We are pleased to present the Wetlands International Annual Review 2019. The Source is our digital magazine showcasing the achievements of Wetlands International and its partners in 2019, and aims to bring to life our annual review and accounts with stories, profiles and figures.
Restoration, creation and management of salt marshes and tidal flats
A collation of evidence-based guidance
Edited by Vanessa Cutts, Paul L.A. Erftemeijer, Lorenzo Gaffi, Ward Hagemeijer, Rebecca K. Smith, Nigel G. Taylor & William J. Sutherland
Editor affiliations
Vanessa Cutts 1 , Paul L.A. Erftemeijer 2 , Lorenzo Gaffi 3 , Ward Hagemeijer 3 , Rebecca K. Smith 1 , Nigel G. Taylor 1 and William J. Sutherland 1
1 Conservation Science Group, Department of Zoology, University of Cambridge 2 School of Biological Sciences and Oceans Institute, University of Western Australia 3 Wetlands International, The Netherlands
Cite as: Cutts V., Erftemeijer P.L.A., Gaffi L., Hagemeijer W., Smith R.K, Taylor N.G. & Sutherland W.J. (eds.) (2024) Restoration, creation and management of salt marshes and tidal flats: A collation of evidence-based guidance . Report of Conservation Evidence, Wetlands International and the World Coastal Forum. https://doi.org/10.52201/CGSCOL1/LCNC6109
Cover photos Front page: Saeftinge, Speelmansgat, The Netherlands. Credit: Edwin Paree Section 1: Marker Wadden nature reserve, The Netherlands. Credit: Paul Erftemeijer Section 2: Credit: Edwin Paree Section 3: Meghna Estuary, Bangladesh. Credit: Sayam Chowdhury Section 4: Shorebirds, Sonadia Island , Cox’s Bazar , Bangladesh. Credit: Sayam Chowdhury. Back page: Bangladesh. Credit: Sayam Chowdhury
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Contents
Section 1 Introduction ............................................................................................................. 6
Scope of the document.......................................................................................................7
Who is this document for? ..................................................................................................8
Structure of guidance .........................................................................................................9
Description of salt marshes and intertidal flats....................................................................10
Why and how are they threatened? ...................................................................................11
Importance of salt marsh and intertidal flat restoration ........................................................13
Feeding, roosting and nesting sites for shorebirds..............................................................15
Other sources of information.............................................................................................19
References ...................................................................................................................... 19
Section 2 Planning ................................................................................................................. 26
Guidance on making evidence-based decisions for conservation management ....................27
Guidance on planning coastal restoration and setting targets..............................................34
Section 3 Restoration approaches....................................................................................... 46
Guidance on facilitating tidal exchange to restore/create salt marshes and intertidal flats......47
Guidance on using sediment to restore/create salt marshes and intertidal flats.....................58
Guidance on reprofiling salt marshes and intertidal flats......................................................71
Guidance on restoring or creating salt marsh vegetation.....................................................77
Guidance on managing vegetation on intertidal flats ...........................................................84
Guidance on chemical control of Spartina spp....................................................................91
Guidance on physical control of Spartina spp. .................................................................... 99
Guidance on integrated control of Spartina spp. ............................................................... 106
Section 4 Management approaches for shorebirds ......................................................... 113
Guidance on managing artificial ponds for shorebirds ....................................................... 114
Guidance on creating islands for shorebirds..................................................................... 122
Guidance on managing/clearing vegetation for shorebirds ................................................ 131
Guidance on reducing disturbance of shorebirds .............................................................. 137
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Advisory group
This document has been reviewed by a group of advisors, each with expertise of some or all aspects of salt marsh and tidal flat restoration. The advisory group gave feedback on the structure and the content. They also provided additional information and recommendations based on their own experience. Errors or omissions from the document remain the responsibility of the authors.
