A six-year (2015–2021) citizen science assessment of anthropogenic litter on Lake Geneva's shorelines

Abstract Background Freshwater systems receive less attention than marine environments despite acting both as major conduits for litter entering the oceans and as long term sinks. Prolonged citizen science monitoring can generate meaningful insights that support environmental policy development. In this study, we analyse freshwater shoreline litter data collected over a 6-year period, focusing on data reliability, temporal variability and site specific accumulation patterns. Results A total of 250 surveys were conducted over 6 years by 6 survey groups. From 79,436 recorded litter items, annual mean densities of 7.4–9.7 pieces per metre were calculated, with medians consistently below means, indicating a strongly right-skewed distribution and no monotonic temporal trend. Cumulative distribution analysis showed substantial overlap among the three groups with the most surveys, suggesting that inter-group differences were small relative to natural environmental variability in shoreline litter counts. Site-specific assessment showed similar litter types dominating the top 10 categories across the lake, with the most abundant items accounting for up to 76% of total observations. Continued monitoring for the duration of the project at Plage de Saint-Sulpice and Parc des Pierrettes demonstrated persistent accumulation of key categories such as cigarette butts, plastics and glass, with observations remaining within each site’s interquartile range over the entire period. Conclusion The findings indicate that long term citizen science data can yield statistically robust insights when appropriate computational methods are applied. The substantial variability in shoreline litter densities appears to arise from environmental features rather than methodological or observer related inconsistencies, supporting the use of multiple survey groups in lake scale measurements. Although no monotonic temporal trend was detected, site-specific differences reflected the influence of land use and shoreline characteristics on accumulation patterns. This 6-year dataset demonstrates that standardised monitoring can provide reliable information for lake scale environmental assessment and evidence based policy development.