Mangrove expansion and salt marsh decline at mangrove poleward limits

Temperature Drives Seagrass Recovery Across the Western North Atlantic

Climate-driven shifts in herbivores, temperature, and nutrient runoff threaten coastal ecosystem resilience. However, ecological resilience, particularly for foundation species, remains poorly understood due to the scarcity of field experiments conducted across appropriate spatial and temporal scales that investigate multiple stressors. This study evaluates the resilience of a widespread tropical marine plant (turtlegrass) to disturbances across its geographic range and examines how environmental gradients in (a)biotic factors influence recovery. We assessed turtlegrass resilience by following recovery rates for a year after a simulated pulse disturbance (complete above- and belowground biomass removal). Contrary to studies in temperate areas, higher temperature generally enhanced seagrass recovery. While nutrients had minimal individual effects, they reduced aboveground recovery when combined with high levels of herbivore grazing (meso and megaherbivore). Belowground recovery was also affected by combined high levels of nutrients and grazing (megaherbivores only). Light availability had minimal effects. Our results suggest that the resilience of some tropical species, particularly in cooler subtropical waters, may initially benefit from warming. However, continuing shifts in nutrient supply and changes in grazing pressure may ultimately serve to compromise seagrass recovery.

Incorporating microbiome analyses can enhance conservation of threatened species and ecosystem functions

Conservation genomics is a rapidly growing subdiscipline of conservation biology that uses genome-wide information to inform management of biodiversity at all levels. Such efforts typically focus on species or systems of conservation interest, but rarely consider associated microbes. At least three major approaches have been used to study how microorganisms broadly contribute to conservation areas: (1) diversity surveys map out microbial species distribution patterns in a variety of hosts, natural environments or regions; (2) functional surveys associate microbial communities with factors of interest, such as host health, symbiotic interactions, environmental characteristics, ecosystem processes, and biological invasions; and (3) manipulative experiments examine the response of changes to microbial communities or determine the functional roles of specific microbes within hosts or communities by adding, removing, or genetically modifying microbes. In practice, multiple approaches are often applied simultaneously. The results from all three conservation genomics approaches can be used to help design practical interventions and improve management actions, some of which we highlight below. However, experimental manipulations allow for more robust causal inferences and should be the ultimate goal of future work. Here we discuss how further integration of microbial research of a host's microbiome and of free living microbes into conservation biology will be an essential advancement for conservation of charismatic organisms and ecosystem functions in light of ongoing global environmental change.

Past, present and future global mangrove primary productivity

Mangrove productivity is crucial for the global carbon cycle, yet previous research has mostly focused on small-scale temporal changes or static global patterns, with limited investigation into global or regional temporal trends. This study used existing data on mangrove leaf litter to model mangrove Net Primary Productivity (NPP) on a monthly timescale from 1980 to 2094 across global regions defined by the Marine Ecoregions of the World framework. The models showed a slight global decrease in NPP of approximately 1.4 %, from 239.2 ± 87.6 Tg yr-1 (1980-1990) to 235.9 ± 81.9 Tg yr-1 (2085-2094). However, significant regional changes were identified, including substantial increases in NPP in the Southwest Australian Shelf (60.58 ± 97.9 %), the Warm Temperate Northeast Pacific (43.75 ± 65.7 %), and the Warm Temperate Northwest Pacific (31.55 ± 55.7 %), as well as decreases in Southeast Asian provinces like the Java Transitional (11.45 ± 6.2 %) and Western Coral Triangle (7.61 ± 9.6 %). These findings highlight previously unreported regional shifts in mangrove productivity, which could significantly impact carbon sequestration and the transfer of organic matter to adjacent ecosystems.

Impact of Salinity Gradients on Seed Germination, Establishment, and Growth of Two Dominant Mangrove Species Along the Red Sea Coastline

BACKGROUND

Mangroves are one of the key nature-based solutions that mitigate climate change impacts. Even though they are halophytic in nature, seedlings are vulnerable to high salinity for their establishment. This study investigated the effects of different salinities on seedling growth and mineral element composition of two dominant species (Avicennia marina and Rhizophora mucronata).

METHODS

The study followed a randomized complete block design, i.e., main treatments (growing environment in greenhouse (GH) or net house (NH)) and four sub-treatments under 21 replicates, i.e., irrigation with 100% freshwater (0.4%o-T1), 100% saline water (35%o-T2), 50% saline water and 50% freshwater (18%o-T3), and brine water (60%o-T4).

RESULTS

Results revealed that A. marina seeds can optimally germinate and survive well reaching 80% in NH under T1. However, T2 and T4 seedlings had the lowest survival. Mineral element analysis showed that A. marina grown under NH recorded higher levels of Ca, Mg, and K which increased with increasing levels of salinity. The opposite was true with Na levels. R. mucronata on the other hand, recorded completely opposite findings with T1 seedlings reaching 95% in the greenhouse while T3 reached almost 60%.

CONCLUSIONS

It can be concluded that mangrove species can optimally germinate and grow in both freshwater and 50% saline water, but growth reduction occurs with seawater and complete growth inhibition with brine water.

A lifecycle model approach for predicting mangrove extent

Tidal dynamics are a well-known driver of mangrove distribution, with most predictive measures using some form of tidal parameter (tidal plane or hydroperiod) to define mangrove extent. However, these methods often fail to consider the causative reason why mangroves thrive or perish at a specific elevation or how mangrove survivability thresholds can differ across a species' lifecycle. The lack of understanding of the drivers influencing mangrove establishment has resulted in poor success rates for mangrove restoration and creation projects worldwide. A novel mangrove lifecycle model that uses a multi-forcing threshold approach is proposed to simulate Avicennia marina viability across establishment and development phases. The lifecycle model includes critical threshold stages for reproduction, seed dispersal, seedling establishment and development, and mature tree survival. The model was validated at 37 sites in eastern Australia to predict mangrove extent across various estuary types and tidal dynamic conditions. The model accurately calculated the upper (RMSE = 0.0676, R2 = 0.8932) and lower (RMSE = 0.0899, R2 = 0.7417) mangrove surface elevations, providing physiological reasoning for establishment and development. Based on the various conditions tested, the model results highlight the highly dynamic spatial and temporal conditions where Avicennia forests thrive. It was found that stressors influencing mangrove establishment were the primary factor for mangrove extent across all sites. However, estuarine typology is important in forcing threshold limits and establishment opportunities. Estuaries with limited tidal decay (from the oceanic forcing) provide more opportunities for mangroves to establish than estuaries with significant tidal attenuation. Regardless of estuary typology, all sites tested had substantial spatial variability through time. Results from the lifecycle model suggest that mature Avicennia forests establish and thrive under a wide range of hydrologic conditions. This resilience suggests that mature mangroves may be able to withstand increases in climatic and hydrologic pressures via biophysical adaptations, although the upper thresholds and acceptable rates of change are difficult to predict. Overall, this study highlights the value of a new causal method for estimating mangrove extent across various lifecycle stages, locations, and time periods.

