Is eutrophication really a major impairment for small waterbodies’ biodiversity? | L'eutrophisation est-elle réellement une menace pour la biodiversité des petits plans d'eaux?

Eutrophication is and will stay a major stress for freshwater biodiversity. Moreover, climate warming, land-use changes and other human pressures are increasing the eutrophication. The deleterious consequences of eutrophication on biodiversity were often evidenced for large waterbodies. For smaller waterbodies, like ponds and small lakes, which can support naturally high levels of nutrients, the response to eutrophication is not necessarily identical. Furthermore, the impact of eutrophication on biodiversity could depend on the scale considered, from local (the waterbody) to regional (the network of waterbodies). It is also unclear if the richness of threatened species responds in the same way than the total species richness. The present study investigated, at the local and regional scale, the consequences of eutrophication on taxonomic richness (all taxa) and conservation value (threatened taxa) from temperate lowland ponds and small lakes. Six taxonomic groups were investigated: macrophytes, aquatic macroinvertebrates as a whole and with a particular focus on gastropods and water beetles, adult dragonflies and amphibians, in two data sets covering a large nutrient gradient from mesotrophic to hypertrophic conditions. At both local and regional scales, biodiversity responses to eutrophication could not be described by a general pattern, because the responses varied highly according to the taxonomic group. For macrophytes, local richness and conservation value decreased with eutrophication and a slight contribution of waterbodies of high nutrient status to the regional diversity was revealed by beta and gamma diversities as well taxon accumulation curves according to increasing or decreasing nutrient status. In contrast, for amphibians, an absence of impairment by eutrophication was revealed at both local and regional scales. Dragonflies, gastropods and water beetles showed intermediate situations. No surrogate taxonomic group describes the impact of eutrophication on small waterbody biodiversity, reinforcing the need to consider different taxonomic groups in conservation action plans. Moreover, eutrophication is not a major impairment for all groups, especially if it does not alter all waterbodies of a regional network. However, as nutrient-rich small waterbodies are expected to be dominant in our future landscape, conservation efforts should favor waterbodies with lower nutrient status in order to favor beta and gamma diversities.