Nature-based solutions for protection and restoration of degraded Bielsko Lake

摘要

Bielsko Lake is large (257.9ha) and deep (max. depth 23m, mean depth 6.2m) lake, located in the Pomerania Region, serving as a receiver of treated municipal sewage as well as stormwater, from the town of Bia?y Bór. Deterioration of water quality has been observed in recent years – the lake was still a in moderate state (III class) in 1997, but now it is in a poor condition (IV class) with low Secchi depth (to 0.3m), high chlorophyll-a content (up to 153μgdm?3) and cyanobacterial water bloom in summer, dominated byPlanktothrix agardhii,Aphanizomenon gracile,Planktolyngbya limneticaandPseudanabaena limnetica. External nutrient load exceeds 3.4 times the permissible TN load and 2.4 times TP load, according to Vollenweider's criteria. To improve the ecological state of Bielsko Lake protective measures against pollution have been proposed. Treated municipal sewage should be diverted from the lake by the transport to specially prepared filtering fields, which will drain the sewage into the ground. Stormwater should be treated in a sedimentation and biofiltration system before being discharged to the lake. As internal loading from bottom sediments is very high, sustainable restoration was also proposed. Soluble reactive phosphorus in the water column could be inactivated using small and accurately calculated doses of iron sulfate and magnesium chloride. A cascading effect of trophic interactions of the food web on phytoplankton could be obtained using piscivorous fish stocking (biomanipulation method).