Showing papers in "Estuarine Coastal and Shelf Science in 2021"

The renaissance of Odum's outwelling hypothesis in 'Blue Carbon' science

Abstract: The term ‘Blue Carbon’ was coined about a decade ago to highlight the important carbon sequestration capacity of coastal vegetated ecosystems. The term has paved the way for the development of programs and policies that preserve and restore these threatened coastal ecosystems for climate change mitigation. Blue carbon research has focused on quantifying carbon stocks and burial rates in sediments or accumulating as biomass. This focus on habitat-bound carbon led us to losing sight of the mobile blue carbon fraction. Oceans, the largest active reservoir of carbon, have become somewhat of a blind spot. Multiple recent investigations have revealed high outwelling (i.e., lateral fluxes or horizontal exports) of dissolved inorganic (DIC) and organic (DOC) carbon, as well as particulate organic carbon (POC) from blue carbon habitats. In this paper, we conceptualize outwelling in mangrove, saltmarsh, seagrass and macroalgae ecosystems, diagnose key challenges preventing robust quantification, and pave the way for future work integrating mobile carbon in the blue carbon framework. Outwelling in mangroves and saltmarshes is usually dominated by DIC (mostly as bicarbonate), while POC seems to be the major carbon species exported from seagrass meadows and macroalgae forests. Carbon outwelling science is still in its infancy, and estimates remain limited spatially and temporally. Nevertheless, the existing datasets imply that carbon outwelling followed by ocean storage is relevant and may exceed local sediment burial as a long-term (>centuries) blue carbon sequestration mechanism. If this proves correct as more data emerge, ignoring carbon outwelling may underestimate the perceived sequestration capacity of blue carbon ecosystems.

Mangroves in arid regions: Ecology, threats, and opportunities

Abstract: Mangroves are one of the few woody ecosystems that grow in hot-arid climates. They can survive extreme conditions of low precipitation, high solar radiation, wide temperature fluctuations and hypersalinity. These unique mangroves have distinct geomorphology, hydrology, forest structure, tree physiology, and soil biogeochemistry. In this review, supported by field data from Australia and Mexico, we explore the characteristics of mangroves in arid climates of the world. These mangroves are mostly tide-dominated with freshwater flows restricted to groundwater and sporadic tropical storms. They form dense forests with stunted growth dominated mainly by trees of the genus Avicennia that co-occur with salt marshes in the high intertidal. Their soils have low nutrient and carbon concentrations, and high soil δ15 N and δ13C values compared to subhumid and humid mangroves. Mangroves in arid climates have relatively low human pressure due to sparse human settlements. Key threats to these mangroves, which often persist at the edge of their physiological tolerances, include extreme drought, reductions in groundwater inputs, altered hydrology, sea-level fluctuations and increases in nutrient loading. Restoration of mangroves in arid climates should focus on restoring their hydrology. Mangroves in arid zones are under-represented in global maps and assessment programs, as they may not be consistent with countries’ definition of “forests”. Improved global representation and understanding of the ecology of mangroves in arid climates could help sustain their valuable ecosystem services.

Long-term changes in red tide outbreaks in Xiamen Bay in China from 1986 to 2017

Abstract: Data on red tide (harmful algae blooms, HABs) from 1986 to 2017 in Xiamen Bay of Fujian province in China was collected. In these 30 years, a total of 53 red tide events were recorded, which mostly happened in West sea (47.3%) and Tong'an Bay (40.5%). And, red tide areas in Xiamen Bay have expanded, with the most prone area changing from West sea to Tong'an Bay. Furthermore, red tide focused on late spring and summer, including the months of May, June and July, with the percentage of occurrence of 13.8%, 26.2% and 27.7%, respectively. By contrast, red tide seldom occurred in autumn. Skeletonema costatum, Chaetoceros curvisetus and Akashiwo sanguinea were dominant red tide species in the bay responsible for red tide events on 24, 8 and 6 occasions so far, respectively. Red tide events caused by Dinoflagellates have been increasing in these 30 years. While red tide events caused by Diatom decreased in 2010s obviously compared to those in 2000s. It was suggested that changes of eutrophication degree and nutrient ratios, climate change, and sediment property maybe important factors to the long-term changes in red tide outbreaks in Xiamen Bay.

Reconciling nature, people and policy in the mangrove social-ecological system through the adaptive cycle heuristic

Abstract: While mangroves are increasingly described as social-ecological systems (SESs), performing SES research is so much more than merely documenting local resource utilisation patterns in case studies. The aim of this paper is to review and show how ecological, human and institutional resilience could be understood and fostered in an era of uncertainty, through the adaptive cycle (AC) heuristic. Uncertainties come in many forms and shapes: climate change, social and economic dynamics, natural disasters, political and institutional disruption and ever-increasing public demands for participation. Social-ecological studies form windows of experimentation that can provide insights beyond their case-specific context. In order to synthesise and structure the cumulative knowledge base arising from existing and future studies, the need for a suitable overarching framework arose. Here, the AC heuristic represents the connectedness between variables of the mangrove SES versus the mangrove's accumulated capital (natural, built, human and social). We posit that the AC heuristic can be used to interpret spatial and temporal changes (ecological, social, economic, political) in mangrove SESs and we exemplify it by using the 2004 Indian Ocean tsunami as well as a century-long silviculture case. The AC, combined with the SES scheme, allows integration of the spato-temporal dynamics and the multi-dimensional character of mangrove SESs. We also reviewed the ecosystem functions, services and disservices of mangrove SESs, linking each of them to SES capital and variable (fast or slow) attributes, which in turn are closely linked to the different axes and phases of the AC. We call upon mangrove scientists from the natural, applied, social and human sciences to join forces in fitting diversified empirical data from multiple case studies around the world to the AC heuristic. The aim is to reflect on and understand such complex dynamic systems with stakeholders having various (mutual) relationships at risk of breaking down, and to prepare for interactive adaptive planning for mangrove forests.

