In English. Summaries in Estonian

Proceedings of the Estonian Academy of Sciences

Biology * Ecology

Volume 49 No. 1 March 2000

Lake Peipsi and its ecosystem; 3–18
Abstract. Lake Peipsi (3555 km2, mean depth 7.1 m), consisting of three parts (L. Peipsi s.s., L. Pihkva, L. Lämmijärv), is located on the border of Estonia and Russia. L. Peipsi s.s. belongs to unstratified eutrophic lakes with mesotrophic features, L. Lämmijärv has some dyseutrophic features, while L. Pihkva is a typical unstratified eutrophic lake. The mean concentrations of total phosphorus and nitrogen in the surface water were 42 and 768 mg m–3, respectively. Biomass of phytoplankton fluctuated between 1 and 125 g m–3, that of zooplankton from 0.088 to 6.344 g m–3, with a summer average of 3.092 g m–3. The total count of bacteria in L. Peipsi s.s. was 2.2 × 106, in L. Pihkva 4.3 × 106, and in L. Lämmijärv 3.9 × 106 cells mL–1. The average abundance of macrozoobenthos (without big molluscs) was 2671 ind. m–2, their biomass being 12.9 g m–2. The respective figures for big molluscs (mostly Dreissena polymorpha) were 312 ind. m–2 and 244 g m–2. Macroflora occupies approximately 5–7.9% of the total area of the lake and is rich in species (129 taxa). Dominant are typical species of eutrophic lakes. The main commercial fishes are dwarf smelt, pikeperch, ruffe, roach, bream, pike, and until the early 1990s also vendace. The total catch of fish has usually been 9000–11000 t (25–31 kg ha–1) a year.
Key words: Lake Peipsi, morphometry, hydrochemistry, biota.

Role of inflows in the phytoplankton composition of Lake Peipsi; 19–33
Abstract. The phytoplankton of the major streams flowing into L. Peipsi–Pihkva (altogether 14 streams out of 237) were studied by Estonian researchers in 1985–87 and by Russian (Pskovian) researchers in 1991–94. Two groups of inflows can be distinguished on the ground of phytoplankton composition: (1) large rivers with true phytoplankton, with an average of 40 (17–87 depending on the season) species in a counted sample of the vegetation period (Emajõgi and Velikaya rivers) and (2) small rivers whose plankton consist mainly of nonplanktonic diatoms and littoral forms and a few true planktonic flagellates or chlorophytes, as a rule less than 30 (8–43) species per sample. Generally, the abundance of algae and the number of species are positively correlated with the length and catchment area of the river. In small rivers diatoms and chlorophytes dominate with respect to the number of species, while diatoms and cryptomonads dominate with respect to biomass; in large rivers cyanobacteria (blue-green algae) prevail. In some cases where samples were collected near river mouths plankton were strongly affected by lake water depending on the direction of the wind. The inflows exert a strong influence on the lake; however, it is expressed mainly as an increase in the amount of several chemical compounds and only to a minor extent as direct introduction of certain algal species. The state of major inflows was quite adequately characterized by the composition and biomass of phytoplankton, which correlate with parameters of bacterioplankton as well as with hydrochemical parameters. Benthic and epiphytic diatoms and the saprobic index were found to reflect the state of small rivers better than phytoplankton.
Key words: inflows, phytoplankton biomass, saprobic index, species number.

Dominant zooplankton species in Lake Peipsi; 34–51
Abstract. Lake Peipsi or Lake Peipsi–Pihkva (Pskovsko-Chudskoe ozero in Russian) is a large (3555 km2) shallow (mean depth 7.1 m) moderately eutrophic waterbody with total nitrogen and phosphorus contcentrations respectively 768 and 42 mg m–3. Planktivorous smelt dominates in the fish fauna. The species of zooplankton whose abundance and biomass amount to 20% or more of total zooplankton are considered dominants. In L. Peipsi both characteristic species of oligo-mesotrophic (Conochilus hippocrepis, C. unicornis, Kellicottia longispina, Bosmina berolinensis) and eutrophic waters (Keratella cochlearis, Daphnia cucullata, Bosmina c. coregoni, sometimes even Anuraeopsis fissa and Keratella tecta) are among the dominants. Rotifer species dominate with respect to number throughout the year, while cladocerans and copepods dominate with respect to biomass. Species of the genera Bosmina and Daphnia and the copepod Eudiaptomus gracilis play very important roles in plankton in summer and autumn. The dominant species have not changed during more than 30 years (since 1965), but the degree of dominance (%) has changed. The percentage of oligotrophic species has decreased, whereas eutrophic species have become more abundant.
Key words: zooplankton, dominant species, moderately eutrophic lake, Lake Peipsi.

