PRIMARY PRODUCTION OF LAKE PEIPSI-PIHKVA

Primary production (PP) in Lake Peipsi-Pihkva, the tripartite border w aterbody between Estonia and Russia, was first measured in 1965-1966. Since 1970 there exists a continuous timeseries of monthly PP measurem ents from May to October. Detailed investigations of the seasonal and daily dynamics as well as the vertical distribution of PP were carried out in 1985-1987. The long-term average values of integral PP (PPint) in Lakes Peipsi and Pihkva were equal (0.8 g C m(-2) d(-1)), although the values per cubic metre (PPmax) differed more than twofold and cha racterized L. Pihkva as a eutrophic lake and L. Peipsi as a transition type between meso- and eutrophic lakes. The years from 1973 to 1980, 1987 and 1991 were of low productivity, while in 1971, 1983, 1988 and 1990 PP peaks occurred in both lakes. In the seasonal pattern PPint ha d peaks in May and July. In June, after the spring bloom, PP as well a s the chlorophyll a (Chi) and ATP content were low. The high Chi peak in autumn was probably built up by the degradation products of chlorop hyll, as neither PP nor ATP increased. Seasonal changes in integral PP in L. Peipsi could be well described (R(2) = 0.91) by an empirical mo del relating PPint to PPmax, Secchi depth (S) and total solar radiatio n (Q). In mixed conditions prevailing in both lakes, 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 inhi bition was between 1200 and 2000 kT m(-2) h(-1) in May and July, and d ecreased to < 500 kJ m(-2) h(-1) in October. As a rule, inhibition sta rted in the morning at a higher irradiance than necessary for keeping it up during evening hours. When compared with PPmax, photosynthesis i n the surface layer at noon was suppressed by 56% in May, by 45% in Ju ly and by 40% in October.