Population Dynamics and Structure of Zooplankton Community of Mandra Reservoir, Bulgaria

The zooplankton community in Mandra Lake has been almost unstudied since it was turned into reservoir in 1963. Human intervention in the natural water regime of the lake has affected the species diversity of the aquatic system. This study was conducted in March 2017 and December 2018 in the reservoir. A total of 64 zooplankton taxa from the groups Protozoa, Rotifera, Cladocera and Copepoda were identified in the studied samples. The majority of these organisms (37%) belonged to the Rotifera group, with most common species in spring – Keratella hiemalis Carlin, 1943 and Polyarthra major Burckhardt, 1900. During the investigation period, the most frequent zooplankton components were Chydorus sphaericus (O.F. Müller, 1776) and Nauplii and Copepodites of Copepoda. 22 species from class Branchiopoda were established. Considerable changes in the quantitative composition of Rotifera at different sampling points and years are indicated by the low values of the Jaccard similarity index. When comparing the two spring seasons (1955 and 2017), the index has a value of less than 9 %. At the same time, the number of species increased 2.5 times.