We thank the following experts for their role in advising on this document:
Malcom Ausden , RSPB, UK
Hyun-Ah Choi , Hanns Seidel Foundation, South Korea
Chi-Yeung Choi , Duke Kunshan University, China
Mark Dixon , RSPB, UK
Micha V. Jackson , CSIRO, Australia
Yifei Jia , Beijing Forest University, China
Qiang He , Fudan University, China
Wenhai Lu , National Marine Data and Information Service, China
David Melville , Global Flyway Network, New Zealand
Spike Millington , International Crane Foundation, USA
Taej Mundkur , Wetlands International, The Netherlands and World Coastal Forum Facilitators Group
Han Winterwerp , Delft University of Technology, The Netherlands
Thomas Worthington , University of Cambridge, UK
Fokko van der Goot , Boskalis and EcoShape, The Netherlands
Hongyan Yang , Beijing Forest University, China
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Acknowledgements
We extend our sincere appreciation to all those who contributed to the development of this product. Their valuable input, whether through insightful discussions, constructive feedback, or practical assistance, has been instrumental throughout the process. Special thanks go:
Dr. Yuan Jun and Prof. Chen Kangjuan, Academy of Inventory and Planning, National Forestry and Grassland Administration, China
Prof. Zhang Mingxiang, Beijing Forestry University, China
Dr. Wen Xianji, World Wildlife Foundation, Hong Kong
Dr. Hou Jun, Panjin Forestry and Wetland Conservation Administration Bureau
Mr. Shen Shoutao, Rudong Natural Resources and Planning Bureau
Pieter Van Eijk, Wetlands International, the Netherlands
We also extend our gratitude to the attendees of our workshops in China and at the 2023 World Coastal Forum, Yancheng, who provided us with feedback and new ideas.
We would also like to thank the core team at Conservation Evidence for their support in the design and structure of this document, as well as all the contributors to the Conservation Evidence database for making the creation of this guidance possible (see https://www.conservationevidence.com/content/page/82). Conservation Evidence is supported by core funding from A.G Leventis Foundation, Cartier Fund for Nature, Formas, Lord and Lady Moran and the Natural Environment Research Council and has previously received support from many others (see https://www.conservationevidence.com/content/ page/24#support-received). Finally, we would like to express our deepest gratitude to Arcadia — a charitable fund of Lisbet Rausing and Peter Baldwin — for their indispensable financial support, without which our work would not have been possible.
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Section 1 Introduction
Scope of the document
Who is this document for?
Structure of guidance
Description of salt marshes and intertidal flats
Why and how are they threatened?
Importance of salt marsh and intertidal flat restoration
Feeding, roosting and nesting sites for shorebirds
Other sources of information
References
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Salt marshes and tidal flats serve as vital habitat for biodiversity and provide extensive and highly valuable ecosystem services. Yet, they have undergone substantial loss and transformation over millennia with impacts increasing in recent decades. Their loss and degradation have impacted species of high conservation concern, including migratory shorebirds and other waterbirds, as well as ecosystem functions and services they provide to humans more broadly. There is increasing recognition of the importance of these systems, and the impact of their loss, and increasing concern and effort in managing and restoring them. These intertidal areas are vital in protecting the coast from erosion, especially during on-shore storm tides, and act as a natural flood defence that can protect built areas such as housing and industry, and other human land uses such as agriculture. As such they can reduce the costs of hard engineering-based coastal protection, and will have an increasing importance as sea levels rise as a result of climate change.
