## 1 Introduction  
Cyanobacterial proliferations often occur in reservoirs used for the supply of drinking water or recreational activities. As some species are able to synthetize harmful toxic compounds, cyanobacterial blooms represent a significant risk to ecosystems and human health [1; 2; 3]. Those proliferations are expected to be enhanced by eutrophication [2] and global warming [4]. In freshwater ecosystems, the hepatotoxins microcystins (MC) are the most frequently produced toxins by cyanobacteria [5]. MC are mainly intracellular during cyanobacterial proliferations, with less than 30% of the total concentration being extracellular, and are mainly dissolved in water only during bloom senescence [6]. MC have a tropism towards organs containing organic-anion-transporting polypeptides (OATPs). OATPs are a superfamily with some (OATP2A1, OATP3A1 and OATP4A1) expressed ubiquitously in humans, and others, as those involved in MC transport (e.g., OATP1B1, OATP1B3 and OATP1A2), are assumed to be organ specific (e.g., intestine, liver or digestive gland, kidney, heart, lung, spleen, pancreas, brain) [7; 8; 9; 10]. In hepatic cells, MC can inactivate protein phosphatases (PPases) [11; 12; 13], causing an increase of cytoskeletal protein phosphorylation, an impairment of the cytoskeleton structure and loss of shape, leading to interstitial haemorrhage into the liver [10; 14]. To evaluate the contamination of water by cyanobacteria and cyanotoxins, traditional surveys are based on a visual monitoring coupled with water sampling. The recommended sampling frequency is twice a month and should be increased when the appearance or turbidity of water is changing [1]. However, a twice-monthly sampling may not be representative of the level of water contamination by intracellular MC (in MC-producing cyanobacteria), as it is possible to miss either an entire bloom occurring in between [15] or one at its early stage. Indeed, pressure exerted by environmental factors can lead to rapid proliferation or sudden collapse of cyanobacterial populations [16]. Moreover, some cyanobacteria (e.g., _Planktothrix_ sp.) are able to display daily vertical migratory cycle and can also move horizontally under the effects of winds and movements of water bodies [17; 18]. Therefore, rapid spatial variations in the localisation of cyanobacterial populations in the water column may increase uncertainties in the representativeness of a punctual sample. Similarly, the toxin production may greatly vary depending on environmental factors or stages of cyanobacterial growth [19]. This study investigated the use of bioindicator species, temporally integrating water contamination levels by toxin-producing cyanobacteria, to overcome this sampling issue. Freshwater bivalves, being filter-feeders of limited mobility, were already proposed as sentinel species [20], and used to detect the presence of metals [21; 22; 23], microplastics [24], organochlorine contaminants [25], parasites [26; 27] and MC-producing cyanobacteria [28]. The zebra mussel _Dreissena polymorpha_ ([PERSON], 1771) is a small (below 50 mm) sessile bivalve originated from the Ponto-Caspian region and nowadays widespread in the northern hemisphere and therefore qualified as invasive [29]. On the opposite, the freshwater mussel _Anodonta anatina_ ([PERSON], 1758) is a vagile bivalve [30] present throughout Europe [31] and classified as least concern by the IUCN [32], although some population are declining [31]. Both _A. anatina_ and _D. polymorpha_ are known to filter colonial (i.e., _Microcystis aeruginosa_) and filamentous (i.e., _P. agardili_) cyanobacteria [33; 34; 35; 36]. After cyanobacterial cell ingestion, intracellular MC are liberated during the digestive process, and mainly accumulate in the digestive gland of invertebrates [37; 38]. MC can bind to proteins through a reversible hydrophobic bond, forming covalent adducts after several hours [10].