Optimal physical design in a new lake for reducing phosphorus pools

Abstract

After 200 years of reclaiming shallow lakes to expand cultivated land in Europe and North America, recent decades have been marked by the establishment of new lakes, this time to stop the decline of freshwater biodiversity and help reduce nutrient transport from land to sea. However, new lakes risk becoming eutrophic and turbid, because they are established mainly on fertile agricultural soils. Minimizing internal nutrient loading from sediments can be accomplished by relocating nutrient-rich sediment to deep water with low release, rapidly exporting nutrients by washout, or immobilizing soil-nutrients before inundation. We studied sediment relocation in relation to sediment shear stress and phosphorus decline in sediment in new Lake Birke, Denmark (area 125 ha, mean water depth 0.56 m, retention time ca. 193 days). Evaluating core samples and other data on two occasions, 116 and 530 days after establishment, we found that sediment density changed towards harder materials in the middle of the lake, which is characterized by high bottom shear stress, while islands and shores exposed to short fetches from the prevailing winds experienced low bottom shear stress and accumulated softer, nutrient-rich organic material. After 530 days, sediment density had become a highly significant linear function of bottom shear stress (P < 0.001, R2 = 0.59). Daily mean sediment P in surface sediment (0–10 cm) decreased by 20.1 mg P m−2 sediment during the first 116 days and 7.9 mg P m−2 during the next 414 days. Overall, 69% of the phosphorus pool in surface sediments was lost and likely washed out over the first 530 days. Managing a lake by keeping initially shallow water and short water retention time, and locating the lake outlet in a wind-exposed area of high particle concentration, may facilitate loss of sediment nutrients and thus promote desired ecological qualities. Moreover, deeper sedimentation areas that are easily accessible to mechanical dredging may be planned to reduce nutrient release of fine, nutrient-rich organic particles.