Temporal variation of diatom assemblages in oligotrophic and eutrophic streams

Abstract

Diatoms are used as indicators of water quality as they respond predictably to many water chemistry variables and their ecology is relatively well known. However, temporal variation in diatom assemblages remains poorly resolved and studies covering large temporal and spatial scales are urgently needed. Here, we studied how diatom assemblages vary within and between three years at eight boreal stream sites located in Finland using Mantel tests and ordination analyses. Our study streams differed greatly in their trophic level and size: Vantaanjoki is mesotrophic to eutrophic and large, whereas Luutajoki and Evojoki are oligotrophic and smaller. We found that species composition reflected well the status of the sampled streams. In Vantaanjoki, the most common diatoms were all typical species for mesotrophic to eutrophic streams, whereas oligotrophic streams were characterized by species with narrow ecological niches confined to nutrient-poor water. According to Non-metric Multidimensional Scaling (NMDS), the between-year variation was nearly comparable at all sites, eutrophic sites generally showing slightly higher between-year variation. The reasons for large between-year differences in assemblages were probably related to different weather conditions between the years. The Mantel tests also showed a notable degree of within-year variation in all assemblages. Therefore, we suggest that ecosystem size and productivity act in different directions in the data, resulting in no major differences in the turnover rates at sampled sites. However, NMDS analyses conducted simultaneously for the whole diatom data showed that temporal variation does not confound the large differences in assemblages between eutrophic and oligotrophic streams and that temporal variation may add slightly less noise in spatial assemblage variation in oligotrophic streams. This suggests that site classifications based on diatoms may be overall slightly more robust in oligotrophic waters, whereas in mesotrophic to eutrophic waters, misclassifications may be more likely due to large temporal variation in assemblages.