Transfer pathways of cyanotoxins throughout pelagic foodwebs of peri-alpine Lakes and their implications in the physiological responses of fish

Many deep peri-alpine lakes which are currently under re-oligotrophication are impacted by Planktothrix rubescens blooms. Among cyanotoxins produced by P. rubescens, microcystins (MCs) are the most commonly studied and are involved in many ecological disturbances. In peri-alpine lakes, these blooms might affect exploited fish species due to toxin contaminations. However, there is still a lack of knowledge regarding both transfer pathways of the toxin and their physiological effects on fish populations. In this work, two models of fish populations were considered (Young-Of-Year (YOY) perch and whitefish which exhibit distinct vertical distributions). The main objectives were on the one hand investigated through an experimental approach to analyze on the two model species, accumulation and detoxification processes as well as genotoxic effects of MCs. From these experiments, we showed that YOY perch and whitefish were able to detoxify part of the MCs but that genotoxic effects were still observed. On the other hand, ecosystemic approaches allowed (1) to explore the potential effects of the presence of bloom on the spatial distribution of fish, (2) to assess the levels of contamination by MCs for both fish species taking into account the seasonal variability, (3) to identify the main zooplanktonic vectors of MCs to fish. Our results revealed that the filaments of P. rubescens are a food resource for zooplankton grazers, and that a trophic transfer of MCs exists from herbivorous to their zooplanktonic predators and in fine to fish. Our results also highlight that the contamination pathways and their intensities could vary both at seasonal and daily scales. These variations are explained by both changes in fish diet, daily variations in the production of MCs by P. rubescens, and abundances and vertical distributions of zooplanktonic preys. Overall, we highlighted the importance of MCs trophic transfer in fish intoxication in the case of chronic contaminations observed in peri-alpine lakes (moderate to severe cyanobacteria/toxins concentrations over several months).