Subject: Impact of climate change on intertidal ecosystems

Author: Alan Southward, Marine Biological Association, Plymouth, UK

The main impact of fluctuating climate on intertidal ecosystems would be through changes in sea level and temperature. There is considerable evidence over past epochs of changes in sea level; deposits of shells of intertidal molluscs and barnacles are found above and below present sea levels, including the well-known ‘submerged forests’ and ‘raised beaches’. There is a raised beach at Saunton Devon, where putative Semibalanus balanoides of age greater than 35,000 y BP is present several metres above the present barnacle zone, still in place on the underlying rock beneath a layer of mollusc shells (Kidson and Wood, 1974; Kidson and Tooley, 1977; and pers. obs.). In theory, intertidal organisms should be able to keep pace with changing sea level, providing it was spread over several generations of the organisms. However, we need to put into place a mechanism for recording response of intertidal life to sea level change. This would involve annual surveys of quantitative transects, including photographs, related to geodetically calibrated land survey datums. There is some urgency in this matter.

We have considerable recent evidence of the effects of small changes in temperature on distribution and abundance of intertidal organisms (Southward, 1991; Southward et al, 2004). However, there is a tendency to regard temperature change in the sea as resulting in latitudinal extension or withdrawal of species. This may well be the case for pelagic fish, but the intertidal zone is extremely indented, markedly so in Europe with its many subsidiary seas, bays and islands. Also, owing to the high heat capacity of the ocean, sea temperature is a more ‘conservative’ parameter, so that at low tide intertidal organisms are exposed to the greater fluctuations of air temperature. Hence, observed changes in intertidal biodiversity can show east/west trends in species range and abundance as well as north/south trends. For example, during rising temperatures in the last two decades, recognised ‘southern’ species of barnacles and molluscs have moved eastward along the English Channel and from north to south down the east coast of Scotland (Hawkins et al., 2003; Herbert et al. 2003, 2007; Hiscock et al., 2004; Mieskowska et al., 2006 ; pers.comm. M.T. Burrows).

Long term biodiversity data for the intertidal zone are mostly for rocky shores, which are easy to survey with minimal destructive sampling. Unfortunately, a good deal of past information on intertidal organisms has tended to be qualitative or at best semi-quantitative. In addition to routine quantitative monitoring to follow the effects of slow changes in climate, we need to record the effects of acute and chronic disturbances that damage keynote species, e.g. oil spills and clean up operations and the use of tri-butyl tin based antifouling coatings. There is a need for annually repeated quantitative transects of the intertidal zone that will also aim to show these effects and any result of change in sea level. An example of a quantitative transect monitored each year is shown in the appended Figure. Such fluctuations in the relative abundance of cold water and warm water barnacle species near Plymouth in the second half of the 20thC are paralleled elsewhere in south west England by corresponding fluctuations in intertidal organisms and by changes in distribution of plankton and fish.

References:
Hawkins, S.J., Southward, A.J. & Genner, M.J. 2003. Detection of environmental change in a marine ecosystem – evidence from the western English Channel. Science of the Total Environment 310, 245-256.
Herbert, R.J.H., Hawkins, S.J., Sheader, M. & Southward, A.J. 2003 Range extension and reproduction of the barnacle Balanus perforatus in the eastern English Channel. Journal of the Marine Biological Association of the United Kingdom 83, 73-82.
Herbert, R.J.H., Southward, A.J., Sheader, M. and Hawkins, S.J. 2007. Influence of recruitment and temperature on distribution of intertidal barnacles in the English Channel. Journal of the Marine Biological Association of the UK 87, 487-499.
Hiscock, K., Southward, A., Tittley, I. & Hawkins, S. 2004. Effects of changing temperature on benthic marine life in Britain and Ireland. Aquatic Conservation: Marine and Freshwater Ecosystems 14, 333-362.
Kidson, C. and Wood, R. 1974. The Pleistocene stratigraphy of Barnstaple Bay. Proceedings of the Geological Association 85, 223-237.
Kidson, C. and Tooley, M.J. (eds) 1977. The Quaternary history of the Irish Sea. Liverpool, Seel House Press.
Mieszkowska, N., Kendall, M.A., Hawkins, S.J., Leaper, R., Williamson, P., Hardman-Mountford, N.J. & Southward, A.J., 2006. Changes in the range of some common rocky shore species. A response to climate change? Hydrobiologia, 555, 241–251.
Southward, A.J. 1991. Forty years of changes in species composition and population density of barnacles on a rocky shore near Plymouth. Journal of the Marine Biological Association of the UK 71, 495-513.
Southward, A.J. et al. 2004. Long-term oceanographic and ecological research in the western English Channel. Advances in Marine Biology 47, 1-104.



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