Taxonomic problems in the genus Polyarthra from Lake Peipsi; 52–61
Abstract. The genus Polyarthra is one of the dominant rotifer genera in Lake Peipsi (Estonia). The genus appeared to be quite interesting taxonomically. Two confusing Polyarthra morphotaxa, camouflaging each other morphologically and ecologically and coexisting with P. remata, were found in the summer rotifer community. Previously these were supposed to be rectangular morphotypes of P. remata, but thorough analysis of their morphology and trophi revealed that actually two distinct species were involved: P. cf. dolichoptera and P. cf. vulgaris. Both are atypical forms without ventral finlets, the latter moreover shows proloba variation. In the present study the range of variation in the external and trophi morphology of these forms is examined to clarify their status.
Key words: Polyarthra, taxonomy, morphology.

Pelagic ciliated protozoa in Lake Peipsi: Community composition and seasonal dynamics; 62–70
Abstract. Seasonal population dynamics and community composition of planktonic ciliates of Lake Peipsi was studied in 1997–98. Ciliate abundance and biomass peaked in spring (May) and in summer (July, August) reaching values up to 18 640 cells L–1 and 587.4 µg L–1. The community of ciliates was dominated by oligotrichs, haptorids, scuticociliates, prostomatids, and peritrichs. Larger herbivorous species dominated in spring. In summer these were replaced by smaller bacterivores. The abundance and biomass of ciliates in Lake Peipsi were in the same range as reported from many temperate lakes, with values typical of mesotrophic waters. Altogether 23 identifiable taxa were found.
Key words: pelagic ciliates, community composition, seasonal dynamics, trophic links.

Current status of an introduced Baikalian amphipod, Gmelinoides fasciatus (Stebbing), in the littoral communities of Lake Peipsi; 71–80
Vadim E. PANOV, Tarmo TIMM, and Henn TIMM
Abstract. Gmelinoides fasciatus, an incidentally introduced gammarid of Baikalian origin, has been known from Lake Peipsi since 1972. It was common and abundant in the shallow stony littoral and in some rivers connected with the lake in 1987 98. However, at open-water stations, the species has become rare in recent years compared with its peak in the early 1990s. Native gammarids have been virtually superseded by Gmelinoides in this lake.
Key words: gammarids, introduced species, lakes, zoobenthos.

Main changes in the ichthyocoenosis of Lake Peipsi since the 1950s; 81–90
Ervin PIHU and Andu KANGUR
Abstract. During the postwar period four drastic and commercially important changes have occurred in the ichthyocoenosis of L. Peipsi, related to dwarf smelt, vendace, and pikeperch. In August 1959 and 1972 mass summer kills of dwarf smelt were registered in the lake, caused by severe night-time water anoxia. The stock of smelt was restored in 2–3 years. In 1990 the abundance of vendace dropped sharply in L. Peipsi in connection with a serious deterioration of its spawning conditions (high mortality of eggs) in successive mild winters. In 1991–94 vendace was not caught but later its stock started to restore gradually. As a result of intensive use of trawls and fine-meshed Danish seines, the stock of pikeperch was strongly suppressed in L. Peipsi in 1957–83. After trawls were prohibited and the number of Danish seines was considerably restricted, the stock and catches of pikeperch began to grow rapidly. Since 1989 pikeperch has become the principal export fish in the lake.
Key words: Lake Peipsi, dwarf smelt, vendace, pikeperch.

Peipsi whitefish, Coregonus lavaretus maraenoides Poljakow, in Lake Peipsi; 91–97
Teet KRAUSE and Anu PALM
Abstract. A descriptive overview is given of the morphology, ecology, and fishery of Peipsi whitefish (Coregonus lavaretus maraenoides Poljakow). Peipsi whitefish is an endemic subspecies that evolved in L. Peipsi after the last Ice Age. The lifetime of Peipsi whitefish is up to 15 years; the biggest specimens are 60 cm long (Fl) with a total weight of 3.5 kg. Age groups 6–8 predominate in commercial catches. Peipsi whitefish spawns in autumn in the southern part of L. Peipsi s.s., where the condition of its spawning places is continuously deteriorating. Its mean absolute fecundity is 23 000 eggs. Peipsi whitefish is an euryphagous fish, consuming different food objects from zooplankton to fish. In L. Peipsi whitefish is seriously infected with Tetracotyle intermedia. In the 1990s catches of whitefish in L. Peipsi were 30–70 t, being limited by the close season and quota. Peipsi whitefish is caught with gill nets mainly at the end on June and at the beginning of July. The catch of whitefish accounts for 1% of the total annual fish catch in L. Peipsi.
Key words: morphology, growth and age, reproduction, feeding, parasites, catches.

