Birdwatch News Archive

Sea Lavender is conspicuous on a natural salt marsh at Holkham, Norfolk, but is notably deficient in marshes created by managed coastal realignment, which have much less biodiversity overall. Photo: David C J White.

Man-made salt marshes inferior

Posted on: 21 Sep 2012

New salt marshes created during coastal realignment have reduced biodiversity according to a new study by the University of East Anglia.

Under the EU Habitats Directive, new salt marsh must be created each time natural salt marsh is lost to coastal development, or to the coastal erosion caused by the sea-level rising. The new marshes must display “equivalent biological characteristics” to their natural counterparts, but the new findings published in the Journal of Applied Ecology, reveal that artificially created salt marshes suffer significantly reduced biodiversity.

The researchers analysed the vegetation of 18 marshes created through deliberate coastal realignment since 1991 and 17 marshes created accidentally by storm surges since 1881. They compared the salt-tolerant plants at these sites with those at 34 natural salt marshes and found that some key species were very poorly represented. These included Sea Lavender, Thrift, Sea Arrowgrass and Sea Plantain.

A created salt marsh in Lincolnshire is dominated by Annual Seablite, Common Saltmarsh Grass and Sea Purslane, but is an otherwise depauperate habitat. Photo: Alastair Grant.

“Salt marshes such as those in North Norfolk, Essex and around much of the coast of England are loved by naturalists and tourists alike for their natural beauty and rich ecology,” said lead author Dr Hannah Mossman of UEA’s School of Environmental Sciences. “These unique tidal areas also provide vital habitat for invertebrates, a staging post for migrant birds and the only environment in which a number of salt-tolerant plants can survive.”

Rising sea levels caused by global warming are posing a growing threat to the country’s coastal salt marshes and increasing the cost of maintaining sea defences. This challenge is increasingly being met in Britain by relocating sea walls further inland (termed 'managed coastal realignment') and allowing new salt marsh to develop.

Prior to this in-depth five-year study, it had been assumed that this process caused no loss of species richness and that man-made marshes compensated well for the loss of natural ones. The researchers found, on the contrary, that plant life on artificially created salt marsh tended to be dominated by early-colonizing plants such as Marsh Samphire because of a lack of oxygen in the sediment. Shrubs such as Sea Purslane can also quickly become unduly dominant. The sites tended to be flat and featureless with scrappy vegetation. Even the older, accidentally-created marshes were deficient in the characteristic salt marsh perennials.

“Our findings demonstrate very clearly that marshes created by managed realignment are not biologically equivalent to natural ones and therefore fail to satisfy the biodiversity requirements of the EU Habitats Directive,” said co-author Professor Alastair Grant of UEA’s School of Environmental Sciences.

Professor Anthony Davy of UEA’s School of Biological Sciences added: “These created marshes are certainly better than losing our precious natural salt marsh altogether, but they are not a good enough replacement. We are currently developing ways of enhancing their biodiversity, such as improving local drainage conditions and planting the deficient species.”

The paper is available here