Limited dispersal of riverine litter onto nearby beaches during rainfall events

Abstract: Rivers are assumed to be one of the major conduits for plastics entering the sea, but little is known about the interchange of plastic litter between rivers and coastal waters. We measured the amount of litter stranding on the beach around the mouth of a seasonal river in Cape Town, South Africa, to infer litter dispersal distances. Prior to the mouth opening, litter was fairly uniformly distributed along the 2.4 km study beach. Once the mouth was opened for the first major winter storm, most litter stranded within 0.5 km of the river mouth. Beach litter loads peaked during rain events and decreased after the first rainfall event, presumably due to flushing of accumulated litter in the coastal wetland. However, litter loads within the wetland remained high, indicating that only a proportion of litter flushes during floods. Experimental deployments of marked litter items confirmed that most floating litter strands close to the mouth, although less buoyant items travelled farther than more buoyant items. Exchanges occurred in both directions, with marine litter found up to 1.2 km inland in the estuary. On a rising tide, more litter items moved into the estuary than into the sea, despite strong offshore winds. Our results illustrate the complex interplay of litter between rivers and the sea, and suggest that a large proportion of floating litter carried down small rivers washes ashore shortly after entering the sea. Cleaning beaches around river mouths, especially after rain events, will help to reduce leakage of plastic from land-based sources into the sea.

Impact of wood harvesting on mangrove forest structure, composition and biomass dynamics in India

Abstract: Mangrove forest are used widely for meeting fuelwood needs of local communities in the tropics. However, the implications of fuelwood extraction on mangrove ecology has received very little attention. In this study, we investigated the impact of such small-scale fuelwood harvesting practices on mangrove forest structure, composition, regeneration, and biomass and carbon stocks in a protected area on the east coast of India. We sampled vegetation plots in areas where harvesting was practised and where no harvesting was observed. We found that species composition differed across forest categories with Exoecaria aggalocha and Heriteria fomes dominating non-harvested forest types and Sonneratia apetela, Sonneratia caseolaris, Avicennia officinalis along with mangrove associates comprising a large proportion of harvested mangrove forests. Similarly, tree, sapling and seedling densities, basal area, and biomass and carbon stocks were also found to vary across forest categories, with overall lower values observed in harvested areas. Lastly, total aboveground (11%) and belowground (18%) biomass and carbon stocks were also lower in harvested forests than non-harvested forests. However, large DBH size-class trees were found in harvested areas implying selective cutting practices that conserved old-growth trees and contributed to larger mean carbon pools. As harvesting is selective, mainly for cooking fuel demands, integrated management plans are required to accommodate local cultural and economic needs of the communities in this region.

Geo-ecological impact assessment of severe cyclonic storm Amphan on Sundarban mangrove forest using geospatial technology

Abstract: We report the damage caused by Amphan, a category five tropical cyclonic storm, to the ecology and shoreline of the Sundarban mangrove ecosystem, located in India and Bangladesh. In this study, net shoreline movement (NSM) combined with vegetation indices, such as normalized difference vegetation index (NDVI) and enhanced vegetation index (EVI), were used to analyze land use and land cover changes and the position of the shoreline in pre- and post-cyclone periods. The results show that the cyclone wreaked havoc in the Sundarban mangrove ecosystem, causing dangerous impacts on vegetation and severe erosion along the shoreline. The impact on the Sundarban mangrove forest, shown by NDVI and EVI values, indicates that the mangrove vegetation suffered degradation and fragmentation. Dense and moderately dense mangrove tree cover area shrank from 77% to 34%, less dense and sparse cover increased from 20% to 63%. More than 68% of shoreline transects registered signs of erosion, with mean shoreline change and erosion rate measurements of −31 m and −48 m, respectively. As a result, agricultural lands and aquaculture ponds suffered inundation, leading to crop damage. A single cyclone episode can damage mangrove cover by knocking down tall trees, killing more salt-sensitive mangrove species such as Sundari, and leads to increased man-animal encounter. We argue that post-cyclone biodiversity loss and ecosystem services loss should be analyzed in tandem and synchrony with human and livestock casualty and economic damage assessment.

