Local deepening of large shallow peat lakes: a measure to improve their ecological status

Fine organic suspended sediments (refractory detritus) play an important role in the underwater light attenuation of large shallow lakes with a peat origin and a eutrophication history. Wind driven resuspension of this material, its flocculation in the water column and the settlement of the formed flocs are the main processes governing the dynamics of this material. For restoration of the macrophyte community in such lakes, reduction of this refractory detritus to improve underwater light climate is the key process when eutrophication reduction measures alone are not effective enough. The shallow Lake Loosdrecht (The Netherlands) was used as case study to illustrate the effects of artificially created deepenings on suspended matter concentrations and the consequences for the underwater light climate. Suspended sediment balances were created for the current situation and the situation with deepenings. Field measurements were taken to quantify various processes and results of the calculations from the suspended sediment balances were used to quantify the effects on light climate and potential habitat for macrophytes. These calculations show that creating deepenings (three sections with a total surface area of 120 ha and a depth of 12 m on a 10% section of the lake) decreases the concentration of organic detritus by 25% and decreases attenuation coefficients from 2.5 m-1 to 2.2 m-1. P-load reductions affecting chlorophyll-a levels lead to a change of attenuation coefficients from 2.5 m-1 to 2.0 m-1. The combination of deepenings with P-load reduction measures gives the most optimal result and leads to a predicted attenuation coefficient of 1.6 m-1. These improvements of the underwater light climate are a first step to the recovery of the submerged macrophyte community.