Water flow across and sediment trapping in submerged macrophyte beds of contrasting growth form

Patterns of wave amplitudes, linear flow velocity, turbidity, sedimentation and periphyton accumulation were quantified across two macrophyte beds of contrasting architecture and adjacent unvegetated sediments, in Lake Veluwe , The Netherlands. Within both a Chara aspera- and an Alisma gramineum-dominated bed, wave amp litudes were progressively reduced. Vertical profiles of linear velocity in the two vegetations were significantly different from each other and from profiles measured outside the vegetation. Compared to unvegetated profiles, flow over the Chara-bed was more rapid in the upper 20 cm of the water but then dropped rapidly. In Alisma, maximum velocity at the surface was alrea dy lower than outside the vegetation. The denser Alisma-bed had a higher bi omass (125 +/- 15 g AFDW m(-2), mean +/- standard error) than the Chara-bed (28 +/- 1), it filled the whole water column, and rapidly reduced light at tenuation (K-d) from 6.4 +/- 1.4 outside to 2.3 +/- 0.3 m(-1) within the ve getation. Also in the Chara-bed, light attenuation and sedimentation rates were significantly reduced (light attenuation: 1.2 +/- 0.1 versus 2.7 +/- 0 .1 m(-1); sedimentation: 2.6 +/- 0.3 versus 7.7 +/- 0.5 mg dry weight cm(-2 ) d(-1), averaged over the growing season). The dense Chara vegetation of 1 0-20 cm height effectively lifted the resuspensive turbulent shear stress a bove the sediment surface, thereby trapping settled material.