Credit: ©ISTOCKPHOTO / OKRAD

Salt marshes and tidal wetlands line many coasts, harbouring wildlife and supporting commercial fisheries. Coastal lands are vulnerable to a range of natural and anthropogenic threats. Thousands of hectares of marshes and wetlands are reclaimed by the open ocean each year through storm surges and flooding, and as increasing atmospheric carbon dioxide levels warm the Earth, rising sea levels may pose yet another threat to the survival of the marshes.

However, J. Adam Langley of the Smithsonian Environmental Research Center, Maryland, and colleagues have now shown that rising levels of carbon dioxide in the atmosphere could actually help tidal marshes keep up with the rising seas (Proc. Natl Acad. Sci. USA 106, 6182–6186; 2009). They exposed patches of the Kirkpatrick Marsh, which borders Chesapeake Bay, to either ambient levels or an extra 340 ppm of carbon dioxide from 2006 to 2007. Both grass and root productivity were significantly higher in the marsh patches exposed to air rich in carbon dioxide, although this effect was muted by a severe drought in the autumn of 2007.

Overall, the patches treated with the higher carbon dioxide concentration gained about 3 mm in elevation per year, whereas the marsh patches at ambient carbon dioxide levels sank by just under 1 mm each year. The researchers attribute most of the land rise to increasing root mass, with some contribution from an accumulation of litter from the grass.

However, the experiments also revealed a destructive effect of anthropogenic nitrogen on the marsh, even within patches where growth had been enhanced with additions of carbon dioxide. Marsh patches exposed to high carbon dioxide and nitrogen showed no overall change in elevation, possibly because the nitrogen stimulated soil decomposition while hindering root productivity. So although coastal marshlands may be more resilient to the total effects of rising carbon dioxide levels in the atmosphere than we had thought, fertilizers running off from lawns and golf courses could prove to be their downfall.