Scientific basis, engineering feasibility and system optimization of green sea dykes for temperate mud coasts: a brief overview

Salt marshes

In this paper we discuss the role of nitrogen in the vegation dynamics of European salt marshes. An overview is presented of various fluxes of nitrogen to and from salt marsh ecosystems. Our primary view is on European salt marshes. A nitrogen budget constructed for salt marsh ecosystems in the Wadden Sea area shows that N-influx rates via floodwater (50–200 kg N ha−1 y−1) and via atmospheric deposition (30 kg N ha−1 y−1), the rate of N assimilation (95–230 kg N ha−1 y−1) and the rate of mineralization (28–225 kg N ha−1 y−1) vary and fluctuate in space and time but are of the same order of magnitude. These N-influx and efflux rates may differ markedly between salt marshes and within salt marshes. Within salt marshes the in and effluxes vary along elevational gradients with varying flooding frequencies and sedimentation rates and along successional gradients. Salt marshes are productive and dynamic ecosystems with variable and extreme environmental conditions. Rather than acting as a source or sink for nitrogen the transformations of organic and inorganic nitrogen tend to characterize salt marsh ecosystems. The cycling of N through a sediment-plant-sea water-atmosphere salt marsh ecosystem is also discussed. In addition, the impact of coastal salt marshes in reducing levels of inorganic N from sea water is assessed, 15N-stable isotope studies indicate that, despite the significant influxes of N via atmospheric deposition and seawater flooding to the salt marsh ecosystems, most inorganic N is taken up via the root system and not through foliar uptake. There is strong evidence that availability of nitrogen limits plant growth and vegetation succession in salt marshes. It is hypothesized that increased input of nitrogen via floodwaters (polluted rivers and estuaries) and atmosphericdeposition (agriculture, industry, traffic) has affected the development of salt marsh vegetation. In particular the increasing dominance of the grass Elymus athericus during the last decades is related to the increased supply of nutrients to salt marshes. The impact of eutrophication via seawater and atmospheric deposition is discussed in relation to changes in species composition of various vegetation zones and successional stages in grazed and ungrazed salt marshes. Analysing changes in species composition over periods of years and decades, longer term changes of other environmental factors such as sedimentation, flooding frequency and flooding duration and nitrogen content of the salt marsh soil are also surveyed. The increased occurrence of Elymus athericus in West European salt marshes may relate to eutrophication. Alternatively, the dominance of the tall and rapidly growing Elymus athericus is linked with the “naturally” enhanced N-soil content and increased rate of N-mineralization in late successional stages of salt marshes.

India, Pakistan, Bangladesh, Sri Lanka and the Maldives have extensive coastal and marine ecosystems. Seagrass beds and salt marshes are coastal ecosystems restricted to the subtidal and intertidal zone ranging from shallow water in the case of seagrass to the high upland of intertidal zone in case of salt marsh. The chapter provides an overview of the salt marsh and seagrass ecosystems in India and other South Asian countries. Despite their significant ecological importance, seagrass and salt marsh ecosystems are relatively under-explored or unexplored, particularly in the South Asian countries. Fifteen species of seagrass are reported from South Asian region, all of which are found in India. They are distributed along the coastal states/union territories except Maharashtra, Daman and Diu, Puducherry and West Bengal. The salt marsh species diversity in India and the Maldives is not reported. The chapter provides checklist of salt marshes of India, represented by 14 species, which are distributed along Gujarat, Daman and Diu, Maharashtra, Tamil Nadu, Puducherry, Andhra Pradesh and Andaman and Nicobar Islands.

