Seasonal microplastic fluxes in sediment traps reveal highly variable pollution rates in sub‑Arctic lakes near Reykjavík, Iceland

Abstract

Abstract Microplastics represent an emerging threat to freshwater ecosystems. However, their seasonal deposition patterns in lake sediments remain poorly understood due to low number of studies, conflicting results and methodological inconsistencies. This study presents the first seasonal microplastic flux records in Icelandic freshwaters, obtained using sediment traps deployed in four lakes near the capital region, including Þingvallavatn, Iceland’s largest natural lake. Materials collected by sediment traps were sampled every six months over two years, yielding 13 samples in an attempt to capture winter versus summer deposition dynamics. All samples underwent standardised laboratory processing (density separation, enzymatic digestion) followed by micro-FTIR analysis (20 µm detection limit). Blank correction was applied based on the Minimum Detectable Amount method. Microplastics were detected in all lakes, with non-zero microplastic fluxes ranging from 54 to 1294 particles m − 2 d − 1 . The highest microplastic flux was recorded in Meðalfellsvatn about 25 km NE of Reykjavík while the lowest non-zero flux was recorded for Hafravatn, located on the outskirts of the capital. The results indicated slightly higher winter fluxes, but also demonstrated the complex processes affecting seasonal patterns, linked to hydrological and meteorological variables. Results also highlighted the importance of using flux alongside concentration metrics, as concentration is more relevant for assessing direct exposure to microplastics, particularly for benthic communities, while flux accounts for differences in sediment accumulation rates and provides the temporal constraint necessary to define and compare pollution rates. These findings validate the reliability of using sediment traps for microplastic studies in low-pollution sub-Arctic systems, establish a first baseline for seasonal microplastic fluxes in Icelandic lakes, and demonstrate that flux metrics are essential to study processes affecting microplastic accumulation in sediments and to quantify microplastic pollution inputs.