Spatiotemporal variability of microplastics in Muskoka-Haliburton headwater lakes, Ontario, Canada

Microplastics (mp) are a growing environmental concern due to their ubiquity in terrestrial and aquatic environments. Nonetheless, there is limited knowledge on their abundance in lakes in rural areas. In this study, we surveyed 14 headwater lakes in Muskoka-Haliburton, Ontario, to assess the spatial and temporal variability of microplastics. The average microplastic concentration across the study lakes was 1.78 mp/L during May–June 2019, with limited spatial variability (coefficient of variation = 22%). Further, microplastic abundance was weakly correlated with lake area (rs: 0.469), the number of shoreline residences (rs: 0.399), and watershed area (rs: 0.350), suggesting that diffusive inputs, such as atmospheric deposition, were the dominant source of microplastics to the study lakes. In contrast, microplastics showed a distinct temporal (seasonal) variability, as the average concentration in August 2019 (0.91 mp/L) was significantly lower (p < 0.05) compared with May and June 2019. While microplastic abundance was generally higher in the metalimnion (0.70 mp/L) and epilimnion (0.67 mp/L), there was no significant difference by stratified layer. The annual percent removal of microplastics in lake sediment was estimated to be 14%, suggesting that for most of the study lakes, sediment burial was not a dominant sink for microplastics. Effective management of microplastic pollution requires an understanding of the nexus between microplastics in the atmosphere, lake water, and sediment. In rural areas, microplastic abundance appears to be dominated by atmospheric inputs, suggesting limited need for spatial monitoring. Temporal monitoring, however, is required to understand seasonal changes and long-term trends in microplastic abundance and delivery.