Global Data Compilation Across Climate Gradients Supports the Use of Common Allometric Equations for Three Transatlantic Mangrove Species

Predicting the distribution, structure, and biomass of mangrove forests is an area of high research interest. Across the Atlantic East Pacific biogeographic region, three species are common and abundant members of local mangrove communities; Rhizophora mangle, Avicennia germinans, and Laguncularia racemosa. Biomass prediction for these species has relied on two approaches: site-specific allometries based on the idea that environmental/climatic differences between sites drive growth differences, or the use of common allometric equations based on the idea that site driven differences are minimal. Meta-analyses of global compilations of interspecific plot level data (e.g., mean canopy height, stand basal area) show trends in size and structure with climatic variables, however this has not been critically evaluated across these species using empirical allometric growth functions. We compared allometric equations derived from 590 individuals within and across nine broadly distributed sites at interspecific and intraspecific levels and explored the influence of climatic variables on allometric slopes and intercepts. Assessing variables that can be used to predict biomass in the field (height, diameter at breast height (DBH), canopy spread), we find interspecific root mean squared errors similar to or smaller than most intraspecific or site-specific equations, particularly when examining sites with sample sizes above recommended values. We also find significant effects of several climatic variables on growth allometries with the strongest effects from minimum temperature followed by precipitation seasonality. Our results suggest that while climate has a clear influence on mangrove allometric growth, common equations may have utility in biomass prediction. Future methodological improvements, particularly larger sample sizes across the entire available size range, combined with data from a broader range of growth conditions will further inform which allometric relationships exhibit the most variability within and across sites and which variables best predict mangrove biomass.

Impact of climate change on Colombian Pacific coast mangrove bivalves distribution

The mangrove bivalves, Anadara tuberculosa and Anadara similis, are pivotal for the Colombian Pacific coast mangrove ecosystems and economies. In this study, the current and future potential distribution of these bivalves is modeled considering climate change. The future models (2030 and 2050) were projected considering the new climate scenarios (SSP1, SSP2, and SSP5) proposed by the IPCC in its sixth report. Our findings reveal areas in the Colombian Pacific coast, notably Nariño, Cauca, southern Valle del Cauca, and Chocó, with high environmental suitability for these bivalves. However, the 2050 projections, especially under the pessimistic SSP5 scenario, indicate potential adverse impacts from climate change. By 2030 and 2050, the species might lean more toward a southwesterly distribution in the Colombian Pacific coast. Climate-induced spatiotemporal mismatches could occur between the bivalves and the mangroves in some areas. These insights are crucial for effective conservation and management strategies for these species.

Hurricanes are limiting the mangrove canopy heights in the Gulf of Mexico

Mangrove canopy height (MCH) has been described as a leading characteristic of mangrove forests, protecting coastal economic interests from hurricanes. Meanwhile, winter temperature has been considered the main factor controlling the MCH along subtropical coastlines. However, the MCH in Cedar Key, Florida (∼12 m), is significantly higher than in Port Fourchon, Louisiana (∼2.5 m), even though these two subtropical locations have similar winter temperatures. Port Fourchon has been more frequently impacted by hurricanes than Cedar Key, suggesting that hurricanes may have limited the MCH in Port Fourchon rather than simply winter temperatures. This hypothesis was evaluated using novel high-resolution remote sensing techniques that tracked the MCH changes between 2002 and 2023. Results indicate that hurricanes were the limiting factor keeping the mean MCH at Port Fourchon to <1 m (2002-2013), as the absence of hurricane impacts between 2013 and 2018 allowed the mean MCH to increase by 60 cm despite the winter freezes in Jan/2014 and Jan/2018. Hurricanes Zeta (2020) and Ida (2021) caused a decrease in the mean MCH by 20 cm, breaking branches, defoliating the canopy, and toppling trees. The mean MCH (∼1.6 m) attained before Zeta and Ida has not yet been recovered as of August 2023 (∼1.4 m), suggesting a longer-lasting impact (>4 years) of hurricanes on mangroves than winter freezes (<1 year). The high frequency of hurricanes affecting mangroves at Port Fourchon has acted as a periodic "pruning," particularly of the tallest Avicennia trees, inhibiting their natural growth rates even during quiet periods following hurricane events (e.g., 12 cm/yr, 2013-2018). By contrast, the absence of hurricanes in Cedar Key (2000-2020) has allowed the MCH to reach 12 m (44-50 cm/yr), implying that, besides the winter temperature, the frequency and intensity of hurricanes are important factors limiting the MCH on their latitudinal range limits in the Gulf of Mexico.

Linking tidal wetland vegetation mosaics to micro-topography and hydroperiod in a tropical estuary

Although saltmarshes are critical coastal ecosystems they are threatened by human activities and sea-level rise (SLR). Long-term restoration and management strategies are often hampered by an insufficient understanding of the past, present, and future processes that influence tidal wetland functionality and change. As understanding vegetation distribution in relation to elevation and tidal hydroperiod is often the basis of restoration and management decisions, this study investigated the relationships between micro-topography, tidal hydroperiod, and the distribution of saltmarshes, mangroves, and unvegetated flats in a tropical estuary situated within a Great Barrier Reef Catchment in North Queensland, Australia. A combination of high-resolution unattended-aerial-vehicle (UAV)-derived digital elevation model (DEMs) and land cover coupled with 2D hydrodynamic modelling was used to investigate these aspects. Zonation was more complex than generally recognised in restoration and legislation, with overlapping distribution across elevation. Additionally, although each type of tidal wetland cover had distinct mean hydroperiods, and elevation and hydroperiods were strongly correlated, elevation explained only 15% of the variability in tidal wetland cover distribution. This suggests that other factors (e.g., groundwater dynamics) likely contribute to tidal wetland cover zonation patterns. These findings underline that simplistic rules in the causality of tidal wetlands need to be applied with caution. Their applicability in management and restoration are likely to vary depending on contexts, as observed in our study site, with varying environmental and biological factors playing important roles in the distribution patterns of tidal wetland components. We also identified strong monthly variability in tidal hydroperiods and connectivity experienced by each tidal wetland cover (e.g., 10.26% of succulent saltmarshes were inundated during lower-than-average tides compared to 66% in higher than-average tides), highlighting the importance of integrating temporal dynamics in tidal wetland research and management. Additionally, we explored the potential effects of sea-level rise (SLR) on the tidal hydroperiods and connectivity of our study site. The results show that the inundation experienced by each tidal wetland cover may increase importantly if vegetation does not keep up with SLR (e.g., under a 0.8 m sea level scenarios, mean maximum depth of succulent saltmarsh in higher-than-average tides is 184.1 mm higher than the current mean-maximum inundation depth of mangroves). This underlines the importance of acquiring detailed spatio-temporally resolved data to enable the development of robust long-term and adaptive saltmarsh management strategies. Our results are discussed from a management and restoration perspective. We highlight the uncertainties and complexities in understanding the processes influencing tidal wetland functionality, and hence, their management and restoration prospects.

Balancing multiple management objectives as climate change transforms ecosystems

As climate change facilitates significant and persistent ecological transformations, managing ecosystems according to historical baseline conditions may no longer be feasible. The Resist-Accept-Direct (RAD) framework can guide climate-informed management interventions, but in its current implementations RAD has not yet fully accounted for potential tradeoffs between multiple - sometimes incompatible - ecological and societal goals. Key scientific challenges for informing climate-adapted ecosystem management include (i) advancing our predictive understanding of transformations and their socioecological impacts under novel climate conditions, and (ii) incorporating uncertainty around trajectories of ecological change and the potential success of RAD interventions into management decisions. To promote the implementation of RAD, practitioners can account for diverse objectives within just and equitable participatory decision-making processes.

Uptake and distribution of metal(loid)s in two rare species of saltmarsh, blackseed samphire, Tecticornia pergranulata, and narrow-leafed wilsonia, Wilsonia backhousei, in New South Wales, Australia

The uptake and distribution of copper, zinc, arsenic, and lead was examined in two rare Australian saltmarsh species, Tecticornia pergranulata and Wilsonia backhousei. The bioconcentration factors and translocation factors were generally much lower than one, except for the Zn translocation factors for T. pergranulata. When compared to other Australian saltmarsh taxa, these species generally accumulated the lowest levels observed among taxa, especially in terms of their BCFs. Essential metals tended to be regulated, while non-essential metals increased in concentration with dose during transport among compartments, a pattern not previously observed in Australian saltmarsh taxa. The uptake of metals into roots was mainly explained by total sediment metal loads as well as more acidic pH, increased soil organic matter, and decreased salinity. The low uptake and limited translocation observed in these rare taxa may offer a competitive advantage for their establishment and survival in the last urbanised populations, where legacy metal contamination acts as a selective pressure.

