Sediment-based investigation of naturally or historically eutrophic lakes -- implications for lake management.

Implementation of the EU Water Framework Directive will call for new lake monitoring and management strategies. Therefore, different methods need to be tested in order to achieve reliable assessment of lake background conditions and water quality. Sediment-based techniques provide one such tool for lake management. In this work, 10 lakes, presumed to be naturally eutrophic, were investigated with a paleolimnological short core study. The aim of the study was to examine the composition of the diatom assemblages in their natural state, estimate their change over time and assess the background nutrient levels. One sediment profile from each lake was divided into six sub-samples that were analyzed for diatoms (60 samples). Diatom-based inference models were applied to reconstruct the past total phosphorus concentration and assess the eutrophication. The results indicated that all the lakes studied had already been nutrient-rich before the impact of modern agriculture. However, diatom assemblages have changed remarkably over time and total phosphorus concentrations have generally increased, so at present only two of the study lakes are close to their natural status. This suggests that naturally eutrophic lakes will probably require management actions to fulfill the new directive requirements in the future.

A new gypsum-based technique to reduce methane and phophorus release from sediments of eutrophied lakes: (gypsum treatment to reduce internal loading).

The release of nutrients from the sediment into the water column, i.e. internal nutrient loading, is an important problem in the restoration of eutrophied lakes. The ebullition of gases, mainly methane, has been identified as the key process in nutrient transportation in the internal loading. We studied whether methane production in lake sediment and the release of nutrients into the water column could be prevented by the addition of gypsum (CaSO4 x 2H2O). Three different gypsum products were examined in a controlled laboratory microcosm with a continuous water flow. The addition of gypsum (especially Fe-gypsum) into the sediment increased the redox potentials and reduced methane production and release of phosphorus from the sediments during an incubation period of 3 months. Gypsum evidently acted as a slow-releasing source of sulphate in sediment, which likely enhanced the activity of sulphate-reducing bacteria and improved the overall mineralisation rate of organic matter. The liberation of H2S and phosphate, which might be an undesired consequence of increased sulphate reduction rates was prevented by the application of Fe-containing gypsum likely as a result of the formation of pyrite and apatite minerals.