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The epilimnetic phytoplankton and its relations to nutrient content in Lake Verevi through the whole vegetation period in 2000 were studied. Lake Verevi (surface 12.6 ha, mean depth 3.6 m, maximum depth 11 m) is a hypertrophic hard-water lake, where the so-called spring meromixis occurs due to an extremely warm spring. Most dissolved nutrients in the epilimnion were low already in spring, and their concentrations were quite stable during the study period. The concentration of total silicon was very low in spring but increased rapidly in summer. Total phosphorus followed the pattern for stratified eutrophic lakes, and total nitrogen was quite high. The stoichiometric N:P ratio fluctuated between 25 and 81. The dynamics of phytoplankton biomass with a spring peak from April to May and a late summer peak from July to August is typical of Estonian eutrophic lakes. Green algae and chrysophytes occurred in the phytoplankton throughout the vegetation period. The spring peak was dominated by diatoms ( var. ) and the summer peak was caused by and . The study showed that in physically stratified systems, the total concentration of limiting resources and plain physical factors (light and temperature) may be more important in the determination of phytoplankton dominants than different resource ratios. A combination of light and temperature optimum, along with nutrient utilization and transport capacity, effectively segregates phytoplankton species and can be used for the explanation of seasonal succession pattern.

The epiphyton on 22 macrophyte species was studied in the hypertrophic stratified Lake Verevi mainly in the midsummer of 2000 and 2001. Some material from 1998 and 1999 was used as well. Chlorophyll (Chl a) level was high: 330–360 µg g dw on emergent plants, and an average of 117–200 µg on floating-leaved plants and 820–920 µg g dw on submerged plants. Biomass was 15–23, 5–10 and 35–53 mg g dw, respectively. The richest in epiphyton were submerged plants with densely growing and fine branchlets such as L. and Rupr. The share of Chl a in biomass was higher in 2001 (2.3%) than in 2000 (1.7%), which can be associated with lower irradiance in summer 2001. Filiform chlorophytes were dominating on most plants; 60% of biomass on submerged, 69% on emergent and 80% on floating-leaved plants; in some cases, the share of filamentous species was 95%. Diatoms formed 29, 12 and 7%, cyanobacteria 8, 16 and 10% of the same ecotopes, respectively. As a rule, the epiphyton was quite sparse on large leaves. Cyanobacteria were more abundant on large and leaves, , on stems of L., and on some emergent plants with a smooth and soft stem surface, as and . Diatoms played the most important role on some species and in single samples of and . The morphology of plant species appears to be the main factor of epiphyton richness in L. Verevi.

The vertical and temporal distribution of metazooplankton in the small hypertrophic, strongly stratified, temperate Lake Verevi (Estonia) was studied during 1998–2001. The zooplankton of Lake Verevi is characteristic of hypertrophic lakes, with a small number of dominant species, rotifers being the main ones, and juveniles prevailing among copepods. In 1999–2001, the average abundance of metazooplankton in the lake was 1570 × 10 ind m; in the epilimnion 2320 × 10 ind m, in the metalimnion 2178 × 10 ind m, and in the hypolimnion 237 × 10 ind m. The average biomass of metazooplankton was 1.75 g m; in the epi-, meta- and hypolimnion, accordingly, 2.16, 2.85 and 0.26 g m. The highest abundances — 19,136 × 10 ind m and 12,008 × 10 ind m — were registered in the lower half of the metalimnion in 24 May and 5 June 2001, respectively. Rotifer (Gosse, 1851) was the dominating species in abundance. In biomass, Gosse, 1850, among the rotifers, and (Lilljeborg, 1888), among the copepods, dominated. According to the data from 2000’2001, the abundance and biomass of both copepods and rotifers were highest in spring. Zooplankton was scarce in the hypolimnion, and no peaks were observed there. During the summers of 1998 and 1999, when thermal stratification was particularly strong, zooplankton was the most abundant in the upper half of the metalimnion, and a distinct peak of biomass occurred in the second fourth of the metalimnion. Probably, the main factors affecting the vertical distribution of zooplankton in L. Verevi are fish, larvae, and chemocline, while food, like phytoplankton, composition and abundance may affect more the seasonal development of zooplankton.

