Planktonic communities in a small post-peat reservoir (Ustka Forest District, Poland)

The aim of this study was to evaluate the present state of a small (area of 0.2 ha), shallow (mean depth of 2 m) and approximately 30-year-old post-peat reservoir located in Bruskowskie Bagno, a Baltic raised mire in northern Poland. The study was conducted during all seasons of the year (August 2019–July 2020). The reservoir was characterised by a yellow to brown water colour, low pH (5.4) and quite low conductivity (40.4 μS cm^-1), which are the main features of dystrophic water bodies. Similar to natural, dystrophic lakes and ponds, the phytoplankton was mainly composed of mixotrophic species like Dinobryon sp. and Gonyostomum semen. The only numerous non-flagellate group within the phytoplankton was desmids, which indicated that the water was influenced by the mire. The reservoir was characterised by a high abundance of ciliates (annual mean of 55.6 cells ml^-1) and a very high abundance of rotifers (annual mean of 3.72 ind. ml^-1). Among ciliates, the most important were prostomatids, accounting for 53% of the mean annual ciliate abundance. The results of our study indicate that artificial, approximately 30-year-old, post-peat reservoir resembled a natural dystrophic water body.