Scope of the document
This document collates evidence-based guidance for site managers and decision makers involved in salt marsh and tidal flat restoration with an ecological focus on shorebirds, a highly threatened group of broad conservation concern. Here, we consider shorebirds in a rather general sense, including all species of the order Charadriiformes. Shorebirds that commonly use or heavily depend on intertidal habitats include waders (e.g. plovers, stilts, oystercatchers, sandpipers), gulls and terns. This document is a collection of smaller stand-alone pieces of guidance, each focusing on a different conservation action. They can therefore be used singularly, or as a collection, depending on the management needs of the user. This document was initiated by concerns about salt marshes and tidal flats in the Yellow Sea region, particularly as habitats for birds. The Yellow Sea is a critical bottleneck for migratory shorebirds and other waterbirds that have suffered extensive loss and degradation of tidal flats and salt marsh (see also Box 1). Therefore, the collated guidance relates to selected actions most relevant to salt marsh and tidal flat restoration, and bird conservation therein, in the Yellow Sea region. The content of each guidance document is, however, global in scope. The guidance does not provide strict protocols that must be followed or detailed practical instructions about how to implement interventions or specific techniques (e.g. how to install a culvert, how to transport sediment, required permits and the application process). Rather, it highlights interventions and restoration techniques that have been demonstrated to be effective in at least some situations. Application and implementation of these techniques requires a thorough understanding of the natural system, both its biotic and abiotic aspects. Interventions that were successful at one site may not be at another because of different local conditions or implementation methods. Evidence for the guidance was gathered primarily from the literature. For evidence on the effects of interventions on biodiversity (focusing on shorebirds, benthic invertebrates and vegetation), we drew from Conservation Evidence syntheses (Sutherland et al. , 2019) where
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available, i.e. the Bird Conservation Synopsis (Williams et al. , 2013) and the Marsh and Swamp Conservation Synopsis (Taylor et al. , 2021). These syntheses are based on systematic literature searches of studies that test the effectiveness of conservation actions (see www.conservationevidence.com). We complemented these with ad hoc searches for further literature, especially on invertebrates. In addition to evidence from the literature, we contacted experts and practitioners to record their experiences of the effects of salt marsh and tidal flat restoration efforts and practical information about implementation. New evidence is continually emerging, and readers should take into consideration that this guidance document is currently underpinned by available evidence up until 2023.
Who is this document for?
This document is for practitioners and policy planners who are responsible for managing intertidal habitats, especially those that may be responsible for overseeing/managing restoration projects on tidal flats and/or salt marshes, and who are looking for practical guidance. The information provided focuses on how these systems can be managed as habitats for shorebirds, but this will also be useful for the conservation management of these habitats more generally. The aim is to allow practitioners to easily consult and evaluate existing evidence and implementation knowledge before considering the practical implications relating to the situation at their site when deciding about future management and restoration.
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Structure of guidance
The guidance documents in sections 3 & 4 follow a set structure:
● Objective: A concise statement of the desired outcome of the intervention. ● Definitions: Of key technical terms used in the guidance document ● Description: A definition of the intervention and what it involves, explanation of the logic behind the intervention, and why the intervention is needed. ● Evidence for effects on biodiversity: Evidence, largely drawn from the scientific literature, about the effects of this intervention on biodiversity and the timescales over which they may occur. There is a focus on three groups that are key indicators of the state and functioning of coastal ecosystems (birds, invertebrates and vegetation) and, where possible, on quantitative evidence. ● Factors that can affect outcomes: A list of some major factors that may affect the outcomes of the intervention: generally related to (a) the local context and (b) how the intervention is done. ● Implementation: Notes about practical implementation to achieve the overall objective, for example, specific techniques that can be used, and practical issues to consider when carrying out the intervention. This is based on published reports, the experience of practitioners, and scientific literature. ● Case study: A specific illustrative example of the implementation of the intervention and its observed effects. ● Other useful sources of information: Sources that provide further detail and/or complement information in the guidance. ● References: Published sources referred to in the preceding text.
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Description of salt marshes and intertidal flats
Salt marshes and tidal flats are found in the intertidal zone, the area between the reach of the highest high tide and the lowest low tide, and are subject to varying amounts of flooding by seawater. The ratio of salt marsh to tidal flat in the intertidal zone can vary (Atkinson et al. , 2001) but together, they provide a variety of different habitats for wildlife, including benthic invertebrates, fish and birds. They also provide important ecosystem services to people through coastal protection, water purification, carbon sequestration, food production and recreation (Barbier et al. , 2011).