Stronger increases but greater variability in global mangrove productivity compared to that of adjacent terrestrial forests

Mangrove forests are a highly productive ecosystem with important potential to offset anthropogenic greenhouse gas emissions. Mangroves are expected to respond differently to climate change compared to terrestrial forests owing to their location in the tidal environment and unique ecophysiological characteristics, but the magnitude of difference remains uncertain at the global scale. Here we use satellite observations to examine mean trends and interannual variability in the productivity of global mangrove forests and nearby terrestrial evergreen broadleaf forests from 2001 to 2020. Although both types of ecosystem experienced significant recent increases in productivity, mangroves exhibited a stronger increasing trend and greater interannual variability in productivity than evergreen broadleaf forests on three-quarters of their co-occurring coasts. The difference in productivity trends is attributed to the stronger CO2 fertilization effect on mangrove photosynthesis, while the discrepancy in interannual variability is attributed to the higher sensitivities to variations in precipitation and sea level. Our results indicate that mangroves will have a faster increase in productivity than terrestrial forests in a CO2-rich future but may suffer more from deficits in water availability, highlighting a key difference between terrestrial and tidal ecosystems in their responses to climate change.

Global potential distribution of mangroves: Taking into account salt marsh interactions along latitudinal gradients

Mangrove is one of the most productive and sensitive ecosystems in the world. Due to the complexity and specificity of mangrove habitat, the development of mangrove is regulated by several factors. Species distribution models (SDMs) are effective tools to identify the potential habitats for establishing and regenerating the ecosystem. Such models usually include exclusively environmental factors. Nevertheless, recent studies have challenged this notion and highlight the importance of including biotic interactions. Both factors are necessary for a mechanistic understanding of the mangrove distribution in order to promote the protection and restoration of mangroves. Thus, we present a novel approach of combining environmental factors and interactions with salt marsh for projecting mangrove distributions at the global level and within latitudinal zones. To test the salt marsh interaction, we fit the MaxEnt model with two predicting sets: (1) environments only and (2) environments + salt marsh interaction index (SII). We found that both sets of models had good predictive ability, although the SII improved model performance slightly. Potential distribution areas of mangrove decrease with latitudes, and are controlled by biotic and abiotic factors. Temperature, precipitation and wind speed are generally critical at both global scale and ecotones along latitudes. SII is important on global scale, with a contribution of 5.9%, ranking 6th, and is particularly critical in the 10-30°S and 20-30°N zone. Interactions with salt marsh, including facilitation and competition, are shown to affect the distribution of mangroves at the zone of coastal ecotone, especially in the latitudinal range from 10° - 30°. The contribution of SII to mangrove distribution increases with latitudes due to the difference in the adaptive capacity of salt marsh plants and mangroves to environments. Totally, this study identified and quantified the effects of salt marsh on mangrove distribution by establishing the SII. The results not only facilitate to establish a more accurate mangrove distribution map, but also improve the efficiency of mangrove restoration by considering the salt marsh interaction in the mangrove management projects. In addition, the method of incorporating biotic interaction into SDMs through establish the biotic interaction index has contributed to the development of SDMs.

Impacts of Climate Change on Marine Foundation Species

Marine foundation species are the biotic basis for many of the world's coastal ecosystems, providing structural habitat, food, and protection for myriad plants and animals as well as many ecosystem services. However, climate change poses a significant threat to foundation species and the ecosystems they support. We review the impacts of climate change on common marine foundation species, including corals, kelps, seagrasses, salt marsh plants, mangroves, and bivalves. It is evident that marine foundation species have already been severely impacted by several climate change drivers, often through interactive effects with other human stressors, such as pollution, overfishing, and coastal development. Despite considerable variation in geographical, environmental, and ecological contexts, direct and indirect effects of gradual warming and subsequent heatwaves have emerged as the most pervasive drivers of observed impact and potent threat across all marine foundation species, but effects from sea level rise, ocean acidification, and increased storminess are expected to increase. Documented impacts include changes in the genetic structures, physiology, abundance, and distribution of the foundation species themselves and changes to their interactions with other species, with flow-on effects to associated communities, biodiversity, and ecosystem functioning. We discuss strategies to support marine foundation species into the Anthropocene, in order to increase their resilience and ensure the persistence of the ecosystem services they provide.

Environmental flow assessment for intermittent rivers supporting the most poleward mangroves

The most vulnerable and dynamic ecosystems in terms of response to climate change and fluctuations in hydrological conditions are mangroves, particularly those located on the edge of their latitudinal range limits. The four primary Iranian mangrove forest sites: Nayband, Qeshm, Gabrik, and Govatr, located in the northern part of the Persian Gulf and the Gulf of Oman already exist near the limit of their tolerance to extreme temperature, precipitation, and salinity. Due to extreme climate conditions at these locations, the mangrove trees are usually smaller and less dense as compared with mangroves closer to the equator complicating their monitoring and mapping efforts. Despite the growing attention to the ecological benefits of mangrove forests and their importance in climate change mitigation, there are still a few studies on these marginal mangroves. Therefore, we investigated whether the variation in mangrove ecosystem health is related to the changes in physical parameters and differs between estuarine and sea-side locations. We developed a comprehensive database on NDVI values, associated rainfall, temperature, and river flow based on in-situ and remote sensing measurements. By understanding the normal hydrologic patterns that control the distribution and growth of mangroves in arid and semi-arid regions, we are questioning the need for environmental flow allocation to restore mangrove ecosystem health. This brings us to the second gap in the literature and the need for further studies on Environmental Flow assessment for intermittent and ephemeral rivers. Alike other mangroves studied, forests showed greenness seasonality, positively correlated with rainfall, and negatively correlated with temperature. As there was no clear difference between estuarine and marine sites, freshwater influence in the form of river flow, unlike temperature, cannot be considered a major limiting factor. Nevertheless, during prolonged droughts mangroves could benefit from the recommended allocation of Environmental Flow during the cold period (November-March).

Mangrove interaction with saltmarsh varies at different life stages

Mangroves and saltmarshes are two of the most relevant coastal habitats for humans. These ecosystems offer several services like coastal protection, climate mitigation, and nursery habitats for many artisanal and commercially exploited fish, crabs, and shellfish. They mostly dominate different latitudinal ranges but in several places around the world they co-occur and interact. Here, we summarize the current scientific knowledge on mangrove-saltmarsh ecological interactions and propose a conceptual model. We screened 1410 articles from 1945 to 2022 and selected 29 experiments that assessed mangrove-saltmarsh ecological interactions. Both positive and negative interactions are observed but there is variation along different mangrove life stages. Higher retention and establishment of mangrove propagules are found inside saltmarshes than on bare flats, i.e. facilitation, and these effects are higher at grass than at succulent saltmarsh species. Mangrove seedlings, saplings, or trees mostly compete with saltmarshes, negatively affecting mangrove growth. We propose a model with different outcomes considering the interaction between different mangrove's life stages and saltmarsh forms and discussed these interactions in the light of anthropogenic threats and climate change.

Contamination in mangrove ecosystems: A synthesis of literature reviews across multiple contaminant categories

Mangrove forests are exposed to diverse ocean-sourced and land-based contaminants, yet mangrove contamination research lags. We synthesize existing data and identify major gaps in research on five classes of mangrove contaminants: trace metals, persistent organic pollutants, polycyclic aromatic hydrocarbons, microplastics, and pharmaceuticals and personal care products. Research is concentrated in Asia, neglected in Africa and the Americas; higher concentrations are correlated with waste water treatment plants, industry, and urbanized landscapes. Trace metals and polycyclic aromatic hydrocarbons, frequently at concentrations below regulatory thresholds, may bioconcentrate in fauna, whereas persistent organic pollutants were at levels potentially harmful to biota through short- or long-term exposure. Microplastics were at variable levels, yet lack regulatory and ecotoxicological thresholds. Pharmaceuticals and personal care products received minimal research despite biological activity at small concentrations. Given potential synergistic effects, multi-contaminant research, increased monitoring of multiple contaminant classes, and increased public outreach and involvement are needed.