Salt marsh plants are salt tolerant rooted vegetation that are found in low-energy transition zone between submerged and emerged environments, occupying the upper margins of the inter-tidal landscape. Salt marshes provide a unique habitat for a large number of species. Jaffna district harbors some of the major salt marshes in Sri Lanka, with large extents of undisturbed salt marshes. Salt marsh diversity and distribution has never been studied in Jaffna district since 1969 due to three-decade long armed conflict that prevailed in the entire northern part of Sri Lanka. The objective of the present study was to identify the dominant salt marsh plant species and their distribution in the Jaffna District. The study was conducted in the Jaffna District from 2014 to 2018. Line transects and spot-check methods were used to determine the distribution and abundance of salt marsh plant species. Distribution of salt marshes in the entire district was surveyed using GPS and the distribution maps were prepared using Q-GIS and ArcView. Then the distribution maps were intersected with Grama Niladhari division maps and salt marsh species distribution was evaluated by Grama Niladhari division level. Salt marsh plant species recorded in the present study are Suaeda maritima, S. vermiculata, S. monoica, Halosarcia indica and Salicornia brachiata. Total extent of the salt marshes in the Jaffna district was estimated to be around 1,105 hectares. Saltmarsh plant species are distributed in 25 Grama Niladhari divisions where large extents (over 100 ha) of salt marshes are found in 3 divisions (totaling 638 ha). Higher number of salt marsh species were found in Mandaitivu, Thanankilappu, Navali South, Arali- Navali, Ariyalai East and Chvachcheri. These species are found in in high salinity areas especially in southern and western parts of Jaffna main land and the islands. Salicornia brachiata is the most common species which was found in the entire study area. In the low salinity areas such as Vallai and Vatharavaththai small extents of salt marsh species were found. Jaffna salt marshes are one of the most overlooked coastal ecosystems. The study revealed that five salt marsh species, belonging to three families occur in the Jaffna District, distributed over a 18 number of Grama Niladhari divisions in Southern coastal line, 5 division in the Islands. At present, improper road constructions, hotel development, and lack of awareness on salt marsh ecosystems have negatively impacted on these ecosystems. Hence, more attention should be given to protect the salt marsh ecosystem and environmentally friendly development activities should be promoted to conserve them. Keywords: Salt marsh, Jaffna district, Grama Niladhari Divison, Mandaitivu, Saliconnia

Soil developments in salt marshes and on artificial islands in the Wadden Sea

Sea-level rise enhances carbon accumulation in United States tidal wetlands

Root system structure and functions across coastal saltmarsh flooding gradients

The importance of intertidal estuarine habitats, like salt marsh and oyster reef, has been well established, as has their ubiquitous loss along our coasts with resultant forfeiture of the ecosystem services they provide. Furthering our understanding of how these habitats are evolving in the face of anthropogenic and climate driven changes will help improve management strategies. Previous work has shown that the growth and productivity of both oyster reefs and salt marshes are strongly linked to elevation in the intertidal zone (duration of aerial exposure). We build on that research by examining the growth of marsh-fringing oyster reefs at yearly to decadal time scales and examine movement of the boundary between oyster reef and salt marsh at decadal to centennial time scales. We show that the growth of marsh-fringing reefs is strongly associated to the duration of aerial exposure, with little growth occurring below mean low water and above mean sea level. Marsh-shoreline movement, in the presence or absence of fringing oyster reefs, was reconstructed using transects of sediment cores. Carbonaceous marsh sediments sampled below the modern fringing oyster reefs indicate that marsh shorelines within Back Sound, North Carolina are predominantly in a state of transgression (landward retreat), and modern oyster-reef locations were previously occupied by salt marsh within the past two centuries. Cores fronting transgressive marsh shorelines absent fringing reefs sampled thinner and less extensive carbonaceous marsh sediment than at sites with fringing reefs. This indicates that fringing reefs are preserving carbonaceous marsh sediment from total erosion as they transgress and colonize the exposed marsh shoreline making marsh sediments more resistant to erosion. The amount of marsh sediment preservation underneath the reef scales with the reef’s relief, as reefs with the greatest relief were level with the marsh platform, preserving a maximum amount of carbonaceous sediments during transgression by buffering the marsh from erosional processes. Thus, fringing oyster reefs not only have the capacity to shelter shorelines but, if located at the ideal tidal elevation, they also keep up with accelerating sea-level rise and cap carbonaceous sediments, protecting them from erosion, as reefs develop along the marsh.