Feeding of pikeperch, Stizostedion lucioperca (L.), in Lake Peipsi; 98–108
Abstract. The present study examines the composition of the diet and size-related as well as seasonal changes in the prey selection of pikeperch in shallow eutrophic Lake Peipsi. A total of 801 pikeperch with a standard length of 9–73 cm were sampled mostly with a bottom seine and an experimental trawl from August 1994 till October 1998 and their stomach content was analysed. The frequency of occurrence and the number and restored weight of prey fishes per individual were used for the description of diet. Fish with Sl < 15 cm had consumed only smelt. With increasing body size the diet of pikeperch was enriched with ruffe, perch, and other small fishes whilst predation pressure on smelt decreased. The main seasonal changes were revealed in the ratio of smelt to percids in the diet of pikeperch.
Key words: pikeperch, diet composition, size-related and seasonal changes, Lake Peipsi.

Food of pike, Esox lucius L., in Lake Peipsi; 109–120
Abstract. The distribution of the predatory pressure of pike over different prey species in large eutrophic L. Peipsi was studied. In 1995–98, 415 pike with a standard length of 21.5–105 cm were dissected and their stomach contents were analysed. Of the examined stomachs 42–45% were empty. The diet of pike included at least eight prey fish species, the most frequent being ruffe, smelt, and perch. The same species dominated in the diet numerically. Smelt was the commonest fish species in the diet of smaller pike (Sl < 40 cm). The proportion of smelt decreased in the food of pike with size, whereas that of ruffe increased. Cannibalism did not play an important role in the recent feeding of pike. In comparison with data from 1960–63 (Pihu, E. 1966. The importance of pike, perch, pikeperch and burbot as biological control in Lake Peipsi–Pskov. In Hydrobiological Researches, Vol. 4, pp. 235–248. Valgus, Tallinn (in Russian)), the proportion of pikeperch in the food of pike was greater.
Key words: pike, diet composition, size-related changes, Lake Peipsi.

Diet composition and food consumption level of ruffe, Gymnocephalus cernuus (L.), in Lake Peipsi; 121–135
Külli KANGUR, Andu KANGUR, and Peeter KANGUR
Abstract. Ruffe has little or no value for commercial and recreational fisheries in L. Peipsi today but it plays a very important role in the food web of this large eutrophic lake. The aim of this study was to determine the diet composition and to quantify the food consumption of ruffe in different seasons. In 1995–99 the stomach content of 387 ruffe with a standard length of 29–148 mm was investigated. The diet of ruffe is diverse (including a wide range of invertebrates, fish ova, and detritus). More than 93% of ruffe had consumed benthic animals, mainly chironomids, forming 7.3% of the total number of food items in the stomachs. The frequency of occurrence of zooplankters was 47% (72% numerically). Fish (mainly smelt) eggs were found in 10% of the ruffes’ stomachs. The feeding activity of ruffe was significantly lower in winter than in summer. Mean consumption level as the food weight percentage of ruffe’s mean body wet weight was 0.95 ±  0.05%, with a maximum of 6.8%. As ruffe grew, the weight of consumed food relative to their body weight decreased, although absolute consumed weight increased.
Key words: ruffe, diet composition, consumption level, Lake Peipsi.

Commented list of macrophyte taxa of Lake Peipsi; 136–154
Abstract. A list summarizing data on all 128 taxa of macroflora that different researchers found in Lake Peipsi in 1962–98 and a brief characterization of the habitats of the majority of the species with their frequency are presented. Comparison of the composition of the macroflora of L. Peipsi with that of L. Võrtsjärv, L. Ladoga, and other large lakes located on the northwestern edge of the Russian Platform (eight lakes) yielded 19 species that occurred in all the lakes studied and 12 that were absent in one or two lakes only. Because of the expansion of emergent vegetation (primarily reeds) to new areas several shallow-water species characteristic of L. Peipsi are receding. The recent composition of macrovegetation gives evidence of the eutrophic character of this waterbody.
Key words:  frequency, common species, specific features, anthropogenic eutrophication.

Instructions to authors; 155–158
Copyright Transfer Agreement; 159