Hydrochemical characteristics and functions of groundwater in southern Laizhou Bay based on the multivariate statistical analysis approach

Abstract: Influenced by paleoclimate, paleogeography and hydrogeology, groundwater with different total dissolved solids (TDS) levels, such as freshwater, brackish water, saline water and brine, widely occurs across southern Laizhou Bay (SLB) . Due to groundwater overexploitation, several groundwater depression cones have gradually formed since the 1970s, resulting in saline water intrusion. In this study, the multivariate statistical analysis (MSA) approach was adopted to identify the hydrochemical characteristics of shallow groundwater (SG) and deep groundwater (DG) in the SLB. Moreover, we applied Gibbs diagrams, hydrochemical facies evolution diagrams (HFE-Diagram) and ion proportion relations to analyze the factors influencing the hydrochemical characteristics. The results showed that hydrochemical zonation of the various water types (ranging from Ca-HCO3 to Na-Cl) from south to north was mainly controlled by migration and mixing of saline water with regional groundwater. Based on the MSA results and hydrochemical characteristics, SG functional areas were divided into the groundwater flow zone, intense groundwater mixing zone and groundwater-seawater interaction zone, and DG functional areas were divided into the groundwater flow zone, groundwater mixing zone and underground brine storage zone. Finally, SG and DG groundwater flow evolution models were developed . The comprehensive method proposed in this study combines the MSA approach with regional geological characteristics and provides a reference for the evaluation of complex multiaquifer systems and analysis of the groundwater evolution characteristics in other regions.

The first operations of Mo.S.E. system to prevent the flooding of Venice: Insights on the hydrodynamics of a regulated lagoon

Abstract: In October 2020, for the first time in its thousand-year-old history, the Venice Lagoon has been temporarily closed. The first operations of the Mo.S.E. system, a set of artificial barriers built to isolate the lagoon from the sea in case of high tides, prevented Venice and the other lagoonal settlements from flooding. Beyond its historical value, the closure of the lagoon inlets has led to unprecedented scenarios from a hydrodynamic standpoint. With the Mo.S.E. system operational, significant high tides can no longer be recorded within the lagoon and the undisturbed tide propagation can only be estimated through hydrodynamic modelling. When the inlets are closed and the effect of tide propagation nullified, the action of wind on cross-lagoon setup is enhanced and becomes more clearly recognizable, allowing for a robust calibration of the wind drag coefficient also for low to moderate wind speed. Furthermore, the data collected during the first closures of the Mo.S.E. gates allowed evaluating the real intra-gate infiltration entering the lagoon through the closed gates, and suggested that the gate operation produces some seaward disturbance as well.

Potential short wave attenuation function of disturbed mangroves

Abstract: Mangroves are able to protect coastal communities through their ability to attenuate incoming long and short waves. Evidence of wave attenuation function and the factors contributing to wave attenuation by mangroves are now well established, especially for undisturbed mangrove stands. However, as tropical coastlines continue to urbanize rapidly, there is interest in understanding the ability of mangroves in wave attenuation along such disturbed coastlines. This study models the potential wave attenuation capacity of disturbed mangroves along the urban coastline of Singapore. Short wave attenuation is estimated under both average and storm (elevated water level) conditions. The percentage of wave height reduction is higher under storm events compared to average conditions. Vegetation drag is the main mechanism of wave energy dissipation under both average and storm conditions, with additional wave dissipation caused by wave breaking under the latter. Mangrove density and width were found to be positively correlated to the percentage of wave height reduction during a storm event. Compared to trunks and canopies, mangrove roots contributed to a larger percentage of wave height reduction. No statistical differences in wave height reduction extent were found between mangrove types, incident wave heights, and water levels respectively. This study has illustrated the potential for the attenuation of short waves by disturbed mangroves, especially during elevated water levels associated with storm events. The findings imply the potential of disturbed mangroves in wave attenuation, and should encourage the stronger incorporation of mangroves into coastal management strategies designed to protect urban communities against coastal hazards.

A cloud computing-based approach to mapping mangrove erosion and progradation: Case studies from the Sundarbans and French Guiana

Abstract: Erosion and progradation are natural dynamics that take place across mangrove landscapes, but anthropogenic pressure may disturb these dynamics and cause increased erosion. The increased erosion of mangrove coastlines leads to mangrove degradation and loss of ecosystem services. However, to map the extent of erosion and progradation in dynamic landscapes is often time and data consuming and challenging to perform at a large scale. This study uses cloud computing-based remote sensing analysis in Google Earth Engine to map the extent of mangrove shoreline dynamics in two contrasting hotspots: the Sundarbans mangrove complex across India and Bangladesh, and the coast of French Guiana. Mapping was performed between 1984 and 2018 using available data on mangrove distribution and surface water change, classifying change of states as permanent, seasonal and ephemeral. Between 1984 and 2018, erosion and progradation accounted for 24.55% and 12.52% of total change, respectively. Contrastingly, in French Guiana erosion and progradation accounted for 9.53% and 4.48% of total change, respectively. The Sundarbans experienced more permanent changes as compared to French Guiana. The permanent loss of mangrove forests in the Sundarbans can be attributed to the reduction in sediment supply and the temporary loss in French Guiana to cyclical patterns of migrating mudbanks. The methods presented in this study performed with an overall accuracy above 90% in different settings of both the case studies. This study provides an important baseline of mangrove erosion and progradation extent and an improved method to analyse coastal dynamics in mangrove landscapes.

