Long-term variations of surface chlorophyllaand primary production in the open Black Sea

Extensive data sets on surface chlorophylla(chla), depth-integrated primary production (DIPP) and phosphate (PO⁴, μM) averaged for the upper 25 m layer in 1964, 1973, 1978 and for a period (1980 to 1996) of regular measurements have been used to evaluate long-term changes in the upper portion of the euphotic layer of the entire open (>1000 m) Black Sea. After preliminary analysis of seasonal dynamics, special attention was given to data obtained during those periods of the year with relatively stable values, revealing interannual and long-term fluctuations and trends. Of 2 phytoplankton characteristics (chlaand DIPP), long-term trends were obtained only for chlasince only these data covered the entire open sea and all periods investiged. A positive correlation was found between DIPP (using 2 different14C methods: actualin situand simulatedin situ) and the more numerous chladata for different monthly intervals, with significant correlation coefficients (r = 0.51 to 0.82). This means that the observed patterns in long-term variability for chlamay be valid also for DIPP. The results show that interannual fluctuations in chlaare more pronounced during the warm months, from approