Nitrogen enrichment changed the biogeochemical role of sesarmid crabs by shifting their diets in tropical mangrove ecosystems.

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.

Response of macrophyte litter decomposition in global blue carbon ecosystems to climate change.

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.

Sediment carbon sequestration and sources in peri-urban tidal flats and adjacent wetlands in a megacity.

We investigated the sediment carbon (C) stocks, sequestration and sources in tidal flats and their adjacent mangroves in two coastal wetlands in Hong Kong (the Mai Po Nature Reserve (MPNR) and Ting Kok (TK)), part of a megacity of ∼20 million. At both locations, the C stock of tidal flats was lower than that of mangroves. In MPNR, tidal flats indicated a higher C burial rate (75.2 g C m-2 yr-1) compared to mangroves (64.3 g C m-2 yr-1). The C of tidal flats mainly originated from mangroves and riverine sources, while those in deeper sediments (below 60 cm) in the mangroves originated from the tidal flats. In TK, the C of tidal flats was dominated by oceanic sources. Our study indicates that tidal flats are also important in coastal C sequestration. On highly urbanized coasts, tidal flats and their connected mangroves can play a critical role for C mitigation.

Coastal Transient Niches Shape the Microdiversity Pattern of a Bacterioplankton Population with Reduced Genomes.

Globally dominant marine bacterioplankton lineages are often limited in metabolic versatility, owing to their extensive genome reductions, and thus cannot take advantage of transient nutrient patches. It is therefore perplexing how the nutrient-poor bulk seawater sustains the pelagic streamlined lineages, each containing numerous populations. Here, we sequenced the genomes of 33 isolates of the recently discovered CHUG lineage (~2.6 Mbp), which have some of the smallest genomes in the globally abundant Roseobacter group (commonly over 4 Mbp). These genome-reduced bacteria were isolated from a transient habitat: seawater surrounding the brown alga, Sargassum hemiphyllum. Population genomic analyses showed that: (i) these isolates, despite sharing identical 16S rRNA genes, were differentiated into several genetically isolated populations through successive speciation events; (ii) only the first speciation event led to the genetic separation of both core and accessory genomes; and (iii) populations resulting from this event are differentiated at many loci involved in carbon utilization and oxygen respiration, corroborated by BiOLOG phenotype microarray assays and oxygen uptake kinetics experiments, respectively. These differentiated traits match well with the dynamic nature of the macroalgal seawater, in which the quantity and quality of carbon sources and the concentration of oxygen likely vary spatially and temporally, though other habitats, like fresh organic aggregates, cannot be ruled out. Our study implies that transient habitats in the overall nutrient-poor ocean can shape the microdiversity and population structure of genome-reduced bacterioplankton lineages. IMPORTANCE Prokaryotic species, defined with operational thresholds, such as 95% of the whole-genome average nucleotide identity (ANI) or 98.7% similarity of the 16S rRNA gene sequences, commonly contain extensive fine-grained diversity in both the core genome and the accessory genome. However, the ways in which this genomic microdiversity and its associated phenotypic microdiversity are organized and structured is poorly understood, which disconnects microbial diversity and ecosystem functioning. Population genomic approaches that allow this question to be addressed are commonly applied to cultured species because linkages between different loci are necessary but are missing from metagenome-assembled genomes. In the past, these approaches were only applied to easily cultivable bacteria and archaea, which, nevertheless, are often not representative of natural communities. Here, we focus on the recently discovered cluster, CHUG, which are representative in marine bacterioplankton communities and possess some of the smallest genomes in the globally dominant marine Roseobacter group. Despite being over 95% ANI and identical in the 16S rRNA gene, the 33 CHUG genomes we analyzed have undergone multiple speciation events, with the first split event predominantly structuring the genomic diversity. The observed pattern of genomic microdiversity correlates with CHUG members' differential utilization of carbon sources and differential ability to explore low-oxygen niches. The available data are consistent with the idea that brown algae may be home to CHUG, though other habitats, such as fresh organic aggregates, are also possible.

Ambitious global targets for mangrove and seagrass recovery.

There is an urgent need to halt and reverse loss of mangroves and seagrass to protect and increase the ecosystem services they provide to coastal communities, such as enhancing coastal resilience and contributing to climate stability.1,2 Ambitious targets for their recovery can inspire public and private investment in conservation,3 but the expected outcomes of different protection and restoration strategies are unclear. We estimated potential recovery of mangroves and seagrass through gains in ecosystem extent to the year 2070 under a range of protection and restoration strategies implemented until the year 2050. Under a protection-only scenario, the current trajectories of net mangrove loss slowed, and a minor net gain in global seagrass extent (∼1%) was estimated. Protection alone is therefore unlikely to drive sufficient recovery. However, if action is taken to both protect and restore, net gains of up to 5% and 35% of mangroves and seagrasses, respectively, could be achieved by 2050. Further, protection and restoration can be complementary, as protection prevents losses that would otherwise occur post-2050, highlighting the importance of implementing protection measures. Our findings provide the scientific evidence required for setting strategic and ambitious targets to inspire significant global investment and effort in mangrove and seagrass conservation.

Fate and Effects of Macro- and Microplastics in Coastal Wetlands.

Coastal wetlands trap plastics from terrestrial and marine sources, but the stocks of plastics and their impacts on coastal wetlands are poorly known. We evaluated the stocks, fate, and biological and biogeochemical effects of plastics in coastal wetlands with plastic abundance data from 112 studies. The representative abundance of plastics that occurs in coastal wetland sediments and is ingested by marine animals reaches 156.7 and 98.3 items kg-1, respectively, 200 times higher than that (0.43 items kg-1) in the water column. Plastics are more abundant in mangrove forests and tidal marshes than in tidal flats and seagrass meadows. The variation in plastic abundance is related to climatic and geographic zones, seasons, and population density or plastic waste management. The abundance of plastics ingested by pelagic and demersal fish increases with fish length and dry weight. The dominant characteristics of plastics ingested by marine animals are correlated with those found in coastal wetland sediments. Microplastics exert negative effects on biota abundance and mangrove survival but positive effects on sediment nutrients, leaf drop, and carbon emission. We highlight that plastic pollution is widespread in coastal wetlands and actions are urged to include microplastics in ecosystem health and degradation assessment.

A functional analysis reveals extremely low redundancy in global mangrove invertebrate fauna.

Deforestation results in habitat fragmentation, decreasing diversity, and functional degradation. For mangroves, no data are available on the impact of deforestation on the diversity and functionality of the specialized invertebrate fauna, critical for their functioning. We compiled a global dataset of mangrove invertebrate fauna comprising 364 species from 16 locations, classified into 64 functional entities (FEs). For each location, we calculated taxonomic distinctness (Δ+), functional richness (FRi), functional redundancy (FRe), and functional vulnerability (FVu) to assess functional integrity. Δ+ and FRi were significantly related to air temperature but not to geomorphic characteristics, mirroring the global biodiversity anomaly of mangrove trees. Neither of those two indices was linked to forest area, but both sharply decreased in human-impacted mangroves. About 60% of the locations showed an average FRe < 2, indicating that most of the FEs comprised one species only. Notable exceptions were the Eastern Indian Ocean and west Pacific Ocean locations, but also in this region, 57% of the FEs had no redundancy, placing mangroves among the most vulnerable ecosystems on the planet. Our study shows that despite low redundancy, even small mangrove patches host truly multifunctional faunal assemblages, ultimately underpinning their services. However, our analyses also suggest that even a modest local loss of invertebrate diversity could have significant negative consequences for many mangroves and cascading effects for adjacent ecosystems. This pattern of faunal-mediated ecosystem functionality is crucial for assessing the vulnerability of mangrove forests to anthropogenic impact and provides an approach to planning their effective conservation and restoration.

Effects of food and feeding regime on CO2 fluxes from mangrove consumers - Do marine benthos breathe what they eat?

Intertidal benthos link tertiary predators and primary producers in marine food webs as well as directly contribute to sediment CO2 emission. However, current methods for studying food sources of marine benthos are time-consuming and does not allow direct estimates on feeding regime-related (including different diets, active versus dormant) CO2 production. We examined the food sources of mangrove crabs and gastropods as well as their corresponding CO2 production using cavity-ring down spectroscopy to measure the δ13C-CO2 respiration for consumers, considering the effects of feeding regime, benthos taxa, and dominant feeding habit. Benthos taxa and feeding habit have significant impact on δ13C-CO2 respiration. Particularly, the δ13C-CO2 respiration for crabs (-23.9 ± 0.4‰) was significantly lower than that for gastropods (-17.5 ± 1.3‰). The δ13C-CO2 respiration for deposit-feeders was significantly higher than that for detritivores. There are significant differences in the amount of CO2 emitted and δ13C-CO2 respiration for crabs under different feeding regimes. The differences reflect diet-switching and fuel-switching by the crabs, i.e. 'you breathe what you eat'. Significant differences in CO2 production of crabs also exist between those feeding on microphytobenthos in the laboratory (0.13 ± 0.02 mmol g-1 day-1) and on field collection (i.e. just collected from the field) (0.31 ± 0.03 mmol g-1 day-1). CO2 production of crabs is strongly related to carapace width and length. The δ13C-CO2 respiration for mangrove crabs reflects their diet while crab-respired CO2 flux is related to crab size. These relationships enable partitioning the feeding habit and food sources of key benthos, and help incorporate their contribution into the overall sediment-atmosphere CO2 fluxes in mangrove forests.

The impact of super-typhoon Mangkhut on sediment nutrient density and fluxes in a mangrove forest in Hong Kong.

Cyclone disturbance results in mangrove foliage loss, tree mortality and other changes in ecosystem processes. However, the impact of cyclones on mangrove sediment nutrient density, sediment-air CO2 and CH4 fluxes and their isotopes remains largely unknown. Super-typhoon Mangkhut (maximum gust 256 km h-1) hit Hong Kong in September 2018. We investigated the influence of the cyclone by comparing pre- and post-cyclone sediment carbon cycling processes as well as nitrogen density during an 8-month period in a mangrove forest at Ting Kok, Hong Kong. Time and/or nitrogen density are the dominant drivers of the variation of dark sediment-air CO2 fluxes (Rd) and sediment nutrient density. Significant changes in Rd and their δ13CO2 values, sediment organic carbon density (SOC) and nitrogen density (SON) occurred after the cyclone. Rd were highest one month after the cyclone (0.05 ± 0.01 mmol m-2 min-1) and lowest before the cyclone (8.32 ± 2.84 µmol m-2 min-1). δ13CO2 of pre-cyclone Rd (-18.2 ± 1.7‰) was significantly higher than that of all post-cyclone fluxes (-22.9 ± 1.5‰ to -23.6 ± 1.8‰). Both SOC and SON were highest one month after the cyclone (23.05 ± 1.92 kg C m-3, 2.42 ± 0.11 kg N m-3, 20-40 cm). A significant positive relationship exists between Rd and SON. Sediment-air CH4 fluxes did not show significant changes over time but along the sea-land gradient (0.28 ± 0.21 to 0.61 ± 0.22 µmol m-2 min-1). Cyclone disturbance results in the pulse input of litter, which may explain the significant increase in post-cyclone Rd and lower δ13CO2 of Rd. With anticipated climate change-driven effects on cyclone occurrence and intensity, our data underscores the significance of incorporating the influence of cyclone disturbance in constraining the global nutrient budgets in mangroves.

Colonization by native species enhances the carbon storage capacity of exotic mangrove monocultures.

BACKGROUND: The fast-growing introduced mangrove Sonneratia apetala is widely used for mangrove afforestation and reforestation in China. Some studies suggested that this exotic species outperforms native species in terms of carbon sequestration potential. This study tested the hypothesis that multi-species mangrove plantations might have higher carbon sequestration potential than S. apetala monocultures. RESULTS: Our field measurements at Hanjiang River Estuary (Guangdong province, China) showed that the carbon stock (46.0 ± 3.0 Mg/ha) in S. apetala plantations where the native Kandelia obovata formed an understory shrub layer was slightly higher than that in S. apetala monocultures (36.6 ± 1.3 Mg/ha). Moreover, the carbon stock in monospecific K. obovata stands (106.6 ± 1.4 Mg/ha) was much larger than that of S. apetala monocultures. CONCLUSIONS: Our results show that K. obovata monocultures may have a higher carbon accumulation rate than S. apetala monocultures. Planting K. obovata seedlings in existing S. apetala plantations may enhance the carbon sink associated with these plantations.

Spatial variation of soil properties impacted by aquaculture effluent in a small-scale mangrove.

Small-scale mangroves serve ecological functions similar to large-scale mangroves regarding biological conservation, environmental purification, and supporting biogeochemical processes. The rising aquaculture neighboring mangroves results in their serving as an important sink for massive nutrients and pollutants from aquaculture effluent. We assessed how long-term aquaculture effluent discharge influenced the soil properties of a mangrove-tidal flat continuum using field survey and geostatistics. Common soil physical-chemical properties presented significant spatial variability. Continued aquaculture effluent discharge caused a significant cumulation of soil total organic carbon (SOC) (64.13 g·kg-1), total nitrogen (TN) (2.44 g·kg-1) and total phosphorus (TP) (1.12 g·kg-1) in the mangrove soil, which were as 2-3 times as those on the mudflat. Most of the soil properties changed significantly with increasing distance from the effluent outlet along a tidal channel, and the maximum concentrations of SOC, TN and TP all occurred at 50 m away from the outlet. The results of principal component analysis indicated that aquaculture effluent significantly affected the spatial pattern of soil properties along the mangrove-tidal flat continuum. Continued aquaculture effluent input rendered extensive accumulation of SOC, TN and TP in the mangroves. The spatial heterogeneity of mangrove is the key driver to process the nutrient input spatially differently.

Fatty acids as dietary biomarkers in mangrove ecosystems: Current status and future perspective.

The paradigm that mangrove carbon supports secondary production in mangrove and adjacent habitats has been debated in recent years. Fatty acids (FA) are one of the classic biomarkers that have been frequently applied to track mangrove carbon pathways and assess trophic relationships. However, most previous studies did not evaluate the validity, potential and limitations of FA as biomarkers. The function and metabolism of long-chain polyunsaturated FA (LC-PUFA) in mangrove fauna have been largely ignored, and overlapping single FA biomarkers were widely used to infer dietary contributions from different sources. This review aims to systematically analyze and assess the application of FA biomarkers to dietary analyses in mangrove ecosystems, with a focus on basal food sources and their consumers. Our results show that basal food sources have distinctive FA profiles, with leaves and litter rich in alpha-linolenic acids (ALA, 18:3n-3), microphytobenthos rich in eicosapentaenoic acids (EPA, 20:5n-3) and suspended particulate organic matter (SPOM), or phytoplankton rich in docosahexaenoic acids (DHA, 22:6n-3). Most consumers contain high contents of LC-PUFA, particularly DHA and EPA, but very low levels of long-chain saturated FA (e.g., 22:0, 24:0, 26:0, 28:0), a biomarker of mangrove leaf litter. Bacterial FA biomarkers are present in all consumers. Four possible carbon pathways are identified and examined, i.e., benthic feeding on mangrove leaves and litter, benthic feeding on microphytobenthos, pelagic feeding on SPOM, and benthic and pelagic feeding on bacteria. Each pathway plays a different nutritional role for consumers, together providing a diversity of carbon sources. We recommend that in future (a) a wide range of basal diet sources should be sampled rather than just "visible" sources; (b) the unique FA characteristics of each diet source and consumer should be recognized with a focus on overall FA profiles and the application of multivariate statistics; (c) controlled feeding trials should be considered for keystone or functionally important consumers before selecting certain FA biomarkers to infer animal diets, and; (d) compound-specific stable isotope analysis should be applied to provide more insights into trophic relationships as well as the FA metabolic pathways in consumers.

Spatiotemporal variability in microphytobenthic primary production across bare intertidal flat, saltmarsh, and mangrove forest of Asia and Australia.