Widespread retreat of coastal habitat is likely at warming levels above 1.5 °C

Several coastal ecosystems-most notably mangroves and tidal marshes-exhibit biogenic feedbacks that are facilitating adjustment to relative sea-level rise (RSLR), including the sequestration of carbon and the trapping of mineral sediment1. The stability of reef-top habitats under RSLR is similarly linked to reef-derived sediment accumulation and the vertical accretion of protective coral reefs2. The persistence of these ecosystems under high rates of RSLR is contested3. Here we show that the probability of vertical adjustment to RSLR inferred from palaeo-stratigraphic observations aligns with contemporary in situ survey measurements. A deficit between tidal marsh and mangrove adjustment and RSLR is likely at 4 mm yr-1 and highly likely at 7 mm yr-1 of RSLR. As rates of RSLR exceed 7 mm yr-1, the probability that reef islands destabilize through increased shoreline erosion and wave over-topping increases. Increased global warming from 1.5 °C to 2.0 °C would double the area of mapped tidal marsh exposed to 4 mm yr-1 of RSLR by between 2080 and 2100. With 3 °C of warming, nearly all the world's mangrove forests and coral reef islands and almost 40% of mapped tidal marshes are estimated to be exposed to RSLR of at least 7 mm yr-1. Meeting the Paris agreement targets would minimize disruption to coastal ecosystems.

Remediation of soil polluted with Pb and Cd and alleviation of oxidative stress in Brassica rapa plant using nanoscale zerovalent iron supported with coconut-husk biochar

Accumulation of toxic elements by plants from polluted soil can induce the excessive formation of reactive oxygen species (ROS), thereby causing retarded plants' physiological attributes. Several researchers have remediated soil using various forms of zerovalent iron; however, their residual impacts on oxidative stress indicators and health risks in leafy vegetables have not yet been investigated. In this research, nanoscale zerovalent iron supported with coconut-husk biochar (nZVI-CHB) was synthesized through carbothermal reduction process using Fe2O3 and coconut-husk. The stabilization effects of varying concentrations of nZVI-CHB and CHB (250 and 500 mg/kg) on cadmium (Cd) and lead (Pb) in soil were analyzed, and their effects on toxic metals induced oxidative stress, physiological properties, and antioxidant defence systems of the Brassica rapa plant were also checked. The results revealed that the immobilization of Pb and Cd in soil treated with CHB was low, leading to a higher accumulation of metals in plants grown. However, nZVI-CHB could significantly immobilize Pb (57.5-62.12%) and Cd (64.1-75.9%) in the soil, leading to their lower accumulation in plants below recommended safe limits and eventually reduced carcinogenic risk (CR) and hazard quotient (HQ) for both Pb and Cd in children and adults below the recommended tolerable range of <1 for HQ and 10-6 - 10-4 for CR. Also, a low dose of nZVI-CHB significantly mitigated toxic metal-induced oxidative stress in the vegetable plant by inhibiting the toxic metals uptake and increasing antioxidant enzyme activities. Thus, this study provided another insightful way of converting environmental wastes to sustainable adsorbents for soil remediation and proved that a low-dose of nZVI-CHB can effectively improve soil quality, plant physiological attributes and reduce the toxic metals exposure health risk below the tolerable range.

An assessment of mangrove vegetation changes in reference to cyclone impacted climatic alterations at land-ocean interface of Indian Sundarbans with application of remote sensing-based analytical tools

Mangrove ecoregions of the Indian Sundarbans (IS) are highly productive ecosystems in the Bengal delta of the Indian subcontinent. These mangroves are crucial in reducing the negative consequences of extreme environmental events like excessive wave movements and periodic storm surges, in addition to serving as an important habitat for a variety of distinct flora and animals. The Bay of Bengal has been increasingly affected by climatic changes like increase in sea surface temperature (SST), salinization, and sediment loads, a decrease in freshwater intake, and sea level rise. In the last two decades (2000-2020), these climatic phenomena have increased the frequency of tropical cyclones. From 2000 to 2020, the loss of landmass has been attributed to exposure to these climate changes. According to open-source satellite imaging data, such losses in land area have also led to a decrease in the amount of mangrove vegetation. Thus, to monitor the health of mangrove vegetation, Landsat-based health indicators like normalized difference vegetation index (NDVI), enhanced vegetation index (EVI), and combined mangrove recognition index (CMRI) have been considered in this study. CMRI, as a mangrove-specific index, was measured on the basis of the difference of NDVI and normalized difference water index for remote sensing of vegetation liquid water from space (NDWI_Gao). Furthermore, datasets for abiotic variables have been extrapolated from remotely sensed data for the said period using specific formulae. Both long-term and short-term temporal trends have been considered to better envisage the impact of episodic cyclonic events on mangrove health (1990-2020). Our findings indicate that cyclones altered the habitat with respect to land area and salinization status which would possibly render the dominance of more halotolerant forms with loss of freshwater mangrove biodiversity. Even though plantation efforts have shown the recovery of mangroves in this area, sudden storm surges and concomitant salinization of habitat put the plantation efforts in vain. A combination of factors like salinization, rise in SST, rainfall reduction in pre- and post-monsoon periods and episodic cyclonic events would probably lead to further deterioration of mangrove health in this area. Since the IS is suffering the most from climatic change and intermittent cyclonic occurrences, it is crucial to consider this when making policy decisions. Appropriate actions must be taken along with stronger conservation techniques, to protect this vulnerable environment. Better conservation tactics and ongoing plantation efforts would stop the loss of mangrove vegetation and its habitat, even though the growing frequency of episodic storm occurrences cannot be stopped.

Biological-physical interactions are fundamental to understanding and managing coastal dynamics

There is an urgent need to address coastal dynamics as a fundamental interaction between physical and biological processes, particularly when trying to predict future biological-physical linkages under anticipated changes in environmental forcing. More integrated modelling, support for observational networks and the use of management interventions as controlled experimental exercises should now be vigorously pursued.

A bibliometric study on carbon cycling in vegetated blue carbon ecosystems

Understanding carbon cycling in blue carbon ecosystems is key to sequestrating more carbon in these ecosystems to mitigate climate change. However, limited information is available on the basic characteristics of publications, research hotspots, research frontiers, and the evolution of topics related to carbon cycling in different blue carbon ecosystems. Here, we conducted bibliometric analysis on carbon cycling in salt marsh, mangrove, and seagrass ecosystems. The results showed that interest in this field has dramatically increased with time, particularly for mangroves. The USA has substantially contributed to the research on all ecosystems. Research hotspots for salt marshes were sedimentation process, carbon sequestration, carbon emissions, lateral carbon exchange, litter decomposition, plant carbon fixation, and carbon sources. In addition, biomass estimation by allometric equations was a hotspot for mangroves, and carbonate cycling and ocean acidification were hotspots for seagrasses. Topics involving energy flow, such as productivity, food webs, and decomposition, were the predominant areas a decade ago. Current research frontiers mainly concentrated on climate change and carbon sequestration for all ecosystems, while methane emission was a common frontier for mangroves and salt marshes. Ecosystem-specific research frontiers included mangrove encroachment for salt marshes, ocean acidification for seagrasses, and aboveground biomass estimation and restoration for mangroves. Future research should expand estimates of lateral carbon exchange and carbonate burial and strengthen the exploration of the impacts of climate change and restoration on blue carbon. Overall, this study provides the research status of carbon cycling in vegetated blue carbon ecosystems, which favors knowledge exchanges for future research.

