Spatial Variability of Selected Physicochemical Parameters Across Contrasting Riparian and Adjacent Land-Use Settings in Post-Extraction Gravel Pit Lakes of Eastern Poland

Local physicochemical variability in small post-extraction gravel pit lakes is often difficult to interpret because water chemistry may reflect both seasonal dynamics and highly local shoreline conditions. This study assessed spatial and temporal patterns in selected physicochemical parameters of near-surface water at eight fixed littoral sampling points representing contrasting riparian and land-use settings in two gravel pit lakes within the Międzyrzeckie Jeziorka lake complex near Międzyrzec Podlaski, eastern Poland. Surface-water monitoring was conducted from 9 November 2024 to 23 September 2025. The analytical dataset covered total organic carbon, inorganic carbon, total carbon, total nitrogen and laboratory-measured pH. The strongest and most consistent changes occurred between sampling dates in both lakes. Differences among fixed points were limited; total nitrogen differed between two points in Gravel pit lake no. 1, while the other parameters showed no date-adjusted point-level pattern over the monitored period. Mean total organic carbon and total carbon concentrations were similar in the two lakes, whereas Gravel pit lake no. 1 showed higher maximum values for most parameters and greater variability in selected carbon and nitrogen fractions. Laboratory-measured pH was generally weakly acidic to near-neutral, and the lowest observations occurred during individual campaigns. The findings show pronounced temporal dynamics and a restricted spatial signal in the monitored littoral zones. Repeated fixed-point sampling provided a clear basis for comparing temporal changes and local differences in these post-extraction lakes. These inferences are restricted to the monitored near-surface physicochemical variables.