SEDIMENT RESUSPENSION AND LIGHT CONDITIONS IN SOME SHALLOW DUTCH LAKES

The relationship between sediment resuspension and light conditions has been studied in three shallow Dutch lakes. In shallow eutrophic lakes, the contributions of algae and suspended sediments to light attenuation are usually dominant. Wind induced resuspension of bottom sediment and sedimentation often determine the dynamics and the spatial variability of the suspended solids concentration and the light attenuation coefficient. A model is discussed relating the suspended solids concentration and light attenuation coefficient to the orbital velocity, induced by waves, and to the fall velocity of solids. Using experimental data, the relationships between fall velocity, organic matter content and the specific vertical attenuation coefficient are analyzed, Differences in the specific vertical attenuation coefficient and in fall velocities of sediment fractions, are related to differences in particle size as well as differences in composition. The contribution of (resuspended) particles to the suspended solids concentration and to light attenuation is primarily due to slow settling sediment fractions which are rich in organic matter. So in modelling light attenuation dynamics especially these fractions are of interest. Heavier fractions can be disregarded. A model for the relationship between resuspension and light attenuation dynamics, based on this conclusion, has been tested successfully for 2 shallow Dutch lakes.