Freshwater diatoms as environmental indicators: evaluating the effects of eutrophication using species morphology and biological indices.

Anthropogenic eutrophication is a major form of perturbation in freshwaters, and several approaches aim to recognise its effects on lake ecosystems. We compared the responses of diatom species morphology, diversity indices and diatom indices to total phosphorus, total nitrogen and distance from a point stressor causing eutrophication in a large lake. We specifically examined the degree to which extent nutrients and distance to the stressor affect variation in the values of various biological indices and diatom valve size. In addition, special attention was given to the adequate repetition of diatom valve width measurements in the context of environmental assessment. Our results showed that diatom valve width was a better indicator of nutrient concentrations than any of the diatom and diversity indices examined. However, the results varied between the two study transects, suggesting that the diatom-based variables not only respond to nutrients but also to other environmental factors (e.g. shoreline morphology). We also found that when using the method based on diatom morphology, one should measure more valves than has been originally suggested to provide a more reliable picture of response to eutrophication. We argue that diatom morphology could be considered as an additional environmental assessment tool, because it may complement the information provided by the traditional diatom indices. Diatom valve width may also be more sensitive to early phases of the eutrophication process and its effects on freshwater ecosystems than various diatom indices that were developed in regional contexts with wide ranges in nutrient levels.