The use of macroalgal species richness and composition on intertidal rocky seashores in the assessment of ecological quality under the European Water Framework Directive.

The EC Water Framework Directive (WFD) suggests using abundance and species composition of intertidal seaweed communities for ecological quality classification of rocky seashores. There are two difficulties with this. According to WFD all sensitive species should be present on a shore. There is no accepted list of sensitive seaweed species and those which may be sensitive in one location may not be so in another. Second, natural successions can result in very large abundance changes of common species, e.g. from almost completely fucoid-dominated shores to almost totally barnacle-dominated shores, without any change in ecological quality. Studies have shown that numerical species richness, not the list of actual species present, is broadly constant in the absence of disturbance. The ephemeral species, possibly the sensitive members of the community, change regularly in such a way as to conserve species richness. It is proposed that species richness on a defined length of shore be used as a criterion of ecological quality. A database of species found on over 300 shores in the British Isles, under strictly controlled sampling conditions, has given ranges of values of species richness to be expected and has allowed for variations in these values due to sub-habitat variability, wave exposure and turbidity to be factored in. A major problem in applying such a tool is the lack of expertise of many workers in critical identification of seaweed species. A reduced species list has been extracted from the database using species commonly present and identifiable with reasonable certainty. A numerical index of ecological quality is proposed based on scores for various aspects of the physical nature of the habitat combined with a score for species richness which may be based on the reduced species list. The scoring system also uses further aspects of community structure, such as ecological status groups and the proportions of rhodophyta, chlorophyta and opportunist species. For this system to be effective there has to be close control of the way in which sampling is carried out to ensure a uniform level of thoroughness.

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