. Our findings highlight how AIS data holds a wealth of information that takes considerable computing power to extract, so we make our processed datasets openly accessible to aid in future marine conservation planning and analyses.

Mangroves are important organic carbon (OC) reservoirs that can mitigate climate changes. Although sedimentary dissolved organic matter (SDOM) has been established as a dominant OC component within these ecosystems, its molecular composition, origin, and fate remain largely unknown. This study examined SDOM molecules from one of the largest peri-urban mangroves and adjacent tidal flats in China by using techniques like absorption fluorescence spectroscopy and ultrahigh-resolution mass spectrometry. We found that SDOM in mangroves differs significantly from that in non-mangrove ecosystems. The presence of mangroves created higher levels of N- and S-containing SDOM molecules with depth and exhibited higher lability. However, the suboxic mangrove sediments may limit microbial activity, promoting labile SDOM preservation. Mangrove litter also contributed to the formation of carboxyl-rich alicyclic molecules in sediments, potentially transforming into biologically refractory SDOM that acts as OC sinks. As the understanding of molecular fingerprints of SDOM in mangroves is still at early stages, these findings provide molecular-level evidence that mangrove sediments promote the OC sequestration through preservation and transformation processes. This study offers crucial insights into the role of SDOM in carbon sequestration and the method could be applied into other blue carbon ecosystems.

Large‐scale restorations being implemented in coastal China involve replacing invasive Spartina alterniflora with mangroves, yet the full effects of such saltmarsh‐mudflat‐mangrove land‐use change on the blue carbon sink are largely unknown. This study, using paired eddy covariance measurements of greenhouse gases (GHGs) before and after S . alterniflora removal, reveals that such restoration efforts through excavation and burial of S . alterniflora inadvertently cause pulse methane emission. The emission negates the carbon sink benefit and causes a significant climate debt, potentially taking over 3 decades to offset. These findings highlight the risk of GHG changes from coastal restoration in neutralizing potential blue carbon sink and call for refining current restoration practices to mitigate unintended environmental impacts. This has important implications for achieving climate benefits along with other ecosystem service co‐benefits in coastal restoration, particularly for China's coastal wetlands where S . alterniflora removal is being implemented as the world's largest ecosystem restoration effort.

Clam digging has a long history in Hong Kong, but unregulated clam digging activities depletes clam populations and threatens the ecosystem. Population genomics is useful to unravel the connectivity of clams at different geographical locations and to provide necessary conservation measures; and yet, only limited number of clams in Hong Kong have genomic resources. Here, we present chromosomal-level genome assemblies for two clams commonly found in Hong Kong, Anomalocardia flexuosa and Meretrix petechialis, using a combination of PacBio HiFi and Omni-C reads. For A. flexuosa, we assembled the genome into 19 pseudochromosomes with a genome size of 1.09 Gb (scaffold N50 = 58.5 Mb), and BUSCO scores of 94.4%. A total of 20,881 gene models were also predicted using the transcriptomes generated in this study. For M. petechialis, the genome was mainly assembled into 19 pseudochromosomes with a genome size of 1.04 Gb (scaffold N50 = 53.5 Mb), and BUSCO scores of 95.7%. A total of 20,084 gene models were also predicted using the transcriptomes generated in this study. The two new genomic resources established in this study will be useful for further study of biology, ecology, and evolution of clams, as well as setting up a foundation for evidence-informed decision making in conservation measures and implementation.

The decline of Indo-Pacific humpback dolphins observed in Hong Kong is a microcosm of cetacean population changes resulting from broader global issues, including habitat degradation, pollution, and unsustainable fishing practices. Identifying and prioritizing the natural and anthropogenic pressures on the species is crucial for effective population conservation. The loss of coastal habitats due to reclamation projects, such as those in Hong Kong, is a significant concern as it disrupts the dolphins' natural environment and displaces them from their traditional feeding and breeding grounds. This habitat loss, coupled with other anthropogenic stressors, has contributed to population declines in various regions, including the Eastern Taiwan Strait, Xiamen, and the Pearl River Estuary. This study investigates the decline of Indo-Pacific humpback dolphins (Sousa chinensis) in Hong Kong, where their numbers have dropped from 158 in 2003 to 37 in 2020. By analyzing dolphin density per survey effort (DPSE) and various environmental variables from 1996 to 2020, we identified several key factors influencing dolphin presence. Temperature, salinity, fish production, suspended solids, nitrogen, E. coli, seawater clarity, and high-speed ferry traffic all play significant roles. Reclamation activities in Hong Kong, in particular, have a substantial impact on dolphin habitats and contribute to their abundance dropped by over 80 % in the past 17 years. Despite the establishment of new marine protected areas, it is insufficient to prevent dolphin decline. This study highlights the need for comprehensive conservation measures, including the preservation of critical habitats and the reduction of human-induced stressors, to ensure the survival of Indo-Pacific humpback dolphins in Hong Kong and beyond.