Salt marsh losses have been documented worldwide because of land use change, wave erosion, and sea-level rise. It is still unclear how resistant salt marshes are to extreme storms and whether they can survive multiple events without collapsing. Based on a large dataset of salt marsh lateral erosion rates collected around the world, here, we determine the general response of salt marsh boundaries to wave action under normal and extreme weather conditions. As wave energy increases, salt marsh response to wind waves remains linear, and there is not a critical threshold in wave energy above which salt marsh erosion drastically accelerates. We apply our general formulation for salt marsh erosion to historical wave climates at eight salt marsh locations affected by hurricanes in the United States. Based on the analysis of two decades of data, we find that violent storms and hurricanes contribute less than 1% to long-term salt marsh erosion rates. In contrast, moderate storms with a return period of 2.5 mo are those causing the most salt marsh deterioration. Therefore, salt marshes seem more susceptible to variations in mean wave energy rather than changes in the extremes. The intrinsic resistance of salt marshes to violent storms and their predictable erosion rates during moderate events should be taken into account by coastal managers in restoration projects and risk management plans.

Coastal ecosystems such as oyster reefs, salt marshes and mangroves are widely recognised as nature-based solutions reducing coastal erosion. Oyster reefs maintain their own habitat and have the ability to grow at the rate of sea level rise, making them self-sustainable, flexible and cost-effective coastal erosion measures in the face of climate change. By attenuating waves and stabilising sediment as well as facilitating and protecting neighbouring ecosystems, they stimulate coastal resilience. However, effective employment of oyster reefs as a nature-based erosion control measure is not trivial and requires the integration of ecological and engineering parameters. Given the satisfaction of these eco-engineering parameters, recent work demonstrates that oyster reefs lead to a four-fold reduction in erosion in the protected area compared to a non-protected area across a decadal period. Despite this apparent effectiveness across a longer time period, it is still poorly understood how effective oyster reefs are in reducing erosion during individual storm events and how large their morphological footprint during these events is. We present the findings of a series of detailed morphological field surveys of the Viane oyster reef in the Eastern Scheldt, the Netherlands, during which three storm events (Ciaran, Gerrit and Henk) were captured. These storms led locally to significant wave heights of 1.3-1.5 m, corresponding to the highest percentile of wave events recorded locally. Results show that storm Ciaran resulted in an transect-average erosion of 0.02-0.05 m for the unprotected areas, corresponding to the typical annual erosion for the intertidal flats of this area. In contrast, the reef-protected areas showed a greatly reduced erosion of maximum 0.02 m but typically 0.01 m. It is important to note that the erosion pattern as a result of this storm event is far from homogeneous: erosion is greatest immediately behind the reef (~first 50 m), then reduces up to 150 m behind the reef, followed by a zone of deposition (150-250 m behind the reef) and then transitions into another zone of erosion (250-450 m behind the reef). Complementary numerical modelling with XBeach will be used to obtain additional insights into the role of wave angle, wave period and tidal timing on the flow, sediment transport and morphological changes caused by the Viane reef structure during storm events.

In many salt marsh ecosystems, crabs are the most prominent members of the macrobenthic community, influencing material cycling and energy flow. Studies on population dynamics and secondary production are important for quantitative evaluation of the ecological function of crabs; however, few studies have been conducted in salt marsh ecosystems. In this study, we investigated the secondary production and population dynamics of two Thoracotremata species in an Asian salt marsh. A year-round sampling on a monthly basis in the Dongtan salt marshes of the Yangtze River estuary yielded 12 species of crabs belonging to 7 families and 10 genera. The dominant crabs were Chiromantes dehaani (67.57%) and Helice tientsinensis (17.28%). The overall size frequency distribution was bimodal for C. dehaani and unimodal for H. tientsinensis. The estimated annual production of C. dehaani and H. tientsinensis was 43.981 g and 4.247 g ash-free dry weight (AFDM) m−2 year−1, respectively. The annual P/B ratio was 1.22 for C. dehaani and 0.75 for H. tientsinensis. In total, the annual secondary production of all crabs at the Dongtan salt marshes was estimated to be approximately 49.405 g AFDM m−2 year−1. The consumption of vascular plants by C. dehaani and H. tientsinensis was 480.831 and 42.640 g AFDM m−2 year−1. About 20.4% of plant primary production was ingested by these two herbivorous crabs. The production of C. dehaani was high compared with those reported from other coastal wetland ecosystems. This suggests that the ecological functions of C. dehaani at the Yangtze River estuary salt marshes need more quantitative studies to be fully described.

