What determines the diatom communities of submerged freshwater plants? Implications for the use of community indices in determining ecological quality

Murray-Darling Freshwater Research Centre

MDFRC item.

The European Water Framework Directive requires establishment of a typology of aquatic habitats, the determination of ecological quality in each type and the use of composition and abundance of flora in doing this. Experienced phycologists are often able to relate species or genera of diatoms to particular environmental conditions but there is a need to formalise this into mechanistic indices which, if they are to be used in statutory ways to define ecological quality, must have high reliability. Features of the epiphytic diatom communities on five freshwater macrophytes have been related to environmental factors reflecting morphometry, major ion (base) status, and nutrient status in statistically similar subsets of 13-19 lakes from a series of 46 relatively fertile shallow Polish and UK lakes. Principal components and redundancy analyses showed that functional groupings of the diatoms were more successfully related to environmental factors than taxonomic features but that even then only a small part of the variation was explained by the environmental drivers. More than half of the variance was attributable to other, likely biotic, features.Use of the diatom flora in determining ecological status must therefore employ very general features of the community. The discrepancy between perception of indicator species by experienced phycologists but not by multivariate statistics lies first in the fact that such indicators are usually of gross base status (and relevant more to typology) rather than to subtleties of fertility that may reflect ecological quality and secondly because a more complex set of algorithms is used by the human brain than yet available in mechanistic indices.