Wind-Driven Circulation in a Shallow Polymictic Lake: The Case of Lake Wolsztyńskie

Wind forcing is a primary driver of lake circulation, yet in shallow basins it is strongly constrained by morphometry, limited depth, and aquatic vegetation. We quantified the velocity and direction of horizontal wind-driven currents in Lake Wolsztyńskie (western Poland) and assessed their spatial and vertical variability in relation to depth, wind speed, and effective fetch. Monthly field measurements (June 2019–May 2020) at eight sites showed a consistent, monotonic decline in current speed with depth across the lake. Mean circulation speed increased with wind, but the relationship was weak, indicating that local controls and non-linear response dominate over simple wind–current scaling. In macrophyte-covered littoral zones, currents were substantially attenuated relative to unvegetated sites of comparable depth. Directional analysis revealed that surface flow aligns with wind-driven transport in fewer than half of observations, while compensating (return) currents with opposing directions near the bottom are frequent. Clockwise veering of current direction with depth—expected under a classical Ekman spiral—was only intermittent, consistent with truncation of Ekman dynamics in a shallow water column and a prevailing two-layer circulation pattern.