Effects of ice cover on sediment resuspension and phosphorus entrainment in shallow lakes: Combining in situ experiments and wind‐wave modeling

In situ experiments on sediment resuspension were carried out along a depth transect in shallow polytrophic Lake Langer See, northeast Germany (area = 1.27 km2, Zmax = 3.8 m), in winter under ice and in summer 2006, using a hydrodynamically calibrated erosion chamber (diameter 20 cm). Shear velocity U* was incrementally increased 11 times for 10 min each (0–2.57 cm s−1) to initiate resuspension. Entrainment rates (E) of suspended particulate matter (ESPM) and total P (ETP) were determined by a mass balance. Sandy nearshore sediments at 1–2 m depth showed low ESPM (0.01–6.64 g m−2 h−1) and ETP (0.2–2.96 mg m−2 h−1). There was no difference in critical shear velocity (U*crit) of incipient resuspension between winter and summer. Muddy offshore sediments at 2–3.8 m depth showed higher ESPM (0.09–106.1 g m−2 h−1) and ETP (0.06–532.3 mg m−2 h−1). Under ice, U*crit was 0.9–1.1 cm s−1 higher than in summer, indicating that the wave-unaffected sediment had ample time for consolidation and biostabilization. Wind-wave modeling, using measured ESPM and ETP, revealed that resuspension is primarily (90%) restricted to the nearshore sediments (17% lake area). Our scenarios show that annual entrainment of SPM and TP increased linearly with the duration of ice-free period. Since ice coverage increased shear resistance, climate-driven absence or shortening of ice-cover would reduce the period for settling of particles and their integration into sediments, lowering their biostabilization, thus facilitating resuspension and phytoplankton recruitment in spring after ice thaw.