Dee Estuary

• Category: Marine and Oceanic Biomes.
• Geographic Location: Europe.
• Summary: A major estuary on the Irish Sea, the Dee supports important bird populations; its history of human development, along with rising sea levels, call for significant mitigation and protective measures.

Emptying into the Irish Sea, the Dee Estuary divides England and Wales on the northwest coast of Britain. It occupies a large and wide basin of some 37,000 acres (15,000 hectares) in area, originally formed by glacial erosion that took place in the ice ages that shaped much of northern Europe. The current geomorphology of the estuary is a result of its more recent geological history; sediment deposition; and a large tidal cycle, which is characterized by a rapid flood and a much longer ebb. Like many of the estuaries around the coasts of Britain, the Dee is an important area for wildlife, particularly for breeding and wintering populations of wildfowl and waders. As a result, it is designated as a legally protected wildlife site at both national and international levels.

The most widespread habitats within the estuary are intertidal areas of mudflat, sand, and salt marsh. The salt marsh forms extensive stands in the Dee Estuary, especially in sheltered areas with low levels of wave action and tidal scour (an erosion process that is carried out by the tidal movement of water).

The salt-marsh vegetation often occurs in a series of zones, from the sea toward land. On recently colonized bare seaward fringes is pioneer vegetation, consisting of glassworts and annual sea blite. This vegetation develops into a more mature salt marsh at a middle elevation on the shoreline. Finally, there is a transition to grassy vegetation at the highest levels, including swamp vegetation usually dominated by common reed or sea club-rush.

In contrast to these large areas of soft substrate, any rocky intertidal habitat is relatively scarce in the estuary. The rocky shores of the sandstone Hilbre and Little Hilbre Islands (immediately offshore and often connected to the mainland at low tide) support a variety of habitats, including rock pools, bedrock ledges, gullies, and crevices. Reefs formed by honeycomb worm are a particular feature of the lower shore of Hilbre Island. Small but important areas of maritime cliff, heath, and grassland vegetation also occur above the tide line.

Wildlife

The Dee Estuary is of major international importance for birds, with more than 130,000 individuals supported in the winter months and during migration. These birds are mostly waders, such as redshank, dunlin, and knot, although wildfowl such as teal and pintail also are common. The birds make use of the intertidal flats and salt marshes for feeding and roosting, and they commute to neighboring land, shingle spits, and islands for roosting at high tide.

In summer, the site supports breeding populations of common tern and little tern at levels of European importance. An area by Gronant, near the mouth of the estuary, is Wales's only colony of breeding little terns; they winter off the shoreline of western Africa before returning to the estuary to nest.

Besides birds, a range of other notable species are supported by the estuary, including grey seal, Atlantic salmon, lamprey, twaite shad, the nationally scarce thumbnail crab, and the endemic (found nowhere else on Earth) rock sea lavender, a perennial herb.

Environmental Threats

Potential influences on the conservation front of the Dee Estuary come from both natural and anthropogenic processes. The Dee Estuary is a naturally accreting ecosystem, with a net inflow of sediment from Liverpool Bay. As a result, a steady increase in the extent of salt-marsh habitats is expected over time, at the expense of intertidal flats. This will decrease the amount of open, invertebrate-rich habitat on which the wintering and migratory bird populations currently depend.

Flocks of birds, whether breeding, wintering, or on passage, need areas free from disturbance in which to nest, feed, and roost. To maintain such populations that currently use the estuary, human activities such as wildfowling, recreation (e.g., angling, sailing, waterskiing, windsurfing), and the operation of the shellfish fishery need to be managed appropriately. All these activities have the potential to disturb birds and, in bad weather, are likely to increase the mortality of birds, especially at favored feeding and roosting sites. To minimize this risk, attempts have been made to reduce disturbance in the estuary by voluntary agreements such as no-shooting zones, education, and visitor management.

