PLANKTON SEASONAL DYNAMICS AND ITS CONTROLLING FACTORS IN SHALLOW POLYMICTIC EUTROPHIC LAKE VORTSJARV, ESTONIA

An analysis of plankton seasonal succession in large shallow eutrophic lake Vortsjarv (270 km(2), mean depth 2.8 m, max, depth 6 m) is prese nted. Weekly samples for 1995 have been analysed using the PEG model a pproach. In winter, light was the main factor controlling phytoplankto n growth. In early spring phytoplankton was mainly resource-controlled , competition for phosphorus being the main driving force. Ciliates (P aradileptus sp., Strobilidium sp. and Vorticella sp.) were the first h erbivores which started to increase in April causing a twofold decline of phytoplankton biomass. The annual maximum of primary production (P P) in early May was probably caused by soluble reactive phosphorus (SR P), regenerated by herbivores, and stirred up from bottom sediments as a result of strong wind stress. This primary production peak provided substrate for further increase of bacterial biomass. The more abundan t food supply supported the development of the second spring peak of h erbivores (ciliates, rotifers, juvenile copepods) which was followed b y the second modest ''clear water phase'' in late May. Silicon was dep leted by the end of May causing a strong decrease in primary productio n of the diatom-dominated community, whereas the biomass of the cyanop hytes increased under the improved nutrient conditions. Some weeks lat er, inorganic N was depleted and the period of N limitation with the a ppearance of N-fixing cyanophytes began. The ciliate collapse at the b eginning of June coincided with the start of the cladoceran developmen t and with the increase of other metazooplankton groups. This explains the further decrease of the biomass of phytoplankton and bacteria in spite of their high production. Beginning from late June, silicon appe ared again and SRP started to occur periodically, while inorganic N re mained close to zero until November. During this period, phytoplankton development relied to a gear extent on the N-fixation and N-regenerat ion potential. The collapse of the ciliate community in September remo ved the top-down control from bacteria and their biomass increased, wh ile the development of cladocerans still suppressed phytoplankton biom ass in spite of a quite high PP. In October phytoplankton biomass and chlorophyll a (Chla) increased, SRP was completely depleted by the mid dle of October reflecting a slow regeneration due to the declined acti vity of zooplankton in cold water. In November nitrates appeared again , and silicon reached the same level as in spring. The biomass of N-fi xing Aphanizomenon skujae decreased while Limnothrix redekei and L. pl anctonica were quite abundant together with diatoms.