Seasonal population dynamics and the vertical distribution of planktonic ciliates in a hypertrophic and strongly stratified temperate lake were studied from April to October in 2000 and from April to June in 2001. In the epi- and metalimnion the ciliate abundance peaked in spring and late summer, reaching maximum values in the metalimnion (86 cells ml) on 7th August 2000. In the epilimnion, the highest biomass content (414 µg C l) was observed on 8th May 2000. In the hypolimnion only a late summer peak occurred and the ciliate numbers were always lower than in the epi- and metalimnion. Five groups dominated the community of ciliates: and , and the community composition varied greatly with depth. In the epilimnion the ciliate numbers were dominated by oligotrichs but small algivorous prostomatids, peritrichs and gymnostomes were also numerous. In the metalimnion these groups were gradually replaced by scuticociliates and mixotrophic spp. In the hypolimnion scuticociliates and species known as benthic migrants dominated. In the epilimnion and upper metalimnion in spring large herbivores and in summer small bacterivores were more numerous.

The aim of study was to bring out changes in the macrophyte vegetation, caused by eutrophication, short-term lowering of the water level and the following restoration of equilibrium in L. Verevi. Also biomass and N and P content of shoots of main submergent species were studied in 1999–2001, to follow the temporal and specific differences. Due to strong eutrophication, the type of the lake changed from a -Charophyta lake to a lake in 1984–1988, obviously owing to the formation of loose organic-rich sediment. Water lowering by 0.7 m during summer months of 1998 facilitated mineralization of sediments, as a consequence of which a mass development of and a temporary increase in the abundance and biomass of other nutrient-demanding species took place during following years. Our data suggest differences in nutrient supply and release of submerged species and the need for more species-related approach to this group. The problem of nutrient supply of unrooted plants at the time of stratification arises. Regarding the increase of biomass of in second half of summer, we suppose that one part of nutrients for this growth may derive from freshly decayed filamentous algae or vascular plants.

An overview on studies of macrozoobenthos in the small, hard-water, stratified and hypertrophic Lake Verevi (South-Eastern Estonia) is given. The list of macroinvertebrates comprises at least 105 taxa. In the open water habitats, the biomass and abundance of macrozoobenthos (except the phantom midge ) was rather constant beginning from the epilimnion up to the upper hypolimnion (depth 2–4 m), but very low in the lower hypolimnion (depth 6 m), which was inhabited mainly by . Comparison with long-term reference data from other Estonian lakes, belonging to similar limnological types, indicated that the total biomass and abundance (without ) in the profundal of Verevi were very low.

The diel migration and spatial distribution of fish were explored using six sequential 4-h sample gillnettings in the pelagic and littoral zones of Lake Verevi (Estonia, 12.6 ha, max. depth 11 m, hard-water, deoxygenated hypolimnion) in August 2001 and July 2002. Considering abundance, two-thirds of the total fish moved to the littoral zone. The biomass of fish was distributed evenly between the littoral and pelagic zones, where the topmost epilimnion accounted for 80–85% leaving 10–15% for the lower epilimnion in the pelagic zone. Just above the thermocline only some large specimens of perch (L.) and roach (L.) (1–5%) during the daytime were captured. No fish movements were recorded under the thermocline. Rudd (L.) inhabited only the littoral zone; all the other species were captured in both zones. Juvenile perch stayed in the littoral zone, whereas juvenile roach was caught in both zones and was active over a 24-h period. Piscivores, perch and pike L., were inactive in the dark. Perch inhabited mostly the littoral zone and the duration of its activity increased with age. In summer-stratified Lake Verevi, sharp change in the values of oxygen in the metalimnion along with species interaction affected the spatial distribution of fish, while diel migration was light-dependent.