An example of a coastal system showing variation in habitat and species present across the tidal range [Credit: Petra Dankers. Adapted from: EcoShape].
Tidal flats are large expanses of temporarily exposed soft substrates (sand or mud) that form where sediment deposits, often at the edge of estuaries or in sheltered sections of coasts. A key feature of tidal flats is that they are regularly inundated with water (Healy et al. , 2002), have sufficiently high mud content for the sediment to exhibit cohesive properties (Dyer et al. , 2000) and have no vegetation cover other than occasional seagrass. The International Union for Conservation of Nature (IUCN) classifies tidal flats as shoreline systems within the marine- terrestrial biome (MT1.2; Bishop et al. , 2020). Salt marshes (also known as tidal marshes) are vegetated areas, typically found in the upper parts of the intertidal zone, experiencing less frequent flooding than tidal flats. Salt marshes naturally occur globally but are more well studied in temperate and northern regions. A key feature of salt marshes is the ‘zonation’ of the vegetation, whereby different plant communities establish in bands following bathymetric patterns, depending how tolerant they are of being submerged by saltwater (Davy, 2000). Vegetation is dominated by salt-tolerant forbs, grasses and shrubs, such as Phragmites spp . and Sueda spp . , but not seagrasses (Keith et al. , 2020a). The IUCN classifies salt marshes as brackish tidal systems within the marine-freshwater- terrestrial biome (MFT1.3; Keith et al. , 2020b).
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Salt marshes and tidal flats attract shorebirds for feeding, roosting and nesting. Different degrees of vegetation cover, fluctuating water depths, and varied sediment composition provide habitats that meet the needs of a wide variety of shorebird and other waterbird species, which have different requirements for foraging, roosting and nesting. Tidal creeks (networks of small drainage channels in tidal areas) provide foraging habitat for fish and invertebrates (Olmstead & Fell, 1974; West & Zedler, 2000). During high tide, these animals are able to invade the salt marsh via tidal creeks in order to feed and are subsequently preyed upon by larger fish and birds (Olmstead & Fell, 1974). Tidal flats have therefore been described as ‘the supermarkets of the sea ’ because of their abundance of food that shorebirds can feed on, such as polychaete worms, molluscs and crustaceans. Many migratory shorebirds use intertidal areas as stepping stones to re-fuel before embarking on, or during, their long migrations and during the non-breeding period. Resident species of shorebirds and other waterbirds depend on these food sources throughout the year.
Tidal flats and salt marshes occur in estuarine systems worldwide, providing both ecological and economic value. However, these systems are vulnerable to a range of threats, which overall have led to a significant reduction in their extent. Top: Salt marshes at Saeftinge, Speelmansgat, The Netherlands [Credit: Edwin Paree]. Left: Shorebirds feeding on a tidal flat on the west coast of South
Korea [Credit: Peter Prokosch, www.grida.no/resources/4394].
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Why and how are they threatened?
Salt marshes and tidal flats are under pressure from a range of threats (Melville et al. , 2016). It has been estimated that 16% of tidal flats were lost globally between 1984 and 2016 (Murray et al. , 2019), while salt marshes are being lost at a rate of 0.3% per year (Campbell et al. , 2022).