A place for subterranean estuaries in the coastal zone

Abstract: Willard S. Moore coined the term subterranean estuary (STE) as that part of the coastal aquifer ‘where groundwater derived from land drainage measurably dilutes seawater that has invaded the aquifer through a free connection to the sea’ more than two decades ago (Moore, 1999). In doing so, Moore was calling attention to the potential importance of this zone, long studied by terrestrial hydrologists, to marine systems. Since then, we have accumulated indisputable evidence of the importance of STEs and their associated submarine groundwater discharge (SGD) in coastal ecosystem function. Yet, subterranean estuaries are often not considered distinct systems in the land-ocean continuum either from an academic viewpoint or for developing integrative monitoring and management. This invisibility means that the impact of STEs on ecosystem state has been neglected when environmental assessments that aim to meet coastal water quality and ecosystem health targets are performed. It is possible that our current broad definition of STEs, as well as misuse and misunderstanding of terminology, partly explain their de-facto transparency to the wider coastal scientific community. Here, we argue for the inclusion of STEs as important, interconnected systems driving water quality, ecology, and biogeochemical cycles in nearby coastal ecosystems. To achieve broader recognition, we first need to address the perceived compartmentalization of surface and groundwater systems, as well as overcome a pigeon-holed approach to research often confined within narrow disciplinary boundaries. The establishment of open-water estuarine science over the last several decades shows that such obstacles are surmountable. A lack of awareness and recognition of STE functions impedes a holistic understanding of coastal ecosystems and slows down the development of cause-effect relationships for a science-based approach to ecosystem management.

Dilkamural: A novel chemical weapon involved in the invasive capacity of the alga Rugulopteryx okamurae in the Strait of Gibraltar

Abstract: The southwestern coasts of Europe (Strait of Gibraltar) are experiencing a severe invasion of the brown alga Rugulopteryx okamurae, original from the northwestern Pacific ocean. Currently there is no clue regarding to the reasons of such huge invasive potential, although the involvement of chemical defenses has recently been suggested. In this context, this study was aimed to investigate the presence and potential role of chemical defenses in the invasive success of R. okamurae. The chemical study of R. okamurae from the Strait of Gibraltar led to the isolation of six secondary metabolites, among which the compound dilkamural stands out because of its high concentration. Later, in a set of feeding deterrent assays, the generalist native herbivore Paracentrotus lividus showed higher consumption over the native alga Ulva sp. than over the non-native R. okamurae. This low consumption was tracked down to dilkamural, which displayed not only deterrent properties but also caused harmful and even lethal effects over the sea urchins. These results are consistent with the novel weapons hypothesis, since dilkamural was not described previously in the invaded area and has a defensive role against generalist herbivores in the new range, thus helping to explain the great expansion of R. okamurae in the Strait of Gibraltar.

Seasonal phosphorus variation in coastal water affected by the land-based sources input in the eutrophic Zhanjiang Bay, China

Abstract: Phosphorus (P), a significant biogenic element, plays a key role in the primary productivity of estuarine and coastal ecosystems. The study determined that the seasonal dynamics of total dissolved P (TDP)in the coastal area of the Zhanjiang Bay (ZJB) were largely affected by land-based sources. The TDP concentration had significant seasonal variations in the ZJB (P

Evaluating mangrove conservation and sustainability through spatiotemporal (1990–2020) mangrove cover change analysis in Pakistan

Abstract: This study provides the first comprehensive mangrove cover change assessment from 1990 to 2020, at five-year intervals, across all five mangrove areas in Pakistan, i.e. Indus Delta, Sandspit, Sonmiani, Kalmat Khor, and Jiwani. Using the Google Earth Engine (GEE) geospatial cloud computing platform, Random Forest (RF) classifier was applied on Landsat 30 m spatial resolution satellite images to classify three major land cover classes: ‘mangrove’, ‘water’ and ‘other’. High temporal and spectral resolutions of Landsat images, with a low saturation level of spectral bands with the integration of indices, are the main factors that ensured >90% overall accuracy of land cover maps. Over the last three decades (1990–2020), the annual rate of change calculation, cross-tabulated method, and fragmentation analysis were carried out to identify the changes in mangrove cover. Overall, the findings of this paper revealed that, at the national scale, an estimated 477.22 km2 was covered with mangrove in 1990, which increased to 1463.59 km2 in 2020, a 3.74% annual rate of change. Mangrove fragmentation mapping results have also showed enhancement in mangrove tree canopy density. Due to planting and effective conservation practices, the current study shows positive changes in mangrove cover across all five study sites. The findings of this study will prove useful for design and implementation of mangrove ecosystem management plans, initiatives for adaptation to extreme weather events, carbon budgeting, and others.

Saline intrusion in the Ganges-Brahmaputra-Meghna megadelta

Abstract: In the fertile Ganges-Brahmaputra-Meghna (GBM) delta, which spans the boundary from West Bengal in India and Bangladesh, the availability of freshwater is crucial to subsistence livelihoods and protected ecosystems. Controlled by large tides and widely variable river discharge, the delta experiences rising river salinity and salt intrusion, as well as seasonal flooding during the monsoon. Future climate change is projected to increase rainfall in South Asia and river discharge in the GBM system. We address how this process might combine with sea-level rise (SLR) to control future river salinity. Model experiments designed using a range of SLR and climate change scenarios are performed to investigate the forces controlling river salinity in the delta. A flexible mesh modelling approach allows us to investigate the impacts at a wide range of time and space scales. In future projections the disparity between wet and dry season salt intrusion intensifies. In the future, SLR acts to increase river salinity in the GBM delta. During the dry season, this effect is worsened by reduced river discharge. In the wet season, this can be mitigated in the eastern part of the delta by larger seasonal river flows. The central and western delta is dominated by SLR, leading to increased salt intrusion all year round, impacting on water resources and agricultural productivity. In the context of an intensifying hydrological cycle, these conclusions have implications for similar tide-dominated deltas, where SLR can increase tidal range, and therefore exacerbate salt intrusion.

