Decline of freshwater gastropods exposed to recurrent interacting stressors implying cyanobacterial proliferations and droughts

Freshwater biota increasingly undergo multiple stressors, but we poorly understand to what extent they influence the dynamics of community structure. Here, we study the impact of combined stressor exposure on gastropods at 9-year interval, through a monthly 1-year (2013) monitoring, also providing data on the occurrence of other macroinvertebrate taxa. Previous study in 2004 showed the occurrence of cyanobacterial proliferations, drought, trematode parasites and invasive non-native pulmonate Physa acuta . During the year 2013, we always detected cyanobacterial microcystins (MCs) in gastropods, from 59 to 4149 ng g −1 fresh mass (vs. 0–246 ng g −1 in 2004), suggesting a continuous and increased MC intoxication. Environmental intracellular MC concentrations were high (8–41 µg L −1 ) from August to October 2013, whereas they were detected only in August 2004 (17 µg L −1 ). In 2013, we recorded no trematodes among the 2490 sampled gastropods, and P. acuta represented 94% of gastropods (vs. 58% in 2004). After August 2013, nearly all gastropods disappeared as most other macroinvertebrates (except Chironomidae, Ephemeroptera and Trichoptera). The whole decline of gastropods and other macroinvertebrates, and the absence of trematodes strongly suggest adverse conditions in the study site. Despite acute stressful conditions suggested above, gastropod abundance was 13-fold higher in June 2013 (vs. 2004), reflecting successful recolonization and efficient breeding. Most gastropods exposed to drought and toxic bloom were young vulnerable stages. Thus, we supposed alternation of local gastropod extinctions versus recolonization that could induce, on a long term, a loss of diversity to the detriment of the most sensitive species.