Some of the main threats include:
Coastal development (including land reclamation): The expansion of human populations on coastlines has put increasing pressure on developing infrastructure and housing in these areas (Charlier et al. , 2005; Lai et al. , 2015; Murray et al. , 2019). Reduced sediment supply: Activities upstream, such as river damming, limit the amount of sediment reaching estuaries, therefore the rate at which sediment is replenished is reduced relative to the rate it is eroded (Syvitski et al. , 2005; Dethier et al. , 2022). Similarly, the removal of sand from rivers is a major global environmental issue (Rentier & Cammeraat 2022) that reduces sediments reaching the coasts, including in the Yangtze and Yellow Rivers (Yang et al. , 2006; Yi et al. , 2022). Sea level rise: Coastal areas are vulnerable to sea level rise, which leads to the erosion of shorelines and increased risk of flooding (Fujii, 2012; Passeri et al. , 2015). The combination of rising sea levels with coastal development, which prevents the landward movement of salt marshes and tidal flats, means there is simply less space available for these habitats. The resulting loss of intertidal habitats is called coastal squeeze (Pontee, 2013). Sinking river deltas: Due to sediment compaction and reduced sediment supply, it is estimated that 85% of the world’s deltas have experienced subsidence (Syvitski et al. , 2009). This impacts salt marshes and tidal flats by increased wave exposure, altered tidal inundation characteristics and increased erosion. Habitat degradation: Intertidal habitats can become degraded from human activities such as bottom trawling, dredging and digging, which impact the benthic fauna (Dieter & McConnaughey, 2003). Many pollutants including heavy metals, pesticides, plastics and excess nutrients end up in estuaries from agriculture, aquaculture and domestic waste (Islam & Tanaka, 2004; Bessa et al. , 2018). Invasive species: Invasive species spread through a variety of routes infiltrating coastlines and outcompeting native species (Reise et al. , 2023). The highly invasive Smooth Cordgrass Spartina alterniflora is a considerable threat to tidal flats and salt marshes on many shorelines including the Chinese (Zuo et al. , 2012; Stokstad, 2023) and Korean (Kim et al. , 2015, 2023) coasts. The deliberate introduction of native species for commercial purposes, for example molluscs, can be a threat to other native species by taking over ecological roles (Peng et al. , 2021).
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Importance of salt marsh and intertidal flat restoration
The movement to restore salt marshes and tidal flats has been driven by increasing recognition of their value (Casagrande, 1997; Barbier et al. , 2011). Their protection and restoration benefits biodiversity by conserving and reinstating habitats, while maintaining vital functions required by people. Around 27% of the human population live near the coast (Kummu et al. , 2016) and depend on a range of services provided by coastal ecosystems. Most recorded coastal restoration projects occur in the USA, Europe and Australia, but this may be a reflection of data availability – more data about the successes and failures of restoration projects are urgently needed (Bayraktarov et al. , 2016). Restoring these habitats can be a cost-effective, nature-based solution to biodiversity loss and climate change.
Important functions and services of these habitats include:
Coastal defence: Salt marshes and tidal flats defend the coastline by slowing the incoming tide and dissipating wave energy, thereby reducing coastal erosion and protecting vulnerable human settlements from flooding (Arkema et al. , 2013; Pontee et al. , 2016; Reed et al. , 2018). They can form part of “green - grey” infrastructure, which mixes natural restoration with structures of the built environment such as seawalls and dikes (Green-Gray Community of Practice, 2020). Carbon storage: Salt marshes and tidal flats are a major contributor to the amount of carbon sequestered in the marine environment, particularly due to their ability to store carbon in their soils and sediments (Duarte et al. , 2005; Chen and Lee, 2022; Maxwell et al. , 2023). As such, they have been identified as important blue carbon ecosystems (Macreadie et al. , 2021).
Many species are almost entirely dependent on coastal habitats during the migration season such as these Whimbrels Numenius phaeopus , Sundarbans, Bangladesh. [Credit: Sayam Chowdhury].
Biodiversity: Being at the interface between the terrestrial and marine worlds, salt marshes and tidal flats support a unique variety of wildlife, which function together as a large ecological complex (Daiber, 1986; Boorman, 2003). Salt-tolerant plants and benthic microalgae that live in the sediments are primary producers (Cloern et al. , 2014), which means they are at the base of the food chain. They support organisms at higher trophic levels, such as fish and mud- dwelling invertebrates, which subsequently provide food for foraging shorebirds and humans. Salt marshes and tidal flats are critical stepping stones in the flyways of migratory shorebirds, connecting breeding grounds at high latitudes with non-breeding grounds at lower latitudes.