Structural characteristics of crab burrows in Hong Kong mangrove forests and their role in ecosystem engineering

Abstract: Crab burrowing activity impacts several important biogeochemical processes within mangrove forests. Burrow morphology has been shown to be variable between brachyuran families, hence the community structure and composition of burrowing crabs may have the potential to impact ecosystems differently. By increasing belowground surface area, volume and bacterial microhabitats, large and complex burrows may differ from smaller and simpler burrows in their impact to nutrient and organic matter distribution. The central aim of this work was to provide a detailed quantification of the scale to which different mangrove crab families impact Hong Kong mangrove ecosystem through their burrowing activities. To achieve this goal, we recorded burrow densities and described specie-specific burrow morphology, through casts and 3D scans, of eight crab species belonging to four families, across three mangrove forests. Where present, the large and complex burrows of Parasesarma bidens (Sesarmidae) increased the total below ground air-sediment surface area per m2 by ~190% and accounted for ~1.9% of excavated volume per m3 of mangrove sediment. On average, the burrows of Metaplax spp. (Varunidae) increased the surface area by ~55%, while the ocypodid and dotillid species accounted for a 10–25% increase across sites. Due to its densities and the complexity of its burrows, P. bidens showed to exert a wide impact on Hong Kong mangrove ecosystems unrivalled by other species. By incorporating species-specific burrow characteristics and actual burrow densities, we were able to accurately estimate the differential bioengineering role of the dominant mangrove crab species. Due to the functional role of crab bioengineering, our estimates are critical to assess Hong Kong mangrove ecosystem functioning and health.

Karenia mikimotoi blooms in coastal waters of China from 1998 to 2017

Abstract: Karenia mikimotoi is a typical ichthyotoxic dinoflagellate species that causes blooms around the world. In China, the first K. mikimotoi bloom was recorded around the Pearl River estuary in 1998. There were 137 K. mikimotoi bloom events recorded in the four seas around China from 1998 to 2017. In the northernmost and southernmost regions, K. mikimotoi blooms occurred off of Qinhuangdao city and Zhanjiang city, respectively, with latitudinal range of approximately 20°. K. mikimotoi blooms were recorded almost every month from spring to autumn except November; however, no blooms occurred in winter during this 20-year period. In China, the 1998–2002 period represented the initial and developmental stages of K. mikimotoi blooms, while the 2003–2017 period represented the burst stage. Although large spatial and temporal distributions have been, K. mikimotoi blooms are generally centered on the period from May–June and on the coasts of the East China Sea, where the blooms have been correlated with conditions including eutrophication, meteorology and climate change, and sediment.

Application of the Variational Mode Decomposition (VMD) method to river tides

Abstract: Tides in fluvial estuaries are distorted by non-stationary river discharge, which makes the analysis of estuarine water levels less accurate when using the conventional tidal analysis method. As a powerful and widely-used method for non-stationary and nonlinear time series, the application of Variational Mode Decomposition (VMD) method to non-stationary tides is nonexistent. This paper aims to illustrate and verify the suitability of the VMD method as a new tidal analysis tool for river tides. The efficiency of VMD is validated by the measurements from the Columbia River Estuary. VMD strictly divides different tidal species into different modes, and thus avoids mode mixing. Compared to VMD, Ensemble Empirical Mode Decomposition (EEMD), which is another commonly-used method, fails to completely solve the problem of mode mixing. The observed water levels at Longview station are decomposed into 12 modes via VMD. Based on the mean periods and amplitudes of each VMD mode, the 12 VMD modes sequentially correspond to the tidal species from the sub-tides (D0), diurnal tides (D1), semi-diurnal tides (D2), and up to D11 tides. The non-stationary characteristics of tides influenced by river discharge are accurately captured by VMD without mode mixing. The results also show that the EEMD and VMD modes can capture the subtidal signals better than the nonstationary tidal harmonic analysis tool (NS_TIDE). As a general method, the VMD model can also be used for other research purposes related to non-stationary tides, such as detiding.

Patterns of structural and functional diversity of macrofaunal communities in a subtropical mangrove ecosystem

Abstract: Biological trait analysis has been successfully used to describe ecological functions in marine benthic ecosystems. However, it has rarely been applied to mangrove habitats. This study analyzes taxonomic and functional diversity and the components thereof in two mangrove habitats in the Hara Biosphere Reserve, Persian Gulf: a vegetated area with both mangrove trees and pneumatophores, and an adjacent mudflat. Species diversity and functional diversity were higher in vegetated habitats and slightly increased in summer. Our study demonstrates the importance of mangrove structural complexity for the structure and functioning of invertebrate communities. The macrofaunal communities were characterized by low diversity and a high dominance of just a few species, such as crab Opusia indica which are specifically adapted to this stressed environment. Burrowers, surface deposit feeders, free-living, and large sized organisms were dominant trait modalities that essentially shape the functional composition of arid mangrove ecosystem of the Persian Gulf. The low functional redundancy among species implies that small changes in species diversity may have marked impacts on the system functional diversity.