The race for space: Modelling the landward migration of coastal wetlands under sea level rise at regional scale

Globally, sea-level rise (SLR) is a major environmental challenge for coastal ecosystems. Of particular concern are the impacts on intertidal wetlands, the loss of which would have detrimental consequences for both human and ecological communities. On the south-east Australian coast, case studies suggest that the future of intertidal wetlands will greatly depend on landward migration as surface accretion may not keep up with the predicted SLR in many estuaries. However, due to differences in geomorphological settings and land-use, estuaries vary in their capacity to accommodate lateral migration. Regional scale assessment of the lateral accommodation space is therefore critical for pre-emptive planning to conserve these valuable coastal ecosystems. In this study, we analysed wetland lateral accommodation space distribution within 110 estuaries under three SLR scenarios and three land management options on the New South Wales coast, south-east Australia. From the wetland distribution predictions, we calculated and mapped the lateral accommodation space in each estuary associated with each sea level and land use scenario. We further investigated the relationships between wetland migration capacity, intertidal hypsometry represented by elevation skewness, and estuary type within a Bayesian analysis framework. Our results showed that while a few large riverine estuaries dominated the state's total accommodation space, saline wetlands were at risk of disappearing from most intermittently closed-open estuaries if they cannot vertically accrete at the pace of SLR. These distinct responses to SLR are due to different elevation distributions. Furthermore, our assessment of land use adaptation options suggested that the promotion of landward migration without impairing other important ecosystems could be achieved by making low intensity land uses available within several riverine estuaries and barrier (open entrance) estuaries. Through identifying migration opportunities and barriers, the findings of the study could support regional scale adaptation strategies to ensure the sustainability of wetland-associated ecosystem goods and services.

Mangrove reforestation provides greater blue carbon benefit than afforestation for mitigating global climate change

Significant efforts have been invested to restore mangrove forests worldwide through reforestation and afforestation. However, blue carbon benefit has not been compared between these two silvicultural pathways at the global scale. Here, we integrated results from direct field measurements of over 370 restoration sites around the world to show that mangrove reforestation (reestablishing mangroves where they previously colonized) had a greater carbon storage potential per hectare than afforestation (establishing mangroves where not previously mangrove). Greater carbon accumulation was mainly attributed to favorable intertidal positioning, higher nitrogen availability, and lower salinity at most reforestation sites. Reforestation of all physically feasible areas in the deforested mangrove regions of the world could promote the uptake of 671.5-688.8 Tg CO₂-eq globally over a 40-year period, 60% more than afforesting the same global area on tidal flats (more marginal sites). Along with avoiding conflicts of habitat conversion, mangrove reforestation should be given priority when designing nature-based solutions for mitigating global climate change.

Bacterial communities vary across populations and tissue type in red mangroves (Rhizophora mangle, Rhizophoraceae) along an expanding front

Plant-associated microbial communities may be important sources of functional diversity and genetic variation that influence host evolution. Bacteria provide benefits for their hosts, yet in most plant systems we know little about their taxonomic composition or variation across tissues and host range. Red Mangrove (Rhizophora mangle L.) is a vital coastal plant species that is currently expanding poleward and with it, perhaps, its microbiome. We explored variability in bacterial communities across tissues, individuals, and populations. We collected samples from six sample types from 5 to 10 individuals at each of three populations and used 16S rRNA gene (iTag) sequencing to describe their bacterial communities. Core community members and dominant bacterial classes were determined for each sample type. Pairwise PERMANOVA of Bray-Curtis dissimilarity and Indicator Species Analysis revealed significant differences in bacterial communities between sample types and populations. We described the previously unexplored microbiome of the reproductive tissues of R. mangle. Populations and most sample types were associated with distinct communities. Bacterial communities associated with R. mangle are influenced by host geography and sample type. Our study provides a foundation for future work exploring the functional roles of these microbes and their relevance to biogeochemical cycling.

Global hotspots of salt marsh change and carbon emissions

Salt marshes provide ecosystem services such as carbon sequestration1, coastal protection2, sea-level-rise (SLR) adaptation3 and recreation4. SLR5, storm events6, drainage7 and mangrove encroachment8 are known drivers of salt marsh loss. However, the global magnitude and location of changes in salt marsh extent remains uncertain. Here we conduct a global and systematic change analysis of Landsat satellite imagery from the years 2000-2019 to quantify the loss, gain and recovery of salt marsh ecosystems and then estimate the impact of these changes on blue carbon stocks. We show a net salt marsh loss globally, equivalent to an area double the size of Singapore (719 km2), with a loss rate of 0.28% year-1 from 2000 to 2019. Net global losses resulted in 16.3 (0.4-33.2, 90% confidence interval) Tg CO2e year-1 emissions from 2000 to 2019 and a 0.045 (-0.14-0.115) Tg CO2e year-1 reduction of carbon burial. Russia and the USA accounted for 64% of salt marsh losses, driven by hurricanes and coastal erosion. Our findings highlight the vulnerability of salt marsh systems to climatic changes such as SLR and intensification of storms and cyclones.

The paleo-ecological application of mollusks in the calculation of saltwater encroachment and resultant changes in depositional patterns driven by the Anthropocene Marine Transgression

Numerous studies address changes in wetland deposition in response to saltwater encroachment driven by the accelerating rate of sea-level rise, by quantifying temporal changes recovered from a vertical sediment sequence. This is the first landscape scale study, based upon 10 core transects representing the heterogeneity of the Southeast Saline Everglades, Florida. By utilizing the known salinity preferences of molluscan assemblages, a Salinity Index was calculated for each core sequence and the recorded salinity changes identified and dated. Radiometric dating utilizing the 210Pb method provides the rate of sediment accumulation and the date of changes identified in the core. The core transects provide the basis for calculation of the rate of saltwater encroachment by comparing the date of saltwater encroachment and the distance between two cores. Thereby, temporal and spatial changes in other sediment parameters in a landscape can also be quantified, such as organic carbon. This paleo-ecological approach to rapidly changing coastal conditions can be utilized to provide scientists and land managers with a record of the past, rate of changing conditions and provide the basis for predicting the future trajectory of their site. Application of this paleo-ecological approach documented increasing rates of saltwater encroachment associated with accelerating rate of sea-level rise: an average rate of 49.1 between 1895 and 1940, 69.2 between 1940 and 1968, 73 between 1968 and 1995 and 131.1 m/yr between 1995 and 2015. Approximately 1.79 km of saltwater encroachment has occurred since 1995, with three partial reversals because of increased freshwater delivery. Associated with saltwater encroachment are changes in sediment organic carbon, decreasing area of marl production and increasing distribution of mangrove. Although the distance of saltwater encroachment is greater in Florida Bay, both changes in sediment organic carbon and mangrove distribution are much less than in Biscayne Bay coastal basins. This heterogeneity is likely the result of differences in tidal ingress efficiency. At the present rate of saltwater encroachment, the freshwater wetlands are predictably lost within a century.

The lunar nodal cycle controls mangrove canopy cover on the Australian continent

Long-phase (interannual) tidal cycles have been shown to influence coastal flooding and sedimentation, but their role in shaping the extent and condition of tidal wetlands has received little attention. Here, we show that the 18.61-year lunar nodal cycle, popularly termed the "lunar wobble," is a dominant control over the expansion and contraction of mangrove canopy cover over much of the Australian continent. Furthermore, the contrasting phasing of the 18.61-year lunar nodal cycle between diurnal and semidiurnal tidal settings has mediated the severity of drought impacts in northern bioregions. Long-phase tidal cycles regulate maximum tide heights, are an important control over mangrove canopy cover, and may influence mangrove ecosystem services including forest productivity and carbon sequestration at regional scales.

