Use of a fluorescent molecular rotor probe for nanoplastics assessment in epiphytic biofilms growing on submerged vegetation of Lake Saint Pierre (St. Lawrence River)

Studies on plastic pollution conducted in freshwaters mainly focused on monitoring plastic debris in the water column or in the sediments. Few studies have investigated the occurrence of plastic debris in benthic biofilms (periphyton). Yet, algal biofilms may potentially act as a sink for plastic debris, trapping it within their mucilaginous or filamentous matrix. Biofilms may also represent a source of plastic debris by sloughing when they become senescent. In addition, plastic debris accumulated within biofilms may enter the food web via primary consumers. Considering these observations, this study aims to quantify nanoplastics (NPs) accumulated in biofilms growing on aquatic vegetation from Lake Saint Pierre (LSP), a fluvial lake of the St. Lawrence River, and its archipelago. Biofilms were removed from submerged plants and the presence of NPs was assessed by spectrophotometry using the fluorescent molecular rotor probe 9(dicyanovinyl)-julolidine (DCVJ). The results of this study confirm the biofilms’ ability to act as a sink for NPs. Despite the fact that the determination of the absolute nanoparticle number and size distribution remains a challenge, we estimated a median concentration of 1.05 × 109 NP/mg of biofilm dry weight (DW) when using 100 nm polystyrene beads for calibration. Concentrations were significantly different between water masses, with higher concentrations in samples collected in the two lateral water masses compared to the central water mass. Our study provides, for the first time, a quantitative assessment of NPs from epiphytic biofilms in a large river under the influence of anthropogenic sources.