RESUSPENSION IN A SHALLOW EUTROPHIC LAKE

The frequency and the importance of wind-induced resuspension were studied in the shallow, eutrophic Lake Arreso, Denmark (41 km2, mean depth 3 m). During storm events in autumn 1988 lake water samples were collected every 2-8 hours by an automatic sampler at a mid-lake station. The concentration of suspended solids and Tot-P was found to increase markedly. During storms up to 2 cm of the superficial sediment was resuspended, and the concentration of resuspended solids in the water column rose to 140 mg l-1. The resuspended particles had a relatively high settling velocity and on average, a relatively short residence time in the water column of 7 hours. A model which describes the concentration of resuspended solids as a function of wind velocity and of settling velocity of the resuspended particles is presented. Using additional wind velocity data from a nearby meteorological station, the model has been used to calculate the frequency of resuspension events and concentration of resuspended solids for the period from May to November 1988. These calculations show that resuspension occurred about 50% of the time. Average flux of suspended solids from the sediment to the water was 300 g m-2 d-1 and during 50% of the time lake water concentration of suspended solids was more than 32 mg l-1. A relationship between concentration of suspended solids and Secchi-depth is presented. Because of resuspension, Secchi-depth in Lake Arreso is reduced to 0.5 m. Resuspension also had a marked effect on Tot-P concentration in the lake water, and P input to the lake water being totally dominated by resuspension events.