Grapsoid crabs (Decapoda: Grapsoidea) inhabiting along the land-sea transition provided various amounts and quality of vascular plant carbon (e.g., fresh mangrove leaf, leaf litter, and mangrove-derived organic carbon) and perform differing levels of herbivory. Other than endogenous cellulase, symbiotic cellulolytic bacteria could also contribute to the crabs' vascular plant carbon assimilation and mineralization. In this study, we isolated culturable cellulolytic bacteria from three gut regions (i.e., stomach, midgut, and hindgut) of 15 species of grapsoid crabs that inhabit in various coastal habitats (i.e., land margin, mangrove forest, tidal flat, and subtidal area). Bacillus, which was isolated from 11 out of the 15 grapsoid crabs, was the most common genus of culturable prominently cellulolytic bacteria among the target species. Seventy to ninety nine percent of culturable cellulolytic bacteria were removed, and the endoglucanase activity of five species was significantly reduced by 14.4-27.7% after antibiotic treatment. These results suggest that cellulolytic bacteria play a role in assisting mangrove carbon utilization in coastal grapsoid crabs, especially those inhabiting mangrove, mudflat, and subtidal areas. The significantly higher abundance of cellulolytic bacteria and the generally higher hydrolytic capacity of the bacteria in mangrove crab species suggest that they receive more contribution from symbionts for mangrove carbon utilization, while semi-terrestrial crabs seem to depend little on symbiotic cellulase due to the lower abundances.

The ecological role of brachyuran crabs in mangroves has been extensively studied, but their role as mediators of biogeochemical processes of essential nutrients remains unclear. We hypothesized that speciose sesarmid crabs in tropical mangroves can improve nutrient transfer rate (NTR) and enhance primary productivity by reducing nutrient recycling bottlenecks. Using stable isotope labels and tidal mesocosms simulating the mangrove environment (with and without crabs), we traced the transfer of carbon (C) and nitrogen (N) across the system (from mangrove leaf litter to mangrove seedlings, the microphytobenthos (MPB), and sediment). We compared NTR between the two treatments by measuring the 13C and 15N enrichment levels (as a proxy for NTR) of mangrove seedlings, MPB, and sediment. The significantly higher enrichment level of sediment, MPB and seedlings (for N) in mesocosms with crabs indicated higher NTR driven by these animals. Such increased NTR also led to higher primary productivity, supporting our hypothesis. A consumer-driven nutrient dynamics (CND) framework was established to provide a comprehensive understanding of the role of sesarmid crabs in mediating biogeochemical processes. It is clear that the reciprocal nutrient flow between MPB and sesarmid crabs, and the biogeochemical hotspots created in surface sediment underpin this CND effect. A system picture emerges whereby nutrient flow is strongly regulated by sesarmid crabs promoting the productivity of mangroves by (1) enhancing nutrient remineralization; (2) retaining nutrients; (3) creating biogeochemical hotspots; and (4) improving overall nutrient transfer rate.

Coastal ecosystems are recognized as important carbon reservoirs. However, assessments of coastal carbon dioxide (CO2) fluxes often neglect unvegetated tidal flats, particularly in peri-urban areas. This study investigated the gross primary production (GPP), net primary production (NPP), and total respiration (TR) of three tidal flats in Hong Kong, one of the largest cities in South China, during both summer and winter seasons to understand their CO2 fluxes. Results showed that GPP of three tidal flats was significantly higher in winter than in summer. However, no significant seasonal variations in TR were observed among the tidal flats. Structural equation modelling was used to examine the drivers of CO2 fluxes in the three tidal flats. The model showed that temperature and microphytobenthos abundance were identified as positive drivers of GPP, while sediment mud content had a positive effect on TR. The estimated daily NPP of these tidal flats ranged from -0.853 to 0.112 g C m−2 d−1, which is lower than the mean value reported for global vegetated coastal wetlands. Despite some seasonal and spatial variations, those peri-urban tidal flats may be considered as weak CO2 sources rather than CO2 sinks.

