Declining atmospheric Pb deposition does not ensure lower streamwater Pb concentrations: evidence from 28 years of forest catchment monitoring.

Declining atmospheric lead (Pb) emissions across Europe have substantially reduced Pb deposition to forest ecosystems, yet the extent to which streamwater Pb concentrations respond to this decline remains unclear. Using 28-year records (1996-2023) from 12 forested headwater catchments of the Czech GEOMON network, we evaluated temporal trends in atmospheric Pb inputs, streamwater Pb concentrations and Pb export fluxes, together with factors associated with their spatial variability and interannual dynamics. Atmospheric Pb inputs and precipitation-weighted mean Pb concentrations declined consistently across all sites. In contrast, streamwater Pb concentrations showed divergent trends, including increases in several catchments despite declining deposition. Mean annual streamwater Pb concentrations ranged from 0.5 to 3.9 μg L-1 among catchments, and Pb export ranged from 0.5 to 27 g ha-1 yr-1. Across sites, long-term differences in streamwater Pb concentrations were strongly associated with dissolved organic carbon (DOC), indicating a close link between Pb and dissolved organic matter. The linear mixed-effects model explained 57% of the variability in Pb concentrations through fixed effects and 65% when persistent differences among catchments were included. Lead export among catchments was primarily related by runoff. Overall, the results indicate that declining atmospheric Pb input does not necessarily lead to proportional decreases in streamwater Pb concentrations. Pb stored and strongly retained in forest soils may therefore continue to influence Pb dynamics in forested catchments. Ongoing changes in DOC mobilisation, hydrological conditions and forest disturbance may further modify Pb transfer from catchments to surface waters even under reduced atmospheric deposition.