Erosion by waves and tides threatens coastal areas here; the scarcity of wildlife at the Hilbre Islands is exacerbated by the rising sea level resulting from warmer temperatures and by the ongoing threat of global climate change. Historical sea defenses are present, such as sandstone walls and cliffs, but their ability to withstand changes in the estuary will reduce over time, and the scope for managed retreat needs to continue to evolve.

The ability of the estuary shores to adapt to these changes is constrained in many places by coastal defenses. As a result, the intertidal area may become squeezed between rising sea levels and fixed flood barriers. This issue will require innovative approaches of coastal management, with flood defenses being relocated, or land being set aside and allowed to develop naturally into new intertidal habitats, such as salt marsh.

The current and historical commercial uses of the Dee Estuary have had, and will continue to have, an effect on the estuary through their impact on the surrounding land as well as on the quality of water and sediment that enters the coastal waters. Heavy industry, coal mining, and metal smelting have left a legacy of contaminated land around the estuary. Although some of this activity has been subject to remediation, other sites remain in poor condition.

An obvious concern for such areas is their possible erosion by the rising sea and the consequent leaching of noxious pollutants into the environment. Possible new risks also have been introduced for the bird populations in the estuary, through the development of three offshore wind farms close to the mouth of the Dee at North Hoyle, Burbo Bank, and Gwynt-y-Mor. There is concern that these sites could disturb roosting birds or increase mortality through collision with turbine rotors, especially if located in critical flight lines or on migration routes. To minimize this risk, their potential effects on the ecology of the estuary have been subject to detailed environmental impact assessment and ongoing monitoring programs.

A proposed tidal lagoon in 2015 by Tidal Lagoon Power in the Dee Estuary concerned environmentalists, who expressed fear it could harm wildlife, in particular birds. A first in Wales, the tidal lagoon would be used to generate electricity, providing it to more than 80,000 homes. Supporters of the plan argued it would be an important source of green energy, thus cutting carbon emissions that cause global warming and climate change, and also recognized its potential economic benefits. However, the project came to a halt when Tidal Lagoon Power lost an appeal to continue development after it failed to commence the project within five years after receiving its development consent order.

Bibliography

Allen, J. R. L. and K. Pye, eds. Saltmarshes: Morphodynamics, Conservation and Engineering Significance. Cambridge University Press, 2009.

Archer, Mike, Mark Grantham, Peter Howlett, and Steven Stansfield. Bird Observatories of Britain and Ireland. London: Poyser, 2010.

Dee River Basin Management Plan 2021 – 2027 Summary. Natural Resources Wales, July 2022, cdn.cyfoethnaturiol.cymru/media/695219/dee-rbmp-2021‗2027-summary.pdf. Accessed 5 Nov. 2024.

“Flintshire Tidal Lagoon Scheme Surges Ahead after Reaching Two Big Milestones.” Deeside, 9 Mar. 2022, www.deeside.com/flintshire-tidal-lagoon-scheme-surges-ahead-after-reaching-two-big-milestones/#google‗vignette. Accessed 5 Nov. 2024.

Guerra-Chanis, Gisselle E., et al. "Saltwater Intrusion in Estuaries with Different Dynamic Depths." Regional Studies in Marine Science, vol. 51, 2022. ScienceDirect, doi.org/10.1016/j.rsma.2022.102186. Accessed 5 Nov. 2024.

Kirby, J. S. The Ornithological Significance of the Mostyn Docks Area of the Dee Estuary to Wildfowl and Waders. Thetford, UK: British Trust for Ornithology, 1993.

Prater, A. J. Estuary Birds of Britain and Ireland. London: A & C Black, 1981.

Thomas, Rob. "Tidal Lagoon: Wildlife Fears over Dee Estuary Plans." BBC, 17 Mar. 2022, www.bbc.com/news/uk-wales-60765985. Accessed 5 Nov. 2024.

“Welsh Government Llaunches £750,000 Tidal Lagoon Challenge.” Nation CYMRU, 3 July 2023, nation.cymru/news/welsh-government-launches-750000-tidal-lagoon-challenge/. Accessed 5 Nov. 2024.