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The loss of intertidal habitats has caused declines in shorebird populations (Piersma et al. , 2016; Studds et al. , 2017) and is a key driver for the focus on their restoration.
Filtering of nitrogen pollution: Salt marshes can act as a buffer from nitrogen pollution, caused by run-off from agricultural areas where fertiliser has been applied. The uptake of nitrogen by salt marsh plants can increase their biomass and reduce the amount of nitrogen entering the ocean (Nelson & Zavaleta, 2012). The presence of filter feeders in tidal flats can significantly reduce nutrient and pollution loads in the water column (Officer et al. , 1982). Supporting human livelihoods: Living in coastal areas provides opportunities for specific economic activities and trade (Kummu et al. , 2016). This is reflected in the fact that many large cities are close to the coast. Many communities are dependent on coastal ecosystems for food. For example, subsistence fishing provides protein-rich food and income in many countries (Bell et al. , 2009) and gleaning (the collection by hand of marine organisms from intertidal areas) is an economic activity specifically dependent on healthy tidal flat systems (Grantham et al. , 2021). Coastal areas also support tourism and recreational activities, although these can have negative consequences on sensitive coastal landscapes (Gormsen et al. , 1997).
Intertidal invertebrates are a key human food supply globally, with many communities being dependant on coastal systems for their livelihood. Left: A crab catcher searches for mud crabs at a tidal flat in Sonadia Island, Cox’s Bazar, Bangladesh. [Credit: Sayam Chowdhury]. Bottom: Traditional shellfish harvesting on the former tidal flats of Seamangeum in South Korea. These tidal flats, along with the shellfish harvesting grounds, have been lost due to the embankment at Seamangeum. [Credit: Ju Yung Ki, www.grida.no/resources/4418]
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Feeding, roosting and nesting sites for shorebirds
The availability of feeding, roosting and nesting sites in coastal habitats is essential for shorebirds. Many migratory shorebirds use coastal habitats as stepping stones in their migratory flyway. They use them as stopover or staging sites, where they stop for a period of days to weeks during migration to feed and refuel before a (often long-haul) flight (Warnock, 2010). Shorebirds mostly forage on tidal flats during low tide, following the tide as it moves across the flat. During periods of high tide they must leave their feeding grounds in intertidal areas. They move to areas where they are safe from high water and threats like predators, mostly to roost, but sometimes to continue feeding. They roost in so-called high-tide roosts (Rogers, 2003), either on exposed ground or in shallow water. The availability of both suitable high-tide roosting sites and feeding sites in a given area will affect bird abundance (Rogers et al. , 2006). Some shorebirds prefer to roost in the upper portion of the tidal flats where they feed, in areas above the water level, but will also roost (and forage) in man-made features in intertidal wetlands (Rosa et al. , 2006; Fidorra et al. , 2015; Scarton & Montanari, 2015), such as aquaculture ponds (such as fish or crab ponds) (Li et al. , 2013) or salt production ponds (Sripanomyom et al. , 2011). Some species actually show a preference for such artificial habitats (Green et al. , 2015). It has been suggested that artificial habitats provide a buffer, a secondary role, or a complementary habitat for shorebirds when natural sites are not available (Li et al. , 2013; Rocha et al. , 2017; Jackson et al. , 2019). There are some concerns about the reliance of shorebirds on artificial wetlands in coastal areas (Jackson et al. , 2020). For example, if aquaculture or salt ponds fall out of use, or if they are converted to other land uses, shorebirds may be at risk. Therefore, management of artificial habitats should be considered alongside natural habitat creation and restoration.
In addition to providing valuable stop over sites for migratory birds, many coastal areas also provide nesting habitat to birds. As roosting and nesting are vulnerable behaviours, shorebirds prefer sites that are safe from disturbance from humans or predators (Rogers et al. , 2006; Rosa et al. , 2006). However, with the continuing loss of coastal habitat, safe and accessible roosting and nesting sites are becoming fewer (Studds et al. , 2017). This means that birds may spend more time flying between foraging and roosting sites, which uses up their precious energy reserves needed for migrating or reproducing.