Cascading effects of climate change on recreational marine flats fishes and fisheries

Tropical and subtropical coastal flats are shallow regions of the marine environment at the intersection of land and sea. These regions provide myriad ecological goods and services, including recreational fisheries focused on flats-inhabiting fishes such as bonefish, tarpon, and permit. The cascading effects of climate change have the potential to negatively impact coastal flats around the globe and to reduce their ecological and economic value. In this paper, we consider how the combined effects of climate change, including extremes in temperature and precipitation regimes, sea level rise, and changes in nutrient dynamics, are causing rapid and potentially permanent changes to the structure and function of tropical and subtropical flats ecosystems. We then apply the available science on recreationally targeted fishes to reveal how these changes can cascade through layers of biological organization-from individuals, to populations, to communities-and ultimately impact the coastal systems that depend on them. We identify critical gaps in knowledge related to the extent and severity of these effects, and how such gaps influence the effectiveness of conservation, management, policy, and grassroots stewardship efforts.

Hindcast-validated species distribution models reveal future vulnerabilities of mangroves and salt marsh species

Rapid climate change is threatening biodiversity via habitat loss, range shifts, increases in invasive species, novel species interactions, and other unforeseen changes. Coastal and estuarine species are especially vulnerable to the impacts of climate change due to sea level rise and may be severely impacted in the next several decades. Species distribution modeling can project the potential future distributions of species under scenarios of climate change using bioclimatic data and georeferenced occurrence data. However, models projecting suitable habitat into the future are impossible to ground truth. One solution is to develop species distribution models for the present and project them to periods in the recent past where distributions are known to test model performance before making projections into the future. Here, we develop models using abiotic environmental variables to quantify the current suitable habitat available to eight Neotropical coastal species: four mangrove species and four salt marsh species. Using a novel model validation approach that leverages newly available monthly climatic data from 1960 to 2018, we project these niche models into two time periods in the recent past (i.e., within the past half century) when either mangrove or salt marsh dominance was documented via other data sources. Models were hindcast-validated and then used to project the suitable habitat of all species at four time periods in the future under a model of climate change. For all future time periods, the projected suitable habitat of mangrove species decreased, and suitable habitat declined more severely in salt marsh species.

Warming northward shifting southern limits of the iconic temperate seagrass (Zostera marina)

Global warming can shift the range edges of numerous species poleward. Here, eelgrass distribution was reinvestigated at its southern limits on the eastern coast of China, which indicated that there has been a northward shift in the southern limit of Z. marina. To determine if regional warming resulted in a northward shift in suitable eelgrass habitats, sixteen transplantations of adult eelgrass shoots and seeds at the historical southern distribution limit of eelgrass were conducted between 2016 and 2021. The results showed that high water temperatures in summer had negative effects on eelgrass growth, and directly triggered shoot mortality during 2016–2018. Under heat stress, antioxidant enzyme activity was initially increased, but then decreased under more stressful heat conditions; and the HSP70 protein and its molecular chaperone protein were highly expressed under heat stress. These results demonstrated that suitable eelgrass habitat was now located further north along the eastern coast of China.

•

High temperatures trigger seagrass (Zostera marina L.) restoration failure

•

None seedlings and adult shoots survived the first or second summer

•

Over-summering shoots with lower density, height, and rhizome diameter

•

Warming northward shifting eelgrass habitat range along the eastern coast of China

Can tolerances of multiple stressors and calculated safety margins in fiddler crabs predict responses to extreme environmental conditions resulting from climate change?

To comprehend mangrove crab responses to predicted global climate changes, we assessed submersion and desiccation survival durations and salinity tolerances and upper thermal limits in fiddler crabs from Isla del Carmen, Yucatán Peninsula. Based on their tolerances of extreme ambient conditions, we also calculated safety margins using abiotic monitoring data. The two most terrestrial species, Minuca rapax and Leptuca panacea, exhibited submersion tolerances of from 22 to 40 h, and desiccation tolerances of from 30 to 55 h; LC₅₀'s were ≈45‰S and UT₅₀'s were ≈40 °C. The two least terrestrial species, M. vocator and L. speciosa, were less tolerant of all experimental challenges, showing submersion and desiccation tolerances of <6 h, and LC₅₀'s of 36‰S and UT₅₀'s of 38 °C. While these fiddler crabs inhabit niches closer to their salinity and desiccation/submersion tolerances than to their temperature limits, all are clearly vulnerable to the multiple stressors that accompany anticipated global climate change.

Early impacts of climate change on a coastal marine microbial mat ecosystem

Among the earliest consequences of climate change are extreme weather and rising sea levels-two challenges to which coastal environments are particularly vulnerable. Often found in coastal settings are microbial mats-complex, stratified microbial ecosystems that drive massive nutrient fluxes through biogeochemical cycles and have been important constituents of Earth's biosphere for eons. Little Ambergris Cay, in the Turks and Caicos Islands, supports extensive mats that vary sharply with relative water level. We characterized the microbial communities across this variation to understand better the emerging threat of sea level rise. In September 2017, the eyewall of category 5 Hurricane Irma transited the island. We monitored the impact and recovery from this devastating storm event. New mat growth proceeded rapidly, with patterns suggesting that storm perturbation may facilitate the adaptation of these ecosystems to changing sea level. Sulfur cycling, however, displayed hysteresis, stalling for >10 months after the hurricane and likely altering carbon storage potential.

High-resolution mapping of losses and gains of Earth's tidal wetlands

Tidal wetlands are expected to respond dynamically to global environmental change, but the extent to which wetland losses have been offset by gains remains poorly understood. We developed a global analysis of satellite data to simultaneously monitor change in three highly interconnected intertidal ecosystem types-tidal flats, tidal marshes, and mangroves-from 1999 to 2019. Globally, 13,700 square kilometers of tidal wetlands have been lost, but these have been substantially offset by gains of 9700 km², leading to a net change of -4000 km² over two decades. We found that 27% of these losses and gains were associated with direct human activities such as conversion to agriculture and restoration of lost wetlands. All other changes were attributed to indirect drivers, including the effects of coastal processes and climate change.

Quantification of blue carbon in tropical salt marshes and their role in climate change mitigation

Vegetated coastal ecosystems (VCE) display a promising potential to act as natural carbon sinks in climate change mitigation. Although growing interest in wetland carbon has intensified the global level carbon stock estimation studies, large knowledge gaps and uncertainties remain, particularly in tropical salt marshes in the South and Southeast Asian regions. Therefore, the current study aims to quantify the organic carbon stocks in the salt marsh habitats on the Northwest coast of Sri Lanka and to showcase the relevance of salt marsh carbon in local and regional contexts. Vegetation and soil up to a depth of 50 cm were sampled from four sites representing the Wedithalathive Nature Reserve (WNR). Species-specific allometric relationships developed for the major succulent halophytic species indicated a significant positive correlation between dry biomass and plant height. The loss-on-ignition (LOI) technique was applied in combination with a carbon conversion factor to calculate the soil organic carbon (SOC) content across 4 depth intervals. The study provided an average total organic carbon (TOC) storage of 73 ± 14.47 Mg C ha^-1 up to a depth of 50 cm, in which the aboveground vegetation accounted for ~2% share. Sri Lankan salt marshes hold 2.01 Tg of organic carbon and directly reflect their potential for inclusion in Nationally Determined Contributions (NDCs) under the Paris Agreement. This has been the first comprehensive study on salt marsh blue carbon stocks in Sri Lanka and the findings of this study will strengthen the knowledge base on regional and global saltmarsh carbon stocks and their potential role in climate change mitigation.