Environmental Context. The pathways by which arsenic is accumulated and transferred in aquatic ecosystems are relatively unknown. Examination of whole marine ecosystems rather than individual organisms provides greater insights into the biogeochemical cycling of arsenic. Saltmarshes with low ecological diversity are an important terrestrial–marine interface about which little is known regarding arsenic concentrations and species distribution. This study examines the cycling of arsenic within Australian saltmarsh ecosystems to further understand its distribution and trophic transfer. Abstract. This paper reports the distribution of total arsenic and arsenic species in saltmarsh ecosystems located in south-east Australia. We also investigated the relationship between arsenic, iron, and phosphorus concentrations in saltmarsh halophytes and associated sediment. Total mean arsenic concentrations in saltmarsh plants, S. quinqueflora and S. australis, for leaves ranged from 0.03 ± 0.05 to 0.67 ± 0.48 µg g-1 and 0.03 ± 0.02 to 0.08 ± 0.06 µg g-1, respectively, and for roots ranged from 2 ± 2 to 6 ± 12 µg g-1 and 0.39 ± 0.20 to 0.57 ± 1.06 µg g-1 respectively. Removal of iron plaque from the roots reduced the arsenic concentration variability to 0.40–0.79 µg g-1 and 0.95–1.05 µg g-1 for S. quinqueflora and S. australis roots respectively. Significant differences were found between locations for total arsenic concentrations in plant tissues and these differences could be partially attributed to differences in sediment arsenic concentrations between locations. For S. quinqueflora but not S. australis there was a strong correlation between arsenic and iron concentrations in the leaf and root tissues. A significant negative relationship between arsenic and phosphorus concentrations was found for S. quinqueflora leaves but not roots. Total mean arsenic concentrations in salt marsh animal tissues (7 ± 2–21 ± 13 µg g-1) were consistent with those found for other marine animals. The concentration of total arsenic in gastropods and amphipods could be partially explained by the concentration of total arsenic in the dominant saltmarsh plant S. quinqueflora. Of the extractable arsenic, saltmarsh plants were dominated by arsenic(iii), arsenic(v) (66–99%), and glycerol arsenoribose (17–35%). Arsenobetaine was the dominant extractable arsenic species in the gastropods Salinator soilda (84%) and Ophicardelus ornatus (89%) and the crab Neosarmatium meinerti (89%). Amphipods contained mainly arsenobetaine (44%) with some phosphate arsenoribose (23%). Glycerol trimethyl arsonioribose was found in both gastropods (0.7–0.8%) and the visceral mass of N. meinerti (0.1%). These results show that arsenic uptake into plants from uncontaminated saltmarsh environments maybe dependent on plant iron uptake and inhibited by high phosphorus concentrations. Arsenic in saltmarsh plants is mainly present as inorganic arsenic, but arsenic in animals that eat plant detritus is present as organo arsenic species, primarily arsenobetaine and arsenosugars. The presence of glycerol trimethyl arsonioribose poses the question of whether trimethylated arsonioriboses are transitory intermediates in the formation of arsenobetaine.

Spatial variation of methane emissions and methane-cycling communities in a coastal salt marsh.

优势种去除对崇明东滩盐沼湿地生态系统的影响

Elevated CO2 and nitrogen addition affect the microbial abundance but not the community structure in salt marsh ecosystem