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Zooplankton-phytoplankton interactions have been studied in three shallow lakes of different trophic state. In strongly eutrophic, large and very shallow Lake Vortsjarv the grazing does not play a leading role in controlling phytoplankton production and its standing stock. Small-size zooplankton can not eat filamentous blue-greens. The nutrient regeneration by zooplankton has a weak impact on phytoplankton, the latter being limited rather by underwater light than by nutrient availability. In large moderately eutrophic Lake Peipsi the presence ofconcentrated zooplankton in the experimental vessel mostly stimulated steady-state phytoplankton growth and negative grazing values were measured. Most probably the nutrients (N, P), excreted by zooplanktonin grazing chamber stimulated the growth of larger phytoplankton which dominated because of heavy grazing pressure on edible forms. In Lake Peipsi phytoplankton seems to be nutrient-limited and heavily controlled by zooplankton community. In eutrophic, macrophyte-dominated Lake Yaskhan zooplankton in grazing chamber also mainly stimulated thegrowth of phytoplankton community which seemed to be nutrient-limited because of strong competition with macrophytes. [ABSTRACT FROM AUTHOR]

164 taxa were identified in the net zooplankton of the pelagial of L. Peipsi-Pihkva in 1909-1987, including 3 species of protozoans, 74 species of rotifers, 58 species of cladocerans, 28 species of copepodsand 1 mollusc. One rotifer species, Ploesoma peipsiense Maemets et Kutikova, has been described as new for science here. The zooplankton of L. Peipsi-Pihkva is remarkably rich in species including rarities in Estonia: Limnosida frontosa, Drepanothrix dentata, Bythotrephes longimanus, B. cederstroemi etc. Due to its large surface area, L. Peipsi-Pihkva provides a large scale of biotopes of a diverse trophic state and humic content, which support species with different ecologicalrequirements. Most of the aquatory of the lake has lately been mesotrophic, favouring the coexistence of indicators of oligo- and mesotrophic state and species preferring a higher trophic state. The occurrece of 10 species of the genus Bosmina including B. berolinensis, B. gibbera, B. lilljeborgi, B. thersites and B. crassicornis, sparse in Estonian lakes, is the most noteworthy feature of the zooplankton of L. Peipsi-Pihkva. The coexistence of B. coregoni and B. berolinensis, B. gibbera, B. lilljeborgi etc. which were earlier regarded as subspecies of B. c. coregoni proves that they are different species producing usually no hybrids. The species composition was subjected to certain changes during the years under consideration. Larvae of Dreissena were first found in zooplankton in 1962. The oligo-mesotrophic indicator Holopedium gibberum occurred in the lake in 1909-1964, but was lacking in later samples. [ABSTRACT FROM AUTHOR]

Primary production (PP) in Lake Peipsi-Pihkva, the tripartite borderwaterbody between Estonia and Russia, was first measured in 1965-1966. Since 1970 there exists a continuous timeseries of monthly PP measurements from May to October. Detailed investigations of the seasonaland daily dynamics as well as the vertical distribution of PP were carried out in 1985-1987. The long-term average values of integral PP (PP{sub}int{end}) in Lakes Peipsi and Pihkva were equal (0.8 g C m{sup}-2{end} d{sup}-1{end}), although the values per cubic metre (PP{sub}max{end},) differed more than twofold and characterized L. Pihkva asa eutrophic lake and L. Peipsi as a transition type between meso- and eutrophic lakes. The years from 1973 to 1980, 1987 and 1991 were oflow productivity, while in 1971, 1983, 1988 and 1990 PP peaks occurred in both lakes. In the seasonal pattern PP{sub}int{end} had peaks in May and July. In June, after the spring bloom, PP as well as the chlorophyll a (Chl) and ATP content were low. The high Chl peak in autumn was probably built up by the degradation products of chlorophyll, as neither PP nor ATP increased. Seasonal changes in integral PP in L. peipsi could be well described R{sup}2{end} = 0.91) by an empiricalmodel relating PP{sub}int{end} to PP{sub}max{end}, Secchi depth (S) and total solar radiation (Q). In mixed conditions prevailing in bothlakes, PP was inhibited in the surface layer and its maximum was located at a depth of 0.25 ... 0.5 S. The threshold total solar radiation level for the onset of inhibition was between 1200 and 2000 kJ m{sup}-2{end} h{sup}-1{end} in May and July, and decreased to less than500 kJ m{sup}-2{end} h{sup}-1{end} in October. As a rule, inhibitionstarted in the morning at a higher irradiance than necessary for keeping it up during evening hours . When compared with PP{sub}max{end} , photosynthesis in the surface layer at noon was suppressed by 56 percent in May, by 45 percent in July and by 40 percent in October. [ABSTRACT FROM AUTHOR]