The impacts of mangrove range expansion on wetland ecosystem services in the southeastern United States: Current understanding, knowledge gaps, and emerging research needs

Climate change is transforming ecosystems and affecting ecosystem goods and services. Along the Gulf of Mexico and Atlantic coasts of the southeastern United States, the frequency and intensity of extreme freeze events greatly influence whether coastal wetlands are dominated by freeze-sensitive woody plants (mangrove forests) or freeze-tolerant grass-like plants (salt marshes). In response to warming winters, mangroves have been expanding and displacing salt marshes at varying degrees of severity in parts of north Florida, Louisiana, and Texas. As winter warming accelerates, mangrove range expansion is expected to increasingly modify wetland ecosystem structure and function. Because there are differences in the ecological and societal benefits that salt marshes and mangroves provide, coastal environmental managers are challenged to anticipate the effects of mangrove expansion on critical wetland ecosystem services, including those related to carbon sequestration, wildlife habitat, storm protection, erosion reduction, water purification, fisheries support, and recreation. Mangrove range expansion may also affect wetland stability in the face of extreme climatic events and rising sea levels. Here, we review the current understanding of the effects of mangrove range expansion and displacement of salt marshes on wetland ecosystem services in the southeastern United States. We also identify critical knowledge gaps and emerging research needs regarding the ecological and societal implications of salt marsh displacement by expanding mangrove forests. One consistent theme throughout our review is that there are ecological trade-offs for consideration by coastal managers. Mangrove expansion and marsh displacement can produce beneficial changes in some ecosystem services, while simultaneously producing detrimental changes in other services. Thus, there can be local-scale differences in perceptions of the impacts of mangrove expansion into salt marshes. For very specific local reasons, some individuals may see mangrove expansion as a positive change to be embraced, while others may see mangrove expansion as a negative change to be constrained.

Vinegar residue biochar: A possible conditioner for the safe remediation of alkaline Pb-contaminated soil

A better understanding how to modulate alkaline soil-plant systems with lead (Pb) toxicity with by vinegar residue biochar is important for the remediation of Pb-contaminated soil. Leaching column and pot experiments were conducted to investigate the effect of vinegar residue biochar on Pb speciation, soil properties, and plant growth under Pb stress. The results indicate that biochar could effectively decrease the exchangeable and carbonated-bound Pb but increase the Fe-Mn oxide and residue fractions in the soil with Pb at 500 mg kg^-1. Biochar did not effectively immobilize Pb in the soil with Pb at 1000 mg kg^-1. After leaching, biochar evidently increased the organic carbon and dissolved organic carbon content of the soil, but slightly affected the pH, cation exchange capacity and carbonate content. The biochar addition at 0.5% had no significant effect on soil aggregates, and biochar at 2.0% and 5.0% significantly decreased soil aggregate stability. The dry weight and soluble protein content of pak choi (Brassica chinensis L.) increased with biochar treatment. Lead assimilation by plants was inhibited by the decreased availability of Pb in biochar-treated soils. Soil enzymes activities also significantly increased, then facilitated biochemical reactions in the soil environment. The applied biochar has shown an important role in mitigating Pb toxicity by increasing the soil organic carbon, dissolved organic carbon content, enzyme activities, and plant growth. The low dose biochar (0.5-2.0%) are recommended as references for subsequent experiments, especially in alkaline loam soil.

Differential Adaptive Potential and Vulnerability to Climate-Driven Habitat Loss in Brazilian Mangroves

Geographic and environmental differences have been identified as factors influencing Brazilian mangrove trees' genetic diversity. Geographically, distinct species have convergent spatial genetic structures, indicating a limited gene flow between northern and southern populations. Environmentally, genomic studies and common garden experiments have found evidence of local adaptations along the latitudinal gradient of the Brazilian coast. However, little is known about how such adaptive heterogeneity could be affected by a rapidly changing climate in the coming decades, and the combination of deforestation and climate-induced habitat loss may affect these forests and their genetic diversity. Here, we applied two genomic-environmental association methods to model the turnover of potentially adaptive alleles for two dominant mangrove trees: Avicennia germinans and A. schaueriana. We analyzed a total of 134 individuals from six populations of A. germinans and 10 populations of A. schaueriana spanning the Brazilian coast from 1 °S to 28 °S. Gradient forest models identified temperature-related variables as the most important predictors for A. germinans outlier loci, whereas both temperature and precipitation were important for A. schaueriana. We modeled allele frequencies and projected them for future climatic scenarios to estimate adaptively driven vulnerability. We assessed climate-driven habitat loss through climate-only distribution models and calculated annual deforestation rates for each sampled region. Finally, to assess the vulnerability of individual populations, we combined the environmental suitability, deforestation data, and adaptive vulnerability projections. For both species, subtropical populations presented a higher vulnerability than equatorial populations to climate-driven habitat loss. We also identified deforestation rates at the sampled sites that were alarmingly higher than the global average mangrove deforestation rate. Our results provide improved estimates of the impacts of ongoing climate change and human-caused habitat loss on the distribution of mangroves and highlight the importance of site-based conservation strategies that consider individual subtropical and equatorial mangrove forests.

An emerging coastal wetland management dilemma between mangrove expansion and shorebird conservation

Coastal wetlands around the world have been degraded by human activities. Global declines in the extent of important habitats including mangroves, salt marsh and tidal flats necessitate mitigation and restoration efforts, however some well-meaning management actions, particularly mangrove afforestation and breakwater construction, can inadvertently cause further loss and degradation if these actions are not planned carefully. In particular, there is a potential conflict between mangrove and shorebird conservation, because mangrove afforestation and restoration may occur at the expense of bare tidal flats, which form the main foraging habitats for threatened coastal migratory shorebirds as well as supporting other coastal organisms. Here, we present several case studies that illustrate the trade-off between mangroves and bare tidal flats. To investigate whether these examples reflect an emerging broad-scale issue, we use satellite imagery to develop a detailed quantification of the change in mangrove habitat extent in 22 important shorebird areas in mainland China between 2000 and 2015. Our results indicate that 1) the extent of mangroves across all sites expanded significantly between 2000 and 2015 (p < 0.01, n = 14) while tidal flat extent in the same areas declined significantly within the same period (p < 0.01, n = 21); 2) among the 14 sites where mangroves were present, the dual threat of mangrove expansion and tidal flat loss have considerably reduced shorebird habitat in eight of these sites. To ensure effective conservation of both mangroves and shorebirds, we propose a decision tree framework for resolving this emerging dilemma between mangrove afforestation and shorebird protection, which requires careful consideration of alternative management strategies. This article is protected by copyright. All rights reserved.

A hurricane alters the relationship between mangrove cover and marine subsidies

As global change alters the composition and productivity of ecosystems, the importance of subsidies from one habitat to another may change. We experimentally manipulated black mangrove (Avicennia germinans) cover in ten large plots and over five years (2014-2019) quantifying the effects of mangrove cover on subsidies of floating organic material (wrack) into coastal wetlands. As mangrove cover increased from zero to 100%, wrack cover and thickness decreased by ~60%, the distance that wrack penetrated into the plots decreased by ~70%, and the percentage of the wrack trapped in the first six m of the plot tripled. These patterns observed during four "normal" years disappeared in a fifth year following Hurricane Harvey (2017), when large quantities of wrack were pushed far into the interior of all the plots, regardless of mangrove cover. Prior to the storm, the abundance of animals collected in grab samples increased with wrack biomass. Wrack composition did not affect animal abundance or composition. Experimental outplants of two types of wrack (red algae and seagrass) revealed that animal abundance and species composition varied between the fringe and interior of the plots, and between microhabitats dominated by salt marsh versus mangrove vegetation. The importance of wrack to overall carbon stocks varied as a function of autochthonous productivity: wrack inputs (per m² ) based on survey data were greater than aboveground plant biomass in the plots (42 × 24 m) dominated by salt marsh vegetation, but decreased to 5% of total aboveground biomass in plots dominated by mangroves. Our results illustrate that increasing mangrove cover decreases the relative importance of marine subsidies into the intertidal at the plot level, but concentrates subsidies at the front edge of the mangrove stand. Storms, however, may temporarily override mangrove attenuation of wrack inputs. Our results highlight the importance of understanding how changes in plant species composition due to global change will impact marine subsidies and exchanges among ecosystems, and foster a broader understanding of the functional interdependence of adjacent habitats within coastal ecosystems.