Morphological evolution of estuarine channels influenced by multiple anthropogenic stresses: A case study of the North Channel, Yangtze estuary, China

Abstract: The Construction of mega-dams has attracted worldwide attention due to their impacts on channel scouring from downstream regions to estuaries. However, the impacts of dams and near-end engineering on the evolution of large alluvial estuarine channels are unknown. In this study, the morphological evolutionary process and response regime of the North Channel (NC) in the Yangtze estuary was examined between 1986 and 2016 by using hydrology, multibeam echo sounding and historical bathymetry datasets. The results showed that the NC exhibited overall erosion with a total volume of 9.09 × 108 m3 over the past 30 years. We concluded that sediment load reduction in the river basin was the main reason for channel scouring. However, the short-term influence of near-end engineering on the local channel evolution process could not be ignored. For example, the construction of the north-south channel bifurcation project resulted in slight siltation of the NC with a total volume of 0.12 × 108 m3 from 2002 to 2007 despite the sediment load decreasing from 3.4 × 108 t to 1.8 × 108 t since the closure of the Three Gorges Dam (TGD). Furthermore, after construction (2007–2016) of the Qingcaosha Reservoir, the largest reservoir in the Yangtze estuary, the NC experienced extensive scouring near the reservoir and limited morphologic changes to the margin channel far from the reservoir. The maximum scouring depth near the reservoir reached ~13.5 m. In terms of the scour rate, the value of the average annual scour depth was 23–24 times the riverbed erosion rate caused by sediment load reduction. Therefore, a re-evaluation of the unexpected impacts of near-end engineering on the local Yangtze estuarine channels is urgently needed.

Low humidity and hypersalinity reduce cold tolerance in mangroves

Abstract: Macroclimatic changes are expected to radically alter coastal wetland ecosystems in the coming century. The trajectory of the response to climate warming may differ based on other concomitantly changing abiotic variables such as soil salinity and relative humidity. Thus, understanding plant responses to multiple interacting stressors is required to accurately predict coastal wetland shifts under climate change. The ongoing poleward shift of mangrove range limits has been linked with a reduction in freeze events, yet interactions between low temperature and other abiotic stressors remain underexplored. We grew two common mangroves (Avicennia germinans and Rhizophora mangle, n = 1222) from propagules for 10 months in environmental growth chambers under experimentally manipulated temperature, salinity, and relative humidity treatments that reflected the range of conditions these species experience in the field. We measured variation in growth and physiological characteristics before, during, and after low temperature exposure. For both species, resistance and resilience to low temperature stress were mediated by salinity and relative humidity conditions. Chronic chilling at 10 °C caused widespread reduction in seedling stem elongation rate, altered leaf gas exchange rates, and increased mortality, particularly under high salinity and low humidity conditions. Additional exposure to an overnight freeze (−4 °C) had relatively minor impacts. Five months after exposure to low temperatures, some R. mangle exhibited the capacity to recover from severe cold damage, but only under optimal humidity and salinity conditions. Although A. germinans were generally more resistant to low temperature stress, severely damaged plants did not recover, even in low salinity and high humidity conditions. We contend that current and future mangrove range limits are the result of interactions between multiple abiotic stressors including temperature, salinity, and relative humidity. Consequently, future modelling approaches to predicting range shifts under climate change need to consider multiple concomitantly changing abiotic variables and their interactions.

Measuring mangrove-fishery benefits in the Peam Krasaop Fishing Community, Cambodia

Abstract: Whilst previous studies have applied economic value to the ecosystem services mangroves provide to fisheries, most quantitative studies in the peer reviewed literature have limited their measurements to the value provided through a single fishing sector, gear or particular target species group. It can be argued that present research into mangrove-fisheries has not yet represented the full complexity that mangrove-fisheries can encompass in terms of the wide range of people and activities that benefit from the mangrove ecosystem. The reported values of mangroves to fishing livelihoods are therefore likely to fall short of a full valuation. The study provides an all-encompassing value of mangrove benefits to fishing, purposefully investigating the value gained from mangroves through all fishing sectors, fishing activities and target species existing in the Peam Krasaop Fishing Community (PKFC), Koh Kong Province, southwest Cambodia. The ecosystem service value of mangroves for fishing to households in the PKFC was calculated using daily landings volumes collected through semi-structured interviews with fishers, scaled to approximated annual catches. Catch figures were converted to economic value, based on the local market prices given by respondents. Results suggested that the PKFC derives approximately 90% of fishing catch, and 85% of gross income, from mangrove-associated species. Fishing activities are diverse within households; they conduct between 1 and 8 different seasonal fishing activities, spread across mangrove gathering, fishing by boat and mariculture. This study provides a higher estimated proportion of mangrove-associated catches than many studies of fishing communities elsewhere. It may be the case that the PKFC does not have higher levels of mangrove dependency than other mangrove-fisheries. Rather, this study may provide a better quantification of mangrove value than has previously been achieved. Further studies along the same lines, taking a similarly holistic approach to mangrove-fishery valuation, are necessary to test this proposition.

