Our study will be focused on both planktonic and periphytic species. We will analyze the present-day composition of these communities for each type of substratum and habitat from four different sites distributed along a south to north gradient (1) Southern shore of Great Whale River, «SAS» site (55°14'N; 77°42'W); 2) Near (~15-20 km NE of) Kuujjuarapik-Whapmagoostui, «KWK» site (55°20'N; 77°30'W); 3) Sheldrake River near Umiujaq, «BGR» site (56°37'N; 76°13'W); 4) Nastapoka River, «NAS» site (56°55'N; 76°23'W)), in order to establish and quantify the relationships between species and environmental parameters. Finally, we will apply the transfer functions on the fossil diatoms preserved in the sedimentary sequences of some select sites. This will allow us to infer the evolution of limnological conditions through time, and to predict the response of these ecosystems to global warming.