Global theories of change (ToCs) can provide broad, overarching guidance for conservation and sustainable use of Earth’s ecosystems. However, broad guidance alone cannot inform how conservation actions will lead to desired socioecological outcomes. Here we develop a framework for translating a global-scale ToC into focused, ecosystem-specific ToCs that consider feasibility of actions, as determined by national socioeconomic and political contexts (that is, enabling conditions). We used coastal wetlands as a case study for developing the framework and identified six distinct multinational profiles of enabling conditions (‘enabling profiles’) for their conservation. For countries belonging to profiles with high internal capacity to enable conservation, we described plausible ToCs that involved strengthening policy and regulation. Alternatively, for profiles with low internal enabling capacity, plausible ToCs typically required formalizing community-led conservation. Our ‘enabling profile’ framework can be applied to other ecosystems to help operationalize the Kunming–Montreal Global Biodiversity Framework and meet sustainable development goals. Theories of change have been a staple of sustainability research, but how to connect such overarching concepts to actionable items can be a struggle. This study uses coastal wetlands to demonstrate a potential framework for integrating indicators of conservation enabling conditions into theories of change.

Mangroves are a critical coastal habitat that provides a suite of ecosystem services and supports livelihoods. We undertake the first global analysis to estimate density and abundance of 37 commercially important fish and invertebrates that are known to extensively use mangroves. Geomorphic mangrove type, sea surface salinity and temperature, and length of mangrove forest edge were important in predicting the density of commercial fish and invertebrates, with deltaic systems supporting the highest densities. The model predicted high densities throughout parts of southeast Asia, the northern coast of South America, the Red Sea, and the Caribbean and Central America. Application of our model onto the global mangrove extent, estimates that mangroves support the annual abundance of nearly 800 billion young-of-year fish and invertebrates contained in our model. Our results confirm the critical role of mangroves globally in supporting fish and fisheries, and further builds the case for their conservation and restoration.### Competing Interest StatementThe authors have declared no competing interest.

Sesarmid crabs modulate nutrient dynamics of tropical mangroves through their leaf-eating habit. How N enrichment may alter this regulatory role, and the implications for mangrove nutrient dynamics, remain unclear. Using a mesocosm experiment, we tested how N enrichment could change the microphytobenthos (MPB) communities, thus modifying the crabs' diet and their role in nutrient dynamics. The factorial experiment combined with field investigation revealed a significant increase in the relative abundance of cyanobacteria. Stable isotope analysis suggested that the main carbon source of crabs shifted from leaf litter to cyanobacteria in mesocosms under both high (20×) and low (2×) N enrichment treatments. The significantly lower total cellulase activity of crabs in the mesocosms might explain the decreased carbon assimilation from leaf litter. The changes in the MPB and the microbiome with N enrichment in the presence of crabs may drive significantly higher carbon processing rate in tropical mangroves.

For four decades, cordgrass (Spartina alterniflora) has invaded salt marshes in the Yellow Sea, altering physical, biogeochemical, and biological processes. Here, we investigated the ecological effects of S. alterniflora invasion on benthic environments compared to native halophytes. S. alterniflora contributed to higher carbon accumulation rates compared with bare tidal flat in sediments (3.4 times), through greater primary production and root biomass, compared to Suaeda japonica (2.5 times) and Phragmites australis (2.4 times) over the given period. The results showed that S. alterniflora eradication treatments inhibited its growth but did not significantly affect the benthic communities. Compared to P. australis and bare tidal flats, S. alterniflora invasion resulted in lower greenhouse gas emission and higher contributions to macrobenthos nutrition, and increased sediment stability and carbon burial. Overall, these multiple lines of evidence provide new insights on S. alterniflora invasion, suggesting that the current eradication policy would be carefully reviewed.

The microphytobenthos (MPB) are known to play important ecological roles in estuarine ecosystems. While tidal flat MPB are widely studied, mangrove MPB are often overlooked due to the common belief that the light-limited environment under the mangrove canopy is not conducive to MPB growth. We assessed the biomass and community characteristics of the MPB in two contrasting mangrove forests (silty substrate and brackish influence versus sandy substrate and oceanic influence), their seasonal photosynthetic performance, as well as their role in mediating porewater carbon flux. Biomass measured as chlorophyll a concentration indicated that MPB abundance was equal, if not higher, in mangroves than on tidal flats. While mangrove sediments fostered an equally rich assemblage of MPB (estimated 23 to 45 genera) as the adjacent tidal flat, the MPB genus composition was different across all habitats, sites, and seasons, indicating the strong environmental influence on MPB assemblages. This study identified porewater salinity, temperature, and pH, as well as irradiance level and surface soil temperature to be accountable for 23.5% of this variation. MPB in all sampling locations were dominated by pennate diatoms, while cyanobacteria were common in the estuarine (as opposed to oceanic) site. Compared to tidal flat MPB, mangrove MPB exhibited photosynthetic performance characteristic of lower light acclimation. A mesocosm experiment using enriched stable isotopes of carbon and nitrogen indicated that a tight coupling exists between mangrove-derived carbon and MPB. Mangrove MPB are abundant and different from tidal flat MPB, and are functional photosynthetic entities with a regulatory role in nutrient cycling in mangrove ecosystems.