The material for pigment analysis was collected 1-3 times a year from Lake Peipsi-Pihkva in 1983, 1987, 1988, 1991 and 1992-1995. Concentrations of chlorophyll a, b and c (Chla, Chlb, Chlc), pheopigment (Pheo) and adenosine triphosphate (ATP) were measured biweekly in 1985-1986. The mean of all Chla values was 20.2 mg m{sup}-1{end} (median 13.3 mg m{sup}-1{end} ) indicating the eutrophic state of the lake. Average Chlb, Chlc, Pheo and carotenoid (Car) contents were 3.7 mg m{sup}-3{end} , 4.1 mg m{sup}-3{end} 3 3.0 mg m{sup}- 3{end} and 4.8 mg m{sup}-3{end} , respectively. The average Chlb/Chla ratio was 22.9 percent, Chlc/Chla 23.4 percent, Pheo/Chla 38 percent, Car/Chla 37 percent and ATP/Chla 3 percent, the medians being 14.3, 13.6, 17.5, 39.4 and 1.9 percent, respectively. The proportion of Chla in phytoplankton biomass was 0.41 percent, median 0.32 percent. There were no significant differences in temperature, oxygen concentration, Chla, and ATP between the surface and bottom water; the lake was polymictic during the vegetation period. The Chla concentration had its first peak in May followed by a decrease in June and July. In late summer Chla increased again achieving its seasonal maximum in late autumn. The ATP concentration was the highest during spring and early summer, decreasing drastically in autumn together with the decline of primary production. ATP/Chla was the highest during the "clear water period" in June and early July, which coincided also with the high proportion of Chla in phytoplankton biomass. The highest Chla occurred in November (average 37.2 mg m{sup}- 3{end}) when Secchi transparency was the lowest (1.05 m). Concentrations of Chlb, Chlc and carotenoids were the highestin August, that of Pheo in June. Concentrations of Chla and other pigments were the lowest in the northern part of Lake Peipsi (mean 14.7mg m{sup}-3{end}, median 12.5 mg m{sup}-3{end} ) and the highest in the southern part of Lake Pihkva (mean 47.9 mg m{sup}- 3{end}, median16 [ABSTRACT FROM AUTHOR]

Lake Peipsi-Pihkva (3555 km{sup}2{end} , mean depth 8.3 m), consisting of three parts, (L. Peipsi, L. Pihkva, L. Lammijarv) is located onthe border of Estonia and Russia. L. Peipsi belongs to unstratified eutrophic lakes with mesotrophic features, L. Lammijarv has some dyseutrophic features, while L. Pihkva is strongly eutrophic. The total annual nutrient load is 15.57 tons N km{sup}-2{end} and 327 kgP km{sup}-2{end} with 74 percent of N and 39 percent of P originating from agriculture. The mean concentrations of total N and P in the lake are 876 mg m{sup}-3{end} -3 and 46 mg m{sup}-3{end} , respectively, both being the highest in L. Pihkva and the lowest in the northern part of L. Peipsi. Average pH is 8.14 and Secchi disk transparency 1.63 m. Diatoms and blue-green algae prevail in phytoplankton biomass. The blue-greens Gloeotrichia echinulata and Aphanizomenon flos-aquae dominatein summer causing the water-blooms. The concentration of Chla was the lowest in the northern part of L. Peipsi (mean 14.7 mg m{sup}-3{end} ) and the highest in the southern part of L. Pihkva (mean 47.9 mg m{sup}-3{end} , median 16.3 mg m{sup}-3{end} ). An increase of Chla and decrease of Secchi depth could be noticed in 1983-1988, while in 1988-1994 the tendency was opposite. The long-term average primary production is 0.8 g C m{sup}-2{end} d{sup}-1{end} . Zooplankton is remarkably rich in species, the average biomass in the vegetative period being 2-3 g m{sup}-3{end} and production 22 g C m{sup}- 2{end}. The role of rotifers in production is 53 percent followed by that of cladocerans (30 percent), copepods (16 percent) and Dreissena polymorpha larvae (1 percent). The total count of bacteria is 1-9 million cells perml. Chironomus plumosus and Potamothrix hammoniensis are dominating in the profundal. The average abundance of macrozoobenthos (without big molluscs) 2617 ind. m{sup}-2{end} , and their biomass 12.34 g m{sup}-2{end} are considered to be the highest among the large lakes of Nort [ABSTRACT FROM AUTHOR]

Global environmental change has influenced lake surface temperatures, a key driver of ecosystem structure and function. Recent studies have suggested significant warming of water temperatures in individual lakes across many different regions around the world. However, the spatial and temporal coherence associated with the magnitude of these trends remains unclear. Thus, a global data set of water temperature is required to understand and synthesize global, long-term trends in surface water temperatures of inland bodies of water. We assembled a database of summer lake surface temperatures for 291 lakes collected in situ and/or by satellites for the period 1985-2009. In addition, corresponding climatic drivers (air temperatures, solar radiation, and cloud cover) and geomorphometric characteristics (latitude, longitude, elevation, lake surface area, maximum depth, mean depth, and volume) that influence lake surface temperatures were compiled for each lake. This unique dataset offers an invaluable baseline perspective on global-scale lake thermal conditions as environmental change continues.