Mangrove biomass estimation using canopy height and wood density in the South East and East Asian regions

Abstract: Mangroves aboveground biomass (ABG) estimation at a large scale is a crucial to understand their role in mitigating climate change. The large-scale estimation of AGB is generally conducted with remote sensing techniques using stand characteristics such as canopy height and species composition. In the present study, we employed Lorey's mean canopy height (Hm) and basal area weighted mean wood density (ρm) to develop models for estimating AGB of mangroves in Asian regions such as Indonesia, Philippines and Japan, considering its application toward large-scale AGB estimation covering different climatic zones of subtropical zone in the Eastern Asia and tropical zone in the Southeastern Asia. The best power function model for AGB estimation with Hm as a single explanatory variable was selected based on AIC ranks. The Hm – AGB relationship model varied significantly among study sites. We successfully developed common allometric model to estimate AGB for closed-canopy mangroves in the Asian regions. The common allometric model of the Hm – AGB relationship tended to show underestimation for old growth mangroves having AGB > ca. 400 Mg ha−1. The common allometric model showed a similar trend with a previously developed model for mangroves in Bangladesh. On the other hand, the common allometric model for mangroves showed distinct difference from the model for terrestrial tropical forests, i.e. the range of Hm was narrower in mangroves than in terrestrial tropical forests, and AGB was quite higher in mangroves than in terrestrial tropical forests at a same range of Hm. The difference of AGB specific to Hm was explained by higher ρm and cumulative basal area (BA) in mangroves than that in terrestrial tropical forests. In particular, mangrove showed quite high BA in a comparison with terrestrial tropical forests. This study confirmed that developing mangrove specific biomass model is important since mangroves showed its unique characteristics through comparisons with terrestrial tropical forests, and limitations of the use of Hm as a single variable. It should be noted that old growth mangrove AGB is underestimated with the developed common allometric models. For old growth mangroves, carbon stock should be fairly evaluated with more accurate models otherwise their contribution to blue carbon would be overlooked, which could lead insufficient efforts for conservation of old growth mangroves. Further accurate models for estimating mangrove AGB at large scale are urgently required.

Mangrove leaf species-specific isotopic signatures along a salinity and phosphorus soil fertility gradients in a subtropical estuary

Abstract: Mangrove ecotypes are distinct monospecific or mix-species assemblages and used as classification criteria to evaluate coastal biogeochemical cycles at the local, regional, and global scales. However, it is not clear how plant nitrogen and carbon content, including bulk δ13C and δ15N and n-alkane δ13C values, vary across species and within species when plants are exposed to the interaction between nutrient (nitrogen-N, phosphorus-P) availability and stressors (i.e., salinity). Here we present significant differences in green leaves wax n-alkane δ13C (δ13Cn-alkane) values and brown-senescent leaves C:N atomic ratios and total phosphorus (TP) concentrations of three mangrove species (Rhizophora mangle, Laguncularia racemosa, and Avicennia germinans) that reflect ecophysiological adaptations to nutrient availability and salinity along the Shark River estuary (SRE), South Florida, USA. Linear models between leaf wax δ13Cn-alkane values and species location along TP fertility and salinity gradients showed distinct differences, particularly between the species A. germinans and R. mangle. Our analyses showed that leaf wax δ13Cn-alkane properly represented major differences in ecophysiological responses by each mangrove species. We also found that both R. mangle and L. racemosa showed different isotopic footprints among the SRE upper, middle and lower estuarine salinity regions. Further, the green leaves bulk δ13C values in R. mangle (−32.3‰ to −27.6‰) were positively correlated with distance from the mouth of the estuary. In contrast, L. racemosa showed a negative relationship with distance and a narrower bulk δ13C range (−29.8‰ to −28.1‰) in comparison to the other two species. A. germinans, a species found only in the brackish (salinity: 18.8 ± 1.2) and saline (30.3 ± 0.53) estuarine regions, also showed a positive bulk δ13C relationship with distance. Because of the well-defined species-specific leaf wax n-alkane δ13C values along both water column/soil pore water salinity and TP gradients, we propose these values as a potential salinity proxy for paleoclimate reconstruction.

Changes in taxonomic structure and functional traits of nematodes as tools in the assessment of port impact

Abstract: Numerous anthropogenic activities take place within or nearby ports, thereby increasing environmental pressure in coastal zone that become the final destination of many environmental contaminants. Thus, port areas are crucial to understand the possible long-term effects of contaminants on metazoans that are at the base of the food webs. The port of Vado Ligure is the outlet to the sea of one of the largest industrial areas of the West Liguria (NW Mediterranean). Sediment quality was evaluated in twenty-eight stations at increasing distance from the port. The nematode fauna was characterised (i.e., assemblage composition, taxonomic diversity and functional traits) and organic matter quantity and composition and contaminant concentrations (heavy-metals, polycyclic aromatic hydrocarbons and heavy, long-chain hydrocarbons) were measured. The area resulted mainly polluted by mercury, followed by arsenic, zinc, and polycyclic aromatic hydrocarbons. The faunal distribution (both from a taxonomical and functional point of view) appeared influenced by the contamination, as well as by sediment granulometry and organic matter content and composition. According to nematode assemblages and environmental variables, two groups of stations showed the most marked differences: one located in front of the Vado Ligure port, characterised by the most contaminated stations, the highest organic matter content, and fine sediments, and the other one, close to Bergeggi Marine Protected Area, characterised by more pristine stations, low organic matter content and coarser sediments. A suite of nematode genera that may be considered as bioindicators of pollution and organic enrichment was selected. Maturity index (MI) resulted negatively affected by contamination, suggesting that the different proportion of the nematode life-strategies is useful in assessing the environmental quality of sediments. The ecological quality status of the sampled stations obtained using abiotic and biotic descriptors (i.e., contamination level, organic matter content and nematode diversity and maturity index) was compared. Both the abiotic and nematode variables showed a general consistency for the stations of the two groups characterised by the highest and the lowest levels of contamination and organic enrichment.

