From freshwaters to bivalves: Microplastic distribution along the Saint-Lawrence river-to-sea continuum

Despite the close connection of freshwaters to human health, the occurrence and fate of microplastics in marine estuaries remain poorly documented. To study these particles in the Saint-Lawrence River (Quebec, Canada), surface water and marine bivalve samples were collected along the river-to-sea continuum. The water samples were subdivided to characterize the large microplastics (LMPs; 300-3200 mu m) and the small microplastics (SMPs; 20-300 mu m). Particles were identified by microscopy and infrared spectroscopy techniques. The concentration of LMPs was higher in the surface water in the downstream stations (0.0319 +/- 0.0147 items.L-1) compared to the upstream stations (0.0007 +/- 0.0006 items.L-1). No clear trend was observed for the SMPs. After digestion of the biological tissues, the microplastics ingested by the bivalves were recovered and characterized by microscopy coupled with infrared spectroscopy. Up to 3 items were found per bivalve suggesting that these particles are also present in the water column of the marine estuary and the gulf. The physico-chemical gradients along the continuum were monitored since they could be directly involved in the vertical and horizontal transport of microplastics. This study provides scarce field data collected along the world's largest estuary and gives new insights concerning the fate of microplastics along a river-to-sea continuum.

Automated summary

Despite the importance of freshwaters to human health, the presence and fate of microplastics in marine estuaries have not been extensively studied. This research focused on the Saint-Lawrence River in Canada, analyzing surface water and marine bivalve samples along the river-to-sea continuum. The concentration of larger microplastics was higher in downstream areas compared to upstream areas, while the trend for smaller microplastics was less clear. Microplastics were also found in the digestive systems of bivalves, indicating their presence in the water column. This study provides valuable data on the fate of microplastics in a river-to-sea continuum.