Maintaining undisturbed roosting sites is essential for shorebirds. Here, Whimbrels Numenius phaeopus are roosting during high tide in the upper tidal flats that have not been covered by seawater in Moreton Bey, Queensland, Australia. [Credit: Micha V. Jackson].
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Land reclamation has led to the loss of non-breeding stopover sites and subsequent population declines for shorebird species such as the endangered Great Knot Calidris tenuirostris (pictured above). Here they are feeding on the former Saemangeum tidal flat in South Korea. Developments on coastal wetlands has led to declines of non-breeding populations in South Korea. [Credit: Ju Yung Ki, www.grida.no/resources/4409].
Maintaining habitat for coastal birds can require sustaining both prey in the water column, as for the globally endangered Indian Skimmers Rynchops albicollis (top photo, Nijhum Dwip National Park, Bangladesh), and prey once the tide has receded, as for the critically endangered Spoon-billed Sandpiper Calidris pygmaea (right photo , Sonadia Island, Cox’s Bazar, Bangladesh). [Credit: Sayam Chowdhury].
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Box 1: The Yellow Sea ecoregion
The Yellow Sea is bordered by eastern China, Democratic People’s Republic of Korea (North Korea) and the Republic of Korea (South Korea). Salt marshes and tidal flats are the principal coastal ecosystems in this region (Murray et al. , 2015), yet nearly 65% of tidal flats and nearly 60% of salt marshes have been lost since the 1950s and 1980s, respectively (Murray et al. , 2014; Gu et al. , 2018). The Yellow Sea tidal flats are now considered an endangered ecosystem under the IUCN Red List of Ecosystems due to the decline in their extent, the severity of their degradation and biotic disruption (Murray et al. , 2015). Land reclamation is one of the main drivers of intertidal habitat loss in this region but existing intertidal habitats also suffer from degradation (Melville et al., 2016; Gu et al. , 2018). In South Korea, around half of the tidal flats have been embanked since the 1970s (Koh & de Jonge, 2014) and in China, the seawall stretches for 13,830 km along the coast (Luo et al. , 2015). A typical salt marsh community in North and South Korea is dominated by Phragmites communis and Suaeda japonica (Kolbek et al. , 1989; Ihm et al. , 2001; Chung et al. , 2021), while in China the most extensive species are Suaeda salsa, Phragmites australis, Aeluropus littoralis, Zoysia maerostachys and Imperata cylindrica (Yang & Chen, 1995). However, there is evidence to suggest that reclamation and embanking could change the distribution of vegetation from a zonal pattern to a mosaic pattern by altering the salinity gradient (Feng et al. , 2018). Now, in parts of the Yellow Sea, there is next to no salt marsh, even in areas where a tidal flat remains (Melville et al. , 2016).
The Saemangeum Seawall on the southwest coast of South Korea separates the former Saegmangeum estuary from the Yellow Sea. Its construction caused controversy and opposition from some citizens and environmental groups. [Credit: Ju Yung Ki, www.grida.no/resources/4415].