Assessment of origin and abundance of beach litter in Homa Lagoon coast, West Mediterranean Sea of Turkey

Abstract: The pollution of beach litter was seasonally evaluated in a selected beach in the West Mediterranean Region of Turkey between December 2019 and September 2020. Beach litter survey was carried out seasonally according to ‘Guidance on Monitoring of Marine Litter in European Seas’ published by European Marine Strategy Framework Directive Technical Subgroup on Marine Litter. During the study, a total of 2.324 items and 105.44 kg were collected. The average litter density was 581.0 ± 60.04 items/m2 and 26.36 ± 0.85 g/m2. The results indicated that the most common type of litter were plastic/polystyrene (35%), unidentifiable (18%) followed by cloth/textile (16%). Litter density showed significant seasonal differences with the highest in summer (P

Mangrove carbon stocks in Pongara National Park, Gabon

Abstract: Mangroves are recognized for their valued ecosystem services to coastal areas, and the functional linkages between those services and ecosystem carbon stocks have been established. However, spatially explicit inventories are necessary to facilitate management and protection of mangroves, as well as providing a foundation for payment for ecosystem service programs such as REDD+. We conducted an inventory of carbon stocks in mangroves within Pongara National Park (PNP), Gabon using a stratified random sampling design based on forest canopy height derived from TanDEM-X remote sensing data. Ecosystem carbon pools, including aboveground and belowground biomass and necromass, and soil carbon to a depth of 2 m were assessed using measurements and samples from plots distributed among three canopy height classes within the park. There were two mangrove species within the inventory area in PNP, Rhizophora racemosa and R. harrisonii. R. harrisonii was predominant in the sparse, low-stature stands that dominated the west side of the park. In the east side of the park, both species occurred in tall-stature stands, with tree height often exceeding 30 m. Canopy height was an effective means to stratify the inventory area, as biomass was significantly different among the height classes. Despite those differences in aboveground biomass, the soil carbon density was not significantly different among height classes. Soils were the main component of the ecosystem carbon stock, accounting for over 84% of the total. The ecosystem carbon density ranged from 644 to 943 Mg C ha−1 among the three height classes. The ecosystem carbon stock within PNP is estimated to be 40,588 Gg C. The combination of pre-inventory information about stand conditions and their spatial distribution within the assessment area obtained from remote sensing data and a spatial decision support system were fundamental to implementing this relatively large-scale field inventory. This work exemplifies how mangrove carbon stocks can be quantified to augment national C reporting statistics, provide a baseline for projects involving monitoring, reporting and verification (i.e., MRV), and provide data on the forest composition and structure for sustainable management and conservation practices.

Nutrient cycling in tropical and temperate coastal waters: Is latitude making a difference?

Abstract: Tropical coastal waters are highly dynamic and amongst the most biogeochemically active zones in the ocean. This review compares nitrogen (N) and phosphorus (P) cycles in temperate and tropical coastal waters. We review the literature to identify major similarities and differences between these two regions, specifically with regards to the impact of environmental factors (temperature, sunlight), riverine inputs, groundwater, lateral fluxes, atmospheric deposition, nitrogen fixation, organic nutrient cycling, primary production, respiration, sedimentary burial, denitrification and anammox. Overall, there are some similarities but also key differences in nutrient cycling, with differences relating mainly to temperature, sunlight, and precipitation amounts and patterns. We conclude that due to the differences in biogeochemical processes, we cannot directly apply cause and effect relationships and models from temperate systems in tropical coastal waters. Our review also highlights the considerable gaps in knowledge of the biogeochemical processes of tropical coastal waters compared with temperate systems. Given the ecological and societal importance of tropical coastal waters, we hope that highlighting the differences and similarities to temperate systems as well as the existing gaps, will inspire further studies on their biogeochemical processes. Such knowledge will be essential to better understand and forecast impacts on tropical coastal nutrient cycling at local, regional, and global scales.

The extended concept of littoral active zone considering soft sediment shores as social-ecological systems, and an application to Brittany (North-Western France)

Abstract: This paper applies and extends the concept of Littoral Active Zone (LAZ) of sandy beaches as a relevant dimension to observe and manage this socio ecological system. The LAZ is a dynamic zone where exchanges across land and sea occur, and this concept was initially proposed on a geomorphological background only. However, to achieve full relevance and timely address management choices tied to the functioning of soft sediment shores, it became appropriate to extend its consideration to both the ecological and social templates co-existing on the same physical unit. Current paradigms around the concept of LAZ were used as a background to organize information from different disciplines and extend the concept to different soft sediment shores (mostly sandy beaches and salt marshes), towards a global, integrated relevance of the concept. As a test for this approach, we 1) struabctured information on the biophysical and social templates around the concept of LAZ, 2) extended it to salt marshes and 3) applied it to the case of the Breton coast for a practical test on information organization. Such an exercise highlighted key characteristics of the LAZ using a multidisciplinary approach, but also gaps to be filled when targeting research, perception, communication, sustainable use and management of the LAZ as a functional unit.