The influence of different recultivation techniques on primary production processes in a degraded urban lake

The study was conducted in Lake Długie located in Olsztyn, the Masurian Lake District. In the 1950s and in the 1960s, Lake Długie was used for 20 years as a collector of raw domestic and storm sewage. This has led to complete degradation of the lake. After preliminary protection measures implemented in the catchment, the lake was restored by artificial aeration and phosphorus inactivation methods. Long-term recultivation of the reservoir resulted in a significant and long-lasting improvement of the water quality. Before restoration, the average concentration of organic phosphorus in the surface water layer amounted to 0.166 mg Porg l−1 and of organic nitrogen — to 3.0 mg Norg l−1. After restoration, these values decreased to 0.058 mg Porg l−1 and 2.0 mg Norg l−1. The results obtained eight years after the recultivation of Lake Długie are extremely interesting. The amounts of chlorophyll a did not exceed 7 μg l−1, water transparency remained at a depth of 2.9 m, and BOD5 did not exceed 2 mg O2 l−1. These observations show that during implementation of recultivation treatments, only chemical parameters of the water significantly changed, while biological elements reacted to the treatments with some delay.