The damming of both the Yellow and Yangtze rivers has drastically reduced the sediment supply to the coastline (Yang et al. , 2006; Wang et al. , 2012). Water use for irrigation and human consumption in the upper reaches of the Yellow River has also considerably reduced the fresh water flow to its delta (Yang et al. , 2020). Coastal groundwater extraction is associated with subsidence of up to 25 cm/year (Higgins et al. , 2013). In China, sewage disposal and the movement of chemical industries to the coast increases the risk of chemical pollution (Melville 2018). There are also huge outbreaks of macroalgae (e.g. Ulva prolifera ) in China and South Korea, thought to be a result of multiple factors, including climate
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change, rising sea temperatures and eutrophication caused by increased nitrogen pollution (Zhang et al ., 2019)
Coastal areas in the Yellow Sea are threatened by the invasive Cordgrass ( Spartina sp. ), a group of grasses native to Atlantic, European and African coasts. Spartina species have been introduced intentionally and unintentionally to many coastal areas globally. Spartina occupies large areas of open tidal flats and can facilitate the accumulation of sediment (Crooks, 2002; Civille et al ., 2005). Spartina can be detrimental to shorebirds, making tidal flats and salt marshes inaccessible (Gan et al. , 2009; Jackson et al. , 2021; Lyu et al. , 2023), as well as by reducing the diversity of benthic macroinvertebrates. Research from the Wadden Sea and Australia shows that the diversity of arthropods and macrobenthos is higher in open tidal flat and native salt marsh than in Spartina -invaded marshes (Tang & Kristensen, 2010; Cutajar et al. , 2012). Spartina can outcompete native plants, including Zostera (Madden et al. , 1993), Suaeda (An et al. , 2007), Phragmites australis , and Scirpus mariqueter (Li et al. , 2022), decreasing the amount of food resources and nesting habitats for birds. The loss of intertidal benthic fauna, such as shellfish, can negatively impact human livelihoods (Gan et al. , 2010; Goss-Custard & Moser, 1988; Jackson et al. , 2021). Spartina is a well-known invasive, but it is not the only species to cause problems in intertidal habitats. For example, Black Swans ( Cygnus atratus ) are increasing in numbers in at least two coastal national nature reserves: Yellow River Delta and Chongming Dongtan (David Melville, pers. comm.). The coastline of the Yellow Sea is a critical area for migrating birds and their reliance on this habitat as a migration stop over is a major cause of their decline (Studds et al. , 2017). The East Asian-Australian Flyway (EAAF) is a major bird migration route, where birds travel from Russia, China and Alaska to South East Asia, Australia and New Zealand. The Yellow Sea stopover accounts for around 40% of the birds travelling on the EAAF, with a yearly influx of around 3 million individual birds (Studds et al. , 2017). It is a critical staging region where the birds stop to feed and refuel while they prepare for next steps in their long-haul migration flight. In response to human population growth, many coastal areas have been converted to aquaculture ponds for food production (Sun et al. , 2015), with China being the leading aquaculture producer in the world (FAO, 2020). Although artificial, aquaculture ponds can in fact provide roosting and foraging sites for shorebirds, depending on how they are managed (e.g. Bohai Bay in China, Lei et al. 2018). Therefore, integrating waterbird conservation with economic productivity is something to be considered (Ma et al. , 2010). A healthy intertidal zone will benefit both birds and commercially important benthos. Overall, the coastal ecosystems in the Yellow Sea ecoregion, and the species within, are under immense anthropogenic pressure. In 2018, China introduced strict regulations on land reclamation, whereby general land reclamation projects will no longer be approved (Miao & Xue 2021). In South Korea, opposition by citizens has had some success, for example a lawsuit by environmental groups was brought against the Saemangeum Reclamation Project, forcing the development to take the environment into account (Koh & de Jonge 2014; Song et al. , 2014). According to IUCN (2023), despite efforts to strengthen protection for habitats in the Yellow Sea, most especially in the intertidal zone, the trends for most species continue to decline (IUCN, 2023).
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Other sources of information
Documents
Techniques for restoring coastal habitats: Ausden M. (2007) Habitat Management for Conservation: A Handbook of Techniques. Oxford University Press, New York.
Videos
An example of the wider benefits of salt marsh and tidal flats: Cowden B. (2022, November 08) Rewilding the Essex coast [video]. Vimeo. www.vimeo.com/768722918
References
An S.Q., Gu B.H., Zhou C.F., Wang Z.S., Deng, Z.F., et al . (2007) Spartina invasion in China: Implications for invasive species management and future research. Weed Research , 47 , 183 – 191. https://doi.org/10.1111/j.1365- 3180.2007.00559.x
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