Water movements and their role in rocky shore ecology

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Abstract
Water movement is considered in terms of water flow and wave crash, although differentation is not always possible. Ocean currents and residual drift mainly effect geographical distribution and the spread of introduced species. Tidal streams, in the absence of wave crash, control local distribution through their influence on chemical conditions, sediment and turbidity, transport of food and reproductive stages. The richest littoral faunas are associated with tidal rapids. The biological effects of wave crash are illustrated by reference to zonation and local distribution. Under the latter heading the physical and biological gradients between exposed and sheltered sites are examined and emphasis is placed upon the transportation of reproductive stages as a major influence determining distribution, and one which can lead to a breakdown in conventional concepts of exposed and sheltered types of populations. Methods of measuring exposure are discussed and it is suggested that the dynamic, fluctuating nature of local distribution makes it inappropriate to devote excessive time and effort to trying to establish quantitative correlations between distribution and wave action.