Assessment of the restoration measures in a man-made reservoir: do oligochaete communities respond to the improvement of water quality?

Studies were carried out in a man-made reservoir that was undergoing restoration using selective hypolimnetic withdrawal (Pławniowice reservoir, Southern Poland). The aim of the research was to assess the changes in the composition of oligochaete fauna in relation to water quality and to estimate whether indices based on oligochaetes may be useful in the assessment of a reservoir's recovery from eutrophication. Samples were collected in the first, fifth, and eighth year of the restoration during the summer stratification at 22 sites along six transects that covered depths from 2 to 15 m. The results of the survey indicated an improvement in some physical and chemical water parameters in respect to those during the period prior to the restoration measures. The present study showed an increase in the oligochaete diversity and a decrease in their abundance accompanied by a shift toward a larger share of taxa that are characteristic of mesotrophic and oligotrophic conditions. This study detected slow changes in the values of the trophic indices based on oligochaetes toward mesotrophic conditions. Discriminant analysis, performed using stepwise forward selection, demonstrated a good separation of samples from 2004 and 2011 according to trophic condition (TC) and biotic quality index (BQI) indices. The present data shows that metrics based on Oligochaeta may be useful for detecting changes in the trophic status.