Underestimation of plastic pollution and implications for water quality assessments in urban lakes

Plastic pollution is increasingly recognized as a pervasive threat to water quality, yet most studies quantify only microplastics (<5 mm), potentially biasing estimates of environmental contamination. Here, we formally evaluated the underestimation bias associated with the intentional exclusion of plastics >5 mm from samples collected across a system of urban freshwater lakes (n = 20). We first conducted surveys of predominant single-use plastics to identify common forms and likely sources (e.g., food containers, tobacco products) present in these environments. We then quantified both microplastics (MP) and total plastics (TP = microplastics + macroplastics) in samples collected from four sampling zones: upland terrestrial vegetation; the shoreline; offshore sediments; offshore surface waters. Because both TP and MP were measured for every sample, we quantified size-selective underestimation bias by calculating count differences (i.e., sample bias = TP − MP) and the percent of large plastics excluded for each sample. Our results demonstrate that the simple omission of plastics >5 mm can produce non-trivial underestimations of plastic pollution within a sample. Fragments and films most frequently exhibited potentially meaningful (10–20%) or substantial (≥20%) underreporting bias. Restricting analyses to MPs may result in underestimation of total plastic burdens, but the magnitude of this underreporting bias was highly context dependent, varying among plastic forms and lake sampling zones. Our findings emphasize the consequences of neglecting larger plastics may influence accurate assessments of plastic burdens. Consequently, assessments based solely on MPs may provide an incomplete characterization of plastic pollution in environments where underreporting bias is moderate to substantial.