Blue carbon ecosystems (BCEs) are important nature‐based solutions for climate change‐mitigation. However, current debates question the reliability and contribution of BCEs under future climatic‐scenarios. The answer to this question depends on ecosystem processes driving carbon‐sequestration and ‐storage, such as primary production and decomposition, and their future rates. We performed a global meta‐analysis on litter decomposition rate constants (k) in BCEs and predicted changes in carbon release from 309 studies. The relationships between k and climatic factors were examined by extracting remote‐sensing data on air temperature, sea‐surface temperature, and precipitation aligning to the decomposition time of each experiment. We constructed global numerical models of litter decomposition to forecast k and carbon release under different scenarios. The current k averages at 27 ± 3 × 10−2 day−1 for macroalgae were higher than for seagrasses (1.7 ± 0.2 × 10−2 day−1), mangroves (1.6 ± 0.1 × 10−2 day−1) and tidal marshes (5.9 ± 0.5 × 10−3 day−1). Macrophyte k increased with both air temperature and precipitation in intertidal BCEs and with sea surface temperature for subtidal seagrasses. Above a temperature threshold for vascular plant litter at ~25°C and ~20°C for macroalgae, k drastically increased with increasing temperature. However, the direct effect of high temperatures on k are obscured by other factors in field experiments compared with laboratory experiments. We defined “fundamental” and “realized” temperature response to explain this effect. Based on relationships for realized temperature response, we predict that proportions of decomposed litter will increase by 0.9%–5% and 4.7%–28.8% by 2100 under low‐ (2°C) and high‐warming conditions (4°C) compared to 2020, respectively. Net litter carbon sinks in BCEs will increase due to higher increase in litter C production than in decomposition by 2100 compared to 2020 under RCP 8.5. We highlight that BCEs will play an increasingly important role in future climate change‐mitigation. Our findings can be leveraged for blue carbon accounting under future climate change scenarios.

Tropical estuaries support wetlands with high biodiversity value and provide essential ecosystem services. Many of these systems, however, are global hotspots for urbanization, particularly in Asia, where this process has resulted in rapid conversion, fragmentation, and degradation of 80 % of the wetlands along the East Asian-Australasian Flyway (EAAF) for migratory birds. However, the impact of such landscape scale changes on migratory birds at a key stopover site along the EAAF has not been evaluated. Here, we used long-term data (> 40 years) from Deep Bay (Hong Kong), a shallow embayment in the Pearl River estuary (PRE) in south China, to investigate the impact of urbanization on (1) catchment land use and water quality, and (2) its impact on the capacity of the wetland to support populations of migratory waterbirds. Deep Bay supports the largest remnants of mangrove forests and tidal mudflats in the PRE and is an important refueling ground along the EAAF. It is also part of the Greater Bay Area (GBA, population 86 million), the world's largest megalopolis. The principal component analysis highlighted the nutrient loading and cleansing effect from seasonal flushing as characterizing variation in water quality in Deep Bay over four decades. Major shifts in water quality during the study period were accompanied by contemporaneous changes in wintering waterbirds numbers. Prior to 2003, the main drivers of water quality were organic nutrients from animal husbandry. Following large-scale reclamation and increases in impervious surface cover post-2003, primarily due to the development of the megacity of Shenzhen (population 17.7 million), the hydrodynamics of Deep Bay have changed, with knock-on effects of sedimentation, input of pollutants, and changes in the macrobenthos. The wintering waterbirds community responded to these changes both in total numbers and the relative importance of feeding guilds. Where total bird counts are positively influenced by benthic biomass, and the benthic biomass is positively correlated to the water quality that is driven by the cleansing effect of tidal flushing. These anthropogenic drivers have negatively impacted migratory birds that use Deep Bay as a refueling station. This study highlights the need for policymakers to control these drivers and limit the level to which sensitive coastlines are urbanized.

