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 5 1.27 km 2 , Z max 5 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 21 ) 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 22 h 21 ) and E TP (0.2-2.96 mg m 22 h 21 ). 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 22 h 21 ) and E TP (0.06-532.3 mg m 22 h 21 ). Under ice, U * crit was 0.9-1.1 cm s 21 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.Shallow lakes, without aquatic macrophytes, often demonstrate enhanced sediment resuspension induced by wind and/or benthic fishes. Particles are resuspended from the bottom if a critical shear stress of erosion is exceeded, leading to sediment-related water quality problems such as reduced water clarity, enhanced nutrient cycling, and high phytoplankton biomass (Bloesch 1995).Due to wind-driven resuspension, considerable quantities of particulate and interstitial phosphorus (P) can be entrained and distributed into the water column. At any given time, this physical upward transport of P may be orders of magnitude greater than other P inputs, from external or internal loading, including diffusion and bioturbation (Havens et al. 2007). However, resuspension may be not only a source (Reddy et al. 1996) but also a sink (Havens et al. 2007), or has no effect on water column P concentration (Peters and Cattaneo 1984). Hence, resuspension needs to be studied, particularly in lakes subject to wind and associated wave-current interactions with the lake bed.Because of its growth-stimulating effect on phytoplankton, P resuspension in shallow lakes was quantified by quite different approaches. There are numerous whole lake studies (Istvá novics et al. 2004;Havens et al. 2007), enclosure experiments (Ogilvie and Mitchell 1998), combinations of in situ and laboratory experiments (Qin et al. 2004), and exclusively laboratory experime...

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