Simulated drawdown and rewetting of littoral sediments: implications for Lobelia lake management

This study aims to answer the question if temporary drawdowns could be a beneficial restoration measure for endangered Lobelia lakes. Intact littoral sediment cores with and without plants were used to simulate a drawdown over an almost 5 months period and a subsequent rewetting. During drawdown, the initial low saturation of sorption sites for phosphorus (P) increased due to increased contents of mobile P forms in the surface layers. Contents of P binding iron forms remained constant. The increase of mobile P forms was mostly derived from decayed plant material since the experimental set up caused dying of plants. Upon rewetting effluxes of total dissolved phosphorus and dissolved organic carbon were observed which are most likely attributable to the degraded plant material. A phosphorus uptake experiment with dried and rewetted sediment cores without plants showed that the initial high P binding capacity had been retained during drying and rewetting. These findings were confirmed by a small scale long-term drying (348 days) and rewetting (228 days) experiment which overall did not show a significant reduction in P adsorption capacity. Therefore, drawdowns can be an appropriate restoration measure for endangered Lobelia lakes given that the isoetid plants spread during the drying period.