Particle trapping on leaves and on the bottom in simulated submerged plant stands

Abstract

Two of the processes that influence the ecological behaviour of lakes are sedimentation and resuspension of fine particulate matter. It is well known that water plants reduce current velocity and alter turbulence structure and, therefore, affect particle transport. Very little is known about short-term sedimentation onto the sediment surface and settling of particles onto plant leaves in submerged macrophyte stands. Therefore, investigations were carried out in a flume. Lake conditions – as they occur in littoral sites not exposed to waves – were simulated by keeping the flow velocity low. Artificial plants without biofilm were used to keep the experiments as simple as possible. As an innovation in sedimentation research, the short-term sedimentation rate onto plant leaves was quantified. To determine current velocities and turbulence, we applied a three-dimensional velocity meter. There was a reduction of current velocity, an increase of shear stress and a doubling of particle retention within the macrophyte stand in all experiments. Rates of sedimentation, both onto leaf surfaces and onto the bottom in the stand, varied significantly in dependence on particle concentration, particle composition and outside flow velocity. Multiple regressions could explain only a part of the measured variance, which is probably due to still insufficient measurements of controlling variables, and processes, such as particle sliding from leaves. No significant spatial pattern of sedimentation in the stand was found. Our measurements showed that plant leaves effectively detain fine particles, even when lacking a biofilm. Our results make clear that sedimentation onto plant leaves must not be neglected when quantifying particle retention in submerged macrophyte stands.