Shear stress and sediment resuspension in relation to submersed macrophyte biomass

We examined the impacts of macrophyte beds dominated by a canopy-forming (Myriophyllum sibiricum) and a meadow-forming (Chara canescens) species on bottom shear stress (tau) and resuspension in shallow Lake Christina, Minnesota (U.S.A.). Studies were conducted in late summer, 1998, when macrophyte biomass levels exceeded 200 g m(-2), and in early summer, 2000, when biomass was greatly reduced (<20 g m(-2)) in both plant beds. The critical shear stress (tau(c)) of sediments, measured experimentally in the laboratory, was low (1.4 dynes cm(-2)) indicating potential for resuspension in the absence of macrophytes. During 1998, turbidity was low at the M. sibiricum and Chara station, rarely increasing when calculated bottom tau(c) (calculated from wave theory assuming no biomass obstruction) exceeded tau(c), indicating that both beds reduced sediment resuspension at high biomass levels. In situ tau (estimated tau), measured via gypsum sphere dissolution, did not exceed to above the sediment interface in either bed during 1998. In contrast, sediment resuspension occurred in both beds during similar high winds in 2000. However, estimated tau was lower than calculated bottom tau, suggesting that at low biomass., macrophytes were having some impact on tau.