Methodological heterogeneity outweighs geography in shaping reported riverine microplastic concentrations: A global meta-analysis and implications for monitoring

Riverine microplastic concentrations reported worldwide span more than nine orders of magnitude, and this dispersion is routinely read as environmental signal. We compiled 509 observations from 309 primary field studies covering 343 rivers in 68 countries, recorded the methodological provenance of every value, harmonised the record to a common basis, and partitioned its variance between environmental and methodological sources. Only 283 observations (56%) could be pooled; 178 (35%) lacked an interpretable unit basis. Pooled geometric means, reported to characterise the analysable record rather than as estimates of true concentration, were 1,386 items m -3 in surface water and 605 items kg -1 dry weight in sediment. Continental region did not explain concentrations in either matrix (Kruskal-Wallis p > 0.05), whereas methodological variables did: a ten-fold finer operative size cut-off was associated with a 3.07-fold higher reported water concentration, and identification without chemical confirmation with concentrations 28 times higher. Heterogeneity was effectively total (I 2 = 99.96% water, 99.82% sediment) and the 95% prediction interval for water spanned 1.2 to 849,000 items m -3 . Method variables explained more variance than geography in both matrices, 70-83% of the variance lay between studies, and restricting the analysis to spectroscopically confirmed observations left all three findings essentially unchanged. The published record is therefore structured more by how it was measured than by where, and cannot yet support numeric benchmarks or global burden estimates. We derive a minimum reporting set from the observed failure modes.