Microplastic mass balances in remote alpine lakes

Microplastics pose a threat to all the environmental compartments and specifically to aquatic ecosystems. Lakes are particularly exposed to it as they act as accumulator of pollution from their watersheds. Unfortunately, the fate of microplastics in lake ecosystems remains poorly understood because of the multiplicity of sources and transfer pathways. For the first time, the Plastilac project focuses on the contamination of high altitude lakes (from 1300 m to 2800 m above the sea level) by microplastics. Remote lakes constitute easier-to-investigate ecosystems because there are fewer potential sources of microplastics in their watershed, namely the atmospheric deposit, the supply from the watershed through the tributaries and the tourist attendance. Thus, both water column and sediment from 10 lakes located across the French Alps were sampled. The results showed that no lake was free from microplastics, proving the ubiquity of this pollution at a regional scale. The abundance of microplastics varied significantly from one lake to another and the concentrations measured in high altitude lakes (around 10 MP.m^-3) were approximately 100 times lower than those reported in the literature for lowland lakes. The water column contamination was not correlated to the vicinity of potential sources (urban areas). On the contrary, higher sediment contaminations were observed in lakes located nearby urban areas. Our analyses also showed that the residence times of microplastics in the water column of these lakes were relatively short, of the order of a few days. In contrast, the residence times of microplastics in the sediments were much longer and lake bottoms retain traces of past contamination. This work constitutes a first for understanding the fate of microplastics in mountainous environments. It provides important information on their dynamics and, in particular, on the temporal dimension of this pollution.