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

Abstract

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, Z max = 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 (E SPM) and total P (E TP) were determined by a mass balance. Sandy nearshore sediments at 1-2 m depth showed low E SPM (0.01-6.64 g m-2 h-1) and E TP (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 E SPM (0.09-106.1 g m-2 h-1) and E TP (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 E SPM and E TP, 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. © 2013, by the Association for the Sciences of Limnology and Oceanography, Inc.