PHYTOPLANKTON PIGMENTS AND ADENOSINE-TRIPHOSPHATE (ATP) IN LAKE PEIPSI-PIHKVA

The material for pigment analysis was collected 1-3 times a year from Lake Peipsi-Pihkva in 1983, 1987, 1988, 1991 and 1992-1995. Concentrat ions of chlorophyll a, b and c (Chla, Chlb, Chic), pheopigment (Pheo) and adenosine triphosphate (ATP) were measured biweekly in 1985-1986. The mean of all Chla values was 20.2 mg m(-1) (median 13.3 mg m(-1)) i ndicating the eutrophic state of the lake. Average Chlb, Chic, Pheo an d carotenoid (Car) contents were 3.7 mg m(-3), 4.1 mg m(-3), 3.0 mg m( -3) and 4.8 mg m(-3), respectively. The average Chlb/Chla ratio was 22 .9 %, Chlc/Chla 23.4%, Pheo/Chla 38%, Car/Chla 37% and ATP/Chla 3%, th e medians being 14.3, 13.6, 17.5, 39.4 and 1.9%, respectively. The pro portion of Chla in phytoplankton biomass was 0.41%, median 0.32%. Ther e were no significant differences in temperature, oxygen concentration , Chla, and ATP between the surface and bottom water; the lake was pol ymictic during the vegetation period. The Chla concentration had its f irst peak in May followed by a decrease in June and July. In late summ er 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 proportio n of Chla in phytoplankton biomass. The highest Chla occurred in Novem ber (average 37.2 mg m(-3)) when Secchi transparency was the lowest (1 .05 m). Concentrations of Chlb, Chic and carotenoids were the highest in August, that of Pheo in June. Concentrations of Chla and other pigm ents were the lowest in the northern part of Lake Peipsi (mean 14.7 mg m(-3), median 12.5 mg m(-3)) and the highest in the southern part of Lake 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, whil e in 1988-1994 the tendency was opposite.