Internal phosphorus loading as the response to complete and then limited sustainable restoration of a shallow lake

The urban Swarzędzkie Lake, into which sewage had been diverted many years ago, was still characterised by low ecological status. Three restoration methods were used in order to improve the water quality, i.e. aeration of the waters overlying the bottom sediments, inactivation of phosphorus in the water column with iron sulphate and magnesium chloride and biomanipulation with pike stocking. The aim of the research was to define seasonal and spatial changes of phosphorus internal loading from bottom sediments and to compare this with previous years. We also considered changes in the process of P release when the restoration treatments were limited after 3 yr from 3 methods to 1 method. The highest phosphorus release from bottom sediments was found in the profundal zone, where in summer periods it would reach up to 29.2 mgP m−2 day−1. The lowest P release was observed in the littoral zone, down to a depth of 3 m, where it did not exceed 10.0 mgP m−2 day−1. 31% of the whole load of P released from the bottom sediments was originated from this zone. The research showed an increase of phosphorus release in the first years of restoration treatment and a systematic decrease at all stations in the following years.