Lake Peipsi is a large (3558 km2)but shallow (up to 15.3 m deep) tripartite waterbody hydrologically investigated already since the 19th century. Surface discharge by rivers accounts for more than 80% of its water balance. The residental time of water is about two years in the whole lake but several times less in its shallower southern parts receiving the biggest rivers. The annual water regime is characterized by the highest water in spring, the average amplitude of yearly level fluctuations being 1.15 m. There are known long-term hydrological cycles of 80-90, about 22,9-11, and even fewer years. Several temporary wind-dependent circular currents exist in the subsurface layers. Alternating transitional currents occur in the narrowest part of the lake. Five different periods are distinguishable in the annual thermic cycle. The duration of the stable ice cover is up to five months (December-April) in the shallower parts but a shorter time in the centre of the lake. The maximum surface temperature in July usually reaches 21-22°C in the open regions but considerably higher (up to 27-28OC in some years) on shallows. The unstable summer stratification is often disturbed by waves and currents. Biological summer, with surface temperatures over 10°C, lasts on an average 134 days.
Lake Peipsi-Pihkva (3555 km2 , mean depth 8 .3 m), consisting of three parts, (L . Peipsi, L . Pihkva, L . Lammijarv) is located on the 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 -2 and 327 kgP km -2 with 74% of N and 39% of P originating from agriculture . The mean concentrations of total N and P in the lake are 876 mg m -3 and 46 mg m-3, 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 dominate in summer causing the water-blooms . The concentration of Chla was the lowest in the northern part of L . Peipsi (mean 14 .7 mg m -3 ) and the highest in the southern part of L . Pihkva (mean 47 .9 mg m -3 , median 16 .3 mg m-3 ) . 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-2 d -1 . Zooplankton is remarkably rich in species, the average biomass in the vegetative period being 2-3 g m-3 and production 22 g C m -2 . The role of rotifers in production is 53% followed by that of cladocerans (30%), copepods (16%) and Dreissena polymorpha larvae (1%) . The total count of bacteria is 1-9 million cells per ml . Chironomus plumosus and Potamothrix hammoniensis are dominating in the profundal . The average abundance of macrozoobenthos (without big molluscs) 2617 ind . m -2 , and their biomass 12 .34 g m -2 are considered to be the highest among the large lakes of North Europe . Macroflora occupies a small percentage of the total lake area but is rich in species . Taxa forming communities are Potamogeton perfoliatus, Phragmites australis, Schoenoplectus lacustris, Potamogeton lucens, Eleocharis palustris, and Polygonum amphibium . Submerged vegetation occupies the first place amongst different growth forms, followed by emergent plants . The main commercial fishes are lake smelt, perch, ruff, roach, bream, pike, vendace and pikeperch. The stock of vendace has sharply decreased in the last years, while the amount of pikeperch has increased . Considering annual fish catches (9000-12000 tons or 25-34 kg ha-1 ), L . Peipsi-Pihkva exceeds all large lakes in North Europe .
Abstract. The relationship between solar activity and the water volumes of lakes is searched here by means of correlational and spectral analysis methods. The level of two lakes, Pátzcuaro in México and Tchudskoye in Russia, together with solar activity indexes are used for the analysis. It is found that the source of the oscillation mechanism of the level of those lakes is the solar activity cycle through its influence on the magnetosphere and the terrestrial atmosphere. The present study allows for the development of long-period prognostic of water volumes of big lakes.
is located on the border of Estonia and Russia. L. Peipsi s.s. belongs to unstratified eutrophic lakes with mesotrophic features, L. Limmijirv 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™, respectively. Biomass of phytoplankton fluctuated between 1 and 125 g m™, that of zooplankton from 0.088 to 6.344 g m™, with a summer average of 3.092 g m™. The total count of bacteria in L. Peipsi s.s. was 2.2 x 10°, in L. Pihkva 4.3 x 106, and in L. Lämmijärv 3.9 x 10°% cells mL™". The average abundance of macrozoobenthos (without big molluscs) was 2671 ind. m™, their biomass being 12.9 g m™. The respective figures for big molluscs (mostly Dreissena polymorpha) were 312 ind. m and 244 g m".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 1990 s also vendace. The total catch of fish has usually been 9000-11000 t (25-31 kg ha™') a year.
En el presente trabajo se analiza la influencia de la actividad solar en la precipitación pluvial, a partir de series de tiempo de Lituania (1910-1993) y Estonia (1866-1993) y utilizando datos del área de las manchas solares (1866-1993). Por medio de métodos espectrales y autoregresivos, se obtienen ciclos significativos en los procesos solares y terrestres con períodos de 6 y 12 meses y de 2.5 y 11 años. El análisis de las series anuales de precipitación de Estonia y de la superficie de las manchas solares revela cambios en el ciclo de 11 años, cuyo período se muestra reducido a 9 años entre 1915 y 1956; se observa también una oscilación con período de 5 años en las últimas seis décadas. Los co-espectros de los procesos solares y climáticos coinciden con los de otros trabajos y indican la naturaleza común de todos los ciclos y la influencia de la actividad solar en la variabilidad climática. En el presente trabajo se analiza la influencia de la actividad solar en la precipitación pluvial, a partir de series de tiempo deLituania (1910-1993) y Estonia (1866-1993) y utilizando datos del área de las manchas solares (1866-1993). Por medio de métodosespectrales y autoregresivos, se obtienen ciclos significativos en los procesos solares y terrestres con períodos de 6 y 12 meses yde 2.5 y 11 años. El análisis de las series anuales de precipitación de Estonia y de la superficie de las manchas solares revelacambios en el ciclo de 11 años, cuyo período se muestra reducido a 9 años entre 1915 y 1956; se observa también una oscilacióncon período de 5 años en las últimas seis décadas. Los co-espectros de los procesos solares y climáticos coinciden con los de otrostrabajos y indican la naturaleza común de todos los ciclos y la influencia de la actividad solar en la variabilidad climática.
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