Carbon gas flux is important for studies on carbon dynamics in mangroves, but the controlling factors have not always been sufficiently understood. In this study, it is suggested that sediment carbon dioxide (CO2) fluxes in a natural mangrove in Southern China are controlled by tidal positions, seasons, species, the densities of crab burrows and pneumatophores, light conditions and sediment temperature. All these factors account for 51.47% variation in CO2 flux from the sediment–air interface. CO2 flux generally decreased along the tidal position from landward to seaward, and was higher in the dry season than in the wet season. CO2 flux was highest in Avicennia marina (grey mangrove) in comparison with Aegiceras corniculatum (river mangrove) and Kandelia obovata. Pneumatophores and crab burrows promoted sediment CO2 flux in the mangrove at a rate of 18.29 and 15.52 mmol m−2 d−1. Dark flux was higher than light flux. Sediment temperature has a negative influence on CO2 flux. Pneumatophores explain the most variation (13.9%) in CO2 flux among the above factors. Our study suggests that the photosynthesis activity of microphytobenthos is an important factor driving the change of CO2 emissions in this natural mangrove. This is of great significance for the study and for the full exploitation of the carbon sink potential of mangroves.

Large-scale anthropogenic mangroves have been constructed in coastal regions worldwide but our understanding of their ecological effects is limited. In particular, the question of whether and how anthropogenic mangroves influence biodiversity patterns remains elusive. Here, we investigated the influence of large-scale anthropogenic mangroves on biodiversity patterns of mangrove macrobenthos. Specifically, we measure and seek to explain differences in species richness, abundance, assemblage composition and distance-decay effect before and after the construction of anthropogenic mangroves. We surveyed assemblages of gastropod, bivalve and crab species over a wide latitudinal extent (24-28 & DEG;N) in subtropical China. For each, we calculated species richness, abundance, assemblage composition and distance-decay relationship before and after the construction of anthropogenic mangroves. After the large-scale anthropogenic mangroves, we found species richness of gastropods, bivalves and crabs increased by 23.81%, 100% and 20%, respectively. The distance-decay effects of gastropods and bivalves decreased by 25% and 91.43%, while that of crabs remained virtually unchanged, which mediated by increased dispersal rate of macrobenthos. With mangrove plantation, compositional similarity of crab and bivalve assemblages increased by 28.57% and 38.46%, suggesting that large-scale monospecific planting exacerbate biotic homogenization. Altogether, these results indicate that large-scale anthropogenic habitats increase the diversity of mangrove macrobenthos and change taxonomic compositions by reducing distance-decay effects and increasing dispersal rate of macrobenthos. Synthesis and applications. We emphasize that afforestation of coastal wetlands can drive major changes in benthonic communities. Monitoring and assessing the ecological effects of the anthropogenic habitats for the presence of functional faunas will be important in determining the future coastal restoration and maintaining economic aquaculture. Quantifying those effects in terms of regional biodiversity composition will contribute to the management of coastal restoration to be based upon macroevidence rather than a one-sided local perspective.

Mangroves are highly efficient in sequestering carbon from the atmosphere and can accumulate carbon in sediments for millennials. However, The fate of mangrove carbon has not been well constrained due to the lack of data on different pools of sediment carbon sinks and sources. This study examined the variation of carbon stocks and fluxes at the water-sediment-air interface in both estuarine mangroves (natural: Mai Po, restored: Gei Wai) and oceanic mangroves (Ting Kok). There are divergent patterns in biogeochemical variables at the sediment-water-air interface, likely due to significant variation within sites. Total sediment carbon stocks (TCs) ranked in the order of restored estuarine mangroves (392.5 ± 8.8 Mg ha-1), natural estuarine mangroves affected by aquaculture (315.2 ± 21.4 Mg ha-1) and oceanic mangroves (229.1 ± 32.3 Mg ha-1). Sediment organic carbon stocks (SOC) and inorganic carbon stocks (SIC) accounted for 84.1-90.2 % and 9.8-15.9 % of TC, respectively. The highest sediment-air CO2 and CH4 fluxes occurred in restored and natural estuarine mangroves affected by aquaculture, respectively. The isotope of CO2 fluxes (δ13C-CO2) indicates higher contributions from the degradation of mangrove-derived organic carbon in restored (-25.94 ‰ ± 3.37 ‰) and natural estuarine mangroves affected by aquaculture (-25.54 ‰ ± 0.96 ‰) than in oceanic mangroves (-21.55 ‰ ± 1.36 ‰). The isotope of CH4 fluxes (δ13C-CH4) indicates CH4 production dominated by acetate fermentation in restored estuarine mangroves but dominated by the reduction of CO2 for other sites. Future studies should better constrain the fate of mangrove carbon by considering local drivers.