Beyond the surface: Stratification and turnover control microplastic dynamics in a seasonally ice-covered sentinel lake

Microplastics are increasingly recognized as emerging contaminants in freshwater ecosystems, yet their vertical and seasonal concentrations remain poorly understood, particularly in seasonally ice-covered lakes. Here we investigated the distribution of microplastics (mp; size detection > 50 µm) in a sentinel lake in south-central, Ontario, Canada, during ice-free (summer) and ice-covered (winter) periods across 15 months (June 2022 to August 2023). Water samples were collected approximately monthly at multiple depths, with snow and ice sampled once at the end of winter. Our findings suggest that vertical stratification and seasonal turnover were key drivers of microplastic accumulation and redistribution. Microplastic concentrations peaked in the hypolimnion during summer stratification (20.8 ± 8.0 mp/L) and in snow and ice at end of winter (22.7 ± 20.2 mp/L) and were lowest following spring melt and turnover (5.21-6.37 mp/L). Consequently, we show that surface sampling underestimates whole-lake lake microplastic concentrations by up to 85 %, especially during periods of stratification or ice cover. Further, we observed substantial interannual variability in particle size, morphology, and polymer composition, with higher concentrations and larger fragments in 2022, compared with 2023, suggesting acute local inputs and reinforcing the need for long-term assessments. Our results demonstrate that seasonal processes and vertical heterogeneity must be considered when assessing microplastic budgets in freshwater systems. This work has important implications for aquatic microplastic assessments, mass balance models, and our understanding of microplastic transport in inland waters, particularly under changing seasonal and climatic conditions.