Many Irish limestone lakes contain a characteristic benthic crust, a microbialite layer composed of cyanobacteria, algae and inorganic mineral deposits. Previous work suggests that these crusts are influenced by lake nutrient status. The purpose of this paper was to test this hypothesis. In order to do this, it was first necessary to describe crust structure, chlorophyll content and species composition. Relative abundances of species were examined, as well as crust cover, thickness and organic content. A diverse community of filamentous and coccoid cyanobacteria was recorded, with chlorophytes and diatoms also present. Crusts were found to contain a high percentage (>75 of dry mass) of mineral particles, bound together by various filamentous cyanobacteria. Significant differences were shown between lakes of different trophic states, based on analyses of biological and physical factors. Strong correlations were found between total P, crust chlorophyll and crust community structure.
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