THE SPECIES COMPOSITION AND SEASONAL DYNA MICS OF PHYTOPLANKTON AND ZOOPLANKTON IN THE SYSTEM OF SELF-PURIFICATION RESERVOIRS OF THE SEWAGE-TREATMENT PLANT

In 1987-1988 seasonal dynamics, species composition and the mutual int eraction of phytoplankton and zooplankton in the system of six self-pu rification reservoirs of the sewage treatment plant of the town Skalic a (West Slovakia) were monitored. During movement through the system, the wastewater self-purification process gradually resulted in the cha nge of water quality. Following differences of plankton association in the reservoirs were found: 1. The N1-N2 reservoirs, the closest to th e tributary from the sewage treatment plant, were characterized by a l ow diversity of plankton association and extremely high values of chlo rophyll a in the spring (2.576 mug.1(-1)) (Fig. 3). In phytoplankton E uglenophyta predominated. In net zooplankton Rotatoria with a high abu ndance prevailed at the beginning of the vegetation period. Cladocera and Copepoda were less frequent, therefore the zooplankton biomass rea ched low values. 2. In the N3-N4 reservoirs with partly pre-purified w ater the plankton association was more abundant. Contrary to the first two reservoirs, Cyanophyta and particularly Chlorophyta occurred. Chl orophyll a values in spring were high, in autumn they markedly decreas ed and remained low till the end of the vegetation period. Net zooplan kton achieved the highest species diversity of different groups. Clado cera dominated and together with Copepoda contributed a great deal to the increase of zooplankton biomass. Rotatoria abundance was higher in the spring time only. 3. The N5-N6 reservoirs with fish culture repre sented a typical fish pond plankton assemblage. All groups of Cyanophy ta and Algae with most abundant Chlorococcales were present in phytopl ankton. The values of chlorophyll a remained even during the whole veg etation period, with a maximum in spring and autumn. Net zooplankton s pecies diversity was rather poor. Rotatoria and Copepoda were dominati ng species. In Cladocera only small species, which resisted the predat ory pressure of fish, occurred. Biomass values were low when compared to abundance. The development of phytoplankton was negatively influenc ed by three main factors: shading by duckweed (Lemna), consumption by Cladocera, and lower availability of nutrients caused by their uptake by filamentous algae and macrophytes. The values of chlorophyll a in t he year 1988 decreased when compared with the preceding year.