Temperature and Climate

Sediment loading

The effects of emersion, wave-action and topography can be modified by other important factors. The geographical situation of any shore will determine the species which are available to colonise it, the degree of exposure to wave action and the stresses which organisms face at higher shore levels.

In general terms, water temperature is the most important factor determining the distribution of marine organisms (Lüning and Asmus, 1991). Air temperatures are also important for littoral species.

Many species reach the northern or southern limits of their distributions around the UK (Lewis, 1964). These limits can fluctuate, responding to fluctuations in climate (Crisp, 1964; Southward et al, 1995). Climate also affects the relative competitive abilities of species. This is particularly well illustrated by the barnacles Semibalanus balanoides, Chthamalus stellatus and C. montagui. On shores on the West coast of Scotland, C. stellatus and C. montagui are restricted to high shores as S. balanoides, the northern species, is competitively superior at these latitudes (Connell, 1961a,b). Further south in the UK, the two genera coexist in the mid shore (Crisp et al., 1981).

Climate can also affect the degree of disturbance suffered by shore communities. The extent of damage inflicted by water-borne debris increases as wave exposure and storms become more severe. At very high latitudes, where sea water freezes, ice scour can cause massive mortality of encrusting species during the winter months, though this rarely affects UK shores. Occasional hot summers can cause a disturbance on British rocky shores (e.g. Hawkins and Hartnoll, 1985)

The supply of particulate organic matter to shores will depend on the local hydrodynamic regime, especially in relation to local processes of erosion and fluvial input. More of this material will arrive at the shore by advection when the prevailing wind blows towards the shore. Species tolerant to low salinities are able to colonise shores in estuaries and bays with substantial freshwater inflow. Offshore events help structure rocky shore communities not only by affecting levels of wave exposure and food supply but also by affecting the survival and onshore transport of larvae.

Next Section                     References