Diatom Red Lists: important tools to assess and preserve biodiversity and habitats in the face of direct impacts and environmental change

Freshwater biodiversity is disproportionally endangered and under-prioritized. Algal Red Lists literature is very limited, and there are only two editions of a single diatom-specific Red List (developed for Germany), in spite of the importance of diatoms for biodiversity and global ecosystem functioning. We used and analysed the new diatom Red List, compared to the previous edition, to show that these diatom microalgae threat status data allow, on one hand, a characterization of the ecological integrity and of the diatom diversity of the different types of inland-water ecosystems (also of practical importance to designate the most relevant habitats for conservation purposes), including a clear assessment of the threat status of the habitat, and, on the other hand, they offer ample possibilities to track the effects of stressors and of environmental change. Our results revealed, among other things, that: (1) threatened taxa concentrate in dystrophic-oligotrophic environments; (2) ‘not threatened’, i.e. tolerant and opportunistic, taxa are most frequent in eutrophic and saline ecosystems; (3) most local diatom extinctions happened in carbonate oligotrophic habitats. In this study, nitrates could be shown to possess a highly significant negative association with the cumulative percentage of diatom taxa in threat categories of the Red List. Having included heterogeneous studies (diverse inland-waters, different geographic areas, from close-to-pristine to impacted, neo- and paleolimnology etc.), we think that this strong negative association is noteworthy and points to the high potential of diatom-Red-List based approaches, especially if these would be tailored for the different biogeographic and climatic ecoregions.