Taxonomy and ecology of epilithic diatoms in Irish lakes

Abstract

Benthic algae are significant primary producers and important in biogeochemical cycling in shallow lakes in particular, but epilithic diatom ecology is poorly understood compared to phytoplankton and higher trophic levels. I explored diatom taxonomy and ecology to investigate the suitability of epilithic diatoms for determining the biological condition of Irish lakes. I examined intraspecific variation in populations of two widespread diatom genera to understand the implication of diatom species-complexes for biodiversity and environmental assessments. Diatom composition in 220 samples from the epilithon of 91 limnologically diverse reference and impacted lakes was analysed, with a replicated spring and summer analysis in my study. Multivariate techniques enabled six characteristic groups of diatom assemblages and their relationship with key abiotic environmental variables to be identified. It revealed that physicochemical gradients of alkalinity-pH, total phosphorus and true colour explained the greatest amount of variation in diatom assemblages in both seasons. Lake classifications are successful when diatom metrics reliably reflect the response of assemblages to environmental stressor gradients. The Lake Trophic Diatom Index was validated for Water Framework Directive classifications after reference condition was assessed and a highly significant relationship with total phosphorus was identified. This represents an improvement on a previous study that found diatom indices to be poor indicators of trophic condition in Irish lakes. A key finding relative to comparable international studies was that the diatom-phosphorus relationship was not confounded by factors such as lake type. The clearest seasonal pattern that emerged was a consistently low variability in index scores in high status lakes, with variability increasing with declining trophic condition thereafter. Nevertheless, averaging seasonal metric scores provided broadly equivalent levels of explained variation relative to that reported for other phototrophic indices. The distribution, morphology and ultrastructure of Brachysira and Encyonopsis in a large number of river and lake populations was assessed. Examination of original type material for morphologically challenging Encyonopsis provided new information and taxonomic uncertainty in E. angusta was resolved by a proposed synonymy with E. krammeri. Four new diatom species were described, and greater morphological diversity was discovered. Further validation of nomenclatural types and molecular characterisation is required before many cosmopolitan morphospecies can be formally named. My analyses provided support for the concept of taxonomic redundancy within diatom complexes given the adequate performance of the diatom metric and comparability of species trophic optima when assessed with weighted averaging. However, I also found evidence for differences in ecological niches amongst new and established species that suggested potential for improvements in precision in environmental indices are also achievable. My study provided a rare example of a contemporary taxonomic investigation for a group of freshwater algae in Irish waters. The broader implication of my findings is that algal diversity in Ireland is actually quite poorly documented. As such, my research provides further opportunities to understand patterns of diatom diversity in the near future. The final chapter of this thesis summarises and discusses the role of diatoms and benthic algal indicators for lake phytobenthos classifications, with suggestions for future research areas.