10. Stable isotope study of the Lake Drūkšiai food web before the Ignalina Nuclear Power Plant closure

The performed stable isotope analyses (SIA) showed the structure of the Lake Drūkšiai food web after the 25-year-long utilisation of the lake as a cooling pond of the Ignalina Nuclear Power Plant (INPP). The natural abundance of carbon (13C/12C) and nitrogen (15N/14N) stable isotopes was estimated in 19 macroinvertebrate and 11 fish species collected in 2009 and six community-wide metrics were assessed based on the mean δ13C and δ15N values of these species. SIA showed that the average trophic chain of the lake was composed of four to five trophic levels. The mean δ13C data of primary consumers revealed low diversification between pelagic and littoral primary production in the lake food web. Therefore, most of invertebrates fall into one cluster in the stable isotope bi-plot. Meanwhile, most of fishes rely on the littoral organic source, which indicates a small input of pelagic primary production to the food web of Lake Drūkšiai. The calculated community-wide food web metrics provides us with the background for the investigation of the lake food web recovering from the impacts of the relatively long-term thermal pollution as the INPP was closed and heated water discharge was completely stopped in 2010.