The Opposite of Biotic Resistance: Herbivory and Competition Suppress Regeneration of Native but Not Introduced Mangroves in Southern China

Mangrove forests are increasingly threatened by plant invasions worldwide, but some mangrove species are invasive and threaten salt marsh and native mangrove ecosystems. The southern coast of China is invaded by the cordgrass Spartina alterniflora, and the mangrove Sonneratia apetala, providing a model system for studying the processes and mechanisms through which non-native species establish and spread. We used a transplant experiment to test the overarching hypothesis that native herbivores and plants provided biotic resistance against invasion by S. apetala, and that the importance of these factors would vary geographically. Survival of transplanted mangrove seedlings was lower in Zhangjiang Estuary (23°55′ N) than in Leizhou Bay (20°56′ N), and varied with species and habitats. S. apetala had higher survival and growth rates than native mangroves at both sites, and S. apetala grew taller than the S. alterniflora canopy at Leizhou Bay in only two growing seasons. In contrast, native mangroves grew poorly in S. alterniflora. Grazing by rodents and insects suppressed the growth and survival of Kandelia obovata and Avicennia marina in Zhangjiang Estuary and Leizhou Bay, but had little effect on S. apetala. Competition with vegetation (S. alterniflora and native mangroves) exacerbated the reduced survival of native mangroves, and these effects varied across study sites. Low survival of non-native S. apetala in vegetated habitats at Zhangjiang Estuary was likely due to a synergistic effect of low winter temperatures and low light intensity. Escape from herbivory (the opposite to biotic resistance) and fast growth may drive the quick expansion of non-native S. apetala in China. Rapid encroachment of S. apetala may transform the native mangrove forests and monospecific intertidal Spartina grasslands into non-native mangrove forests in the southern coast of China.

Modeling the Climate Suitability of Northernmost Mangroves in China under Climate Change Scenarios

Mangroves are important wetland ecosystems on tropical and subtropical coasts. There is an urgent need to better understand how the spatial distribution of mangroves varies with climate change factors. Species distribution models can be used to reveal the spatial change of mangroves; however, global models typically have a horizontal resolution of hundreds of kilometers and more than 1 km, even after downscaling. In the present study, a maximum entropy model was used to predict suitable areas for the northernmost mangroves in China in the 2050s. An approach was proposed to improve the resolution and credibility of suitability predictions by incorporating land-use potential. Predictions were made based on two CMIP6 scenarios (i.e., SSP1-2.6 and SSP5-8.5). The results show that the northern edge of the natural mangrove distribution in China would migrate from 27.20° N to 27.39° N–28.15° N, and the total extent of suitable mangrove habitats would expand. By integrating 30 m resolution land-use data to refine the model’s predictions, under the SSP1-2.6 scenario, the suitable habitats of mangroves are predicted to be 13,435 ha, which would increase by 33.9% compared with the current scenario. Under the SSP5-8.5 scenario, the suitable area would be 23,120 ha, with an increased rate of 96.5%. Approximately 40–44% of the simulated mangrove patches would be adjacent to aquacultural ponds, cultivated, and artificial land, which may restrict mangrove expansion. Collectively, our results showed how climate change and land use could influence mangrove distributions, providing a scientific basis for adaptive mangrove habitat management despite climate change.

A 1900 Year Sediment Record Suggests Recent Establishment of Black Mangrove (Avicennia Germinans) Stands within a Salt Marsh in St. Augustine, Florida, USA

Salt marshes and mangroves are currently being affected by rising temperatures. Mangroves thrive below −29° N latitude in Florida, USA, and have a low tolerance for extreme cold events, whereas salt marshes dominate further north. One potential effect of climate change is a reduction in the frequency of extreme cold events, which may lead to mangrove expansion into salt marsh systems. Our research identified sediment proxy indicators of salt marsh and mangrove environments. These indicators were applied to soil cores from intertidal wetlands near the current northern limit of mangrove presence on the east coast of Florida, to determine if mangrove expansion into salt marsh environments has precedence in the deeper past. Our findings suggest that mangrove and salt marsh sediments can be distinguished using a combination of stable carbon isotope ratios of sedimentary organic matter and macroscopic plant fragments, and our results showed that a mangrove stand that we cored established only recently. This result is consistent with other work in the southeastern United States that suggests that mangroves established at the current boreal limit only recently after the end of the Little Ice Age, and that the current mangrove expansion may be fueled by anthropogenic climate change.

Salinity exacerbates oil contamination effects in mangroves

The effects of salinity (10 and 50% seawater) and oil in combination on three mangroves, Avicennia marina, Bruguiera gymnorrhiza, and Rhizophora mucronata, were investigated. In all species, plant height, number of leaves, and CO₂ exchange were generally higher in 10% than in 50% seawater. Salinity and oil decreased plant height, number of leaves, chlorophyll content, and CO₂ exchange, with reductions being greater at the higher salinity. In a second experiment, the effects of salinity (0, 10, and 50% seawater) and oil on concentrations of ions, polycyclic aromatic hydrocarbons (PAHs), leaf ultrastructure, and salt secretion in A. marina were investigated. Salinity and oil in combination increased concentrations of Na⁺ but decreased those of K⁺, Ca²⁺, and Mg²⁺. PAHs caused damage to cell membranes, disrupted ion concentrations, and reduced salt secretion. This study demonstrated that increase in salinity reduces growth of mangroves and that salinity and oiling in combination exacerbate growth reduction. In A. marina, oil was absorbed and translocated to the leaves where it disrupted membranes, ion accumulation, and salt secretion.

Carbon and nitrogen pools and mobile fractions in surface soils across a mangrove saltmarsh ecotone

In the subtropics, climate change is pushing woody mangrove forests into herbaceous saltmarshes, altering soil carbon (C) and nitrogen (N) pools, with implications for coastal wetland productivity and C and N exports. We quantified total C and N pools, and mobile fractions including extractable mineral N, extractable organic C and N, and active (aerobically mineralizable) C and N, in surface soils (top 7.6 cm) of adjacent mangrove (primarily Avicennia germinans) and saltmarsh (Juncus roemerianus) vegetation zones in tidal wetlands of west-central Florida (USA). We tested whether surface-soil accumulations of C, N, and their potentially mobile fractions are greater in mangrove than in saltmarsh owing to greater accumulations in the mangrove zone of soil organic matter (SOM) and fine mineral particles (C- and N-retaining soil constituents). Extractable organic fractions were 39-45% more concentrated in mangrove than in saltmarsh surface soil, and they scaled steeply and positively with SOM and fine mineral particle (silt + clay) concentrations, which themselves were likewise greater in mangrove soil. Elevation may drive this linkage. Mangrove locations were generally at lower elevations, which tended to have greater fine particle content in the surface soil. Active C and extractable mineral N were marginally (p < 0.1) greater in mangrove soil, while active N, total N, and total C showed no statistical differences between zones. Extractable organic C and N fractions composed greater shares of total C and N pools in mangrove than in saltmarsh surface soils, which is meaningful for ecosystem function, as persistent leaching of this fraction can perpetuate nutrient limitation. The active (mineralizable) C and N fractions we observed constituted a relatively small component of total C and N pools, suggesting that mangrove surface soils may export less C and N than would be expected from their large total C and N pools.