The ecological role of intertidal microphytobenthos (MPB) is increasingly recognized in coastal production systems. MPB primary production (PP) measured in coastal wetlands of Korea, Cambodia, and Australia confirmed large variability at the global scale. Surprisingly, MPB biomass in mangrove forests almost doubled those measured in nearby bare tidal flats. However, MPB productivity (Pb) in vegetated habitats was significantly reduced (by ~50%) compared to that on bare tidal flats. Extensive measurements of MPB biomass, PP, and Pb across 12 Korean tidal flats revealed large spatiotemporal variations, suggesting complex sediment-MPB coupled dynamics. The key factors included sediment type, tide, bed elevation, irradiation, temperature, and vegetation. Winter MPB blooms and the elevated Pb seem to be unique characteristics of the Korean intertidal flats. The present study provides the baseline data of MPB PPs in mudflat, saltmarsh, and mangrove habitats in the highly productive zones of the Western Indo-Pacific Rim.

Species choice in mangrove reforestation may influence the quantity and quality of long-term carbon sequestration and storage.

Despite carbon sequestration being an important service of mangrove ecosystems, many mangrove reforestation projects have little consideration of the carbon sequestration capacity of species to be planted. Species selection is mostly based on growth rate and convenience in planting. In this study, to compare the quantity and quality of carbon stored in soil, four habitats were selected in Haijiang River Estuary, southern China to assess the contribution by different mangrove species to sediment carbon pool. Two 12-year-old mangrove forests of the exotic Sonneratia apetala and native Kandelia obovata, respectively, and the adjacent sandflat and mudflat as unvegetated referencing sites had been studied. The total sedimentary organic carbon and active sedimentary organic content in sediment suggested that after 12 years of growth, (1) mangrove forests significantly increased the organic carbon content of sediment; (2) total organic carbon in the K. obovata forest was higher than that of the S. apetala forest; but (3) the carbon pool of the K. obovata forest was less stable than that of the S. apetala forest. These results corroborated with other studies that the sediment carbon pool of S. apetala forests reached a stable state after 13 years of growth, while that of K. obovata forests gradually stabilised upon long-term (>13 years) growth. Our study confirms that K. obovata is more conducive to capture carbon in long-term mangrove reforestation projects, demonstrating that the provision of this service may not be directly related to apparently relevant plant traits such as growth rate.

Mangroves give cause for conservation optimism, for now.

Friess et al. discuss the results of conservation efforts for mangrove forests in recent years.

Improved estimates on global carbon stock and carbon pools in tidal wetlands.

Tidal wetlands are global hotspots of carbon storage but errors exist with current estimates on their carbon density due to the use of factors estimated from other habitats for converting loss-on-ignition (LOI) to organic carbon (OC); and the omission of certain significant carbon pools. Here we show that the widely used conversion factor (LOI/OC = 1.724) is significantly lower than our measurements for saltmarsh sediments (1.92 ± 0.01) and oversimplifies the polynomial relationship between sediment OC and LOI for mangrove forests. Global mangrove OC stock in the top-meter sediment reaches 1.93 Pg when corrected for this bias, and is 20% lower than the previous estimates. Ecosystem carbon stock (living and dead biomass, sediment OC and inorganic carbon) is estimated at 3.7-6.2 Pg. Mangrove deforestation leads to carbon emission rates at 23.5-38.7 Tg yr-1 after 2000. Mangrove sediment OC stock has previously been over-estimated while ecosystem carbon stock underestimated.

Spatially-explicit valuation of coastal wetlands for cyclone mitigation in Australia and China.

Coastal wetlands are increasingly recognised for their pivotal role in mitigating the growing threats from cyclones (including hurricanes) in a changing climate. There is, however, insufficient information about the economic value of coastal wetlands for cyclone mitigation, particularly at regional scales. Analysis of data from 1990-2012 shows that the variation of cyclone frequencies is related to EI Niño strength in the Pacific Ocean adjacent to Australia, but not China. Among the cyclones hitting the two countries, there are significant relationships between the ratio of total economic damage to gross domestic production (TD/GDP) and wetland area within cyclone swaths in Australia, and wetland area plus minimum cyclone pressure despite a weak relationship in China. The TD/GDP ratio is significantly higher in China than in Australia. Despite their extensive and growing occurrence, seawalls in China appear not to play a critical role in cyclone mitigation, and cannot replace coastal wetlands, which provide other efficient ecosystem services. The economic values of coastal wetlands in Australia and China are respectively estimated at US$52.88 billion and 198.67 billion yr-1 for cyclone mitigation, albeit with large within-country geographic variation. This study highlights the urgency to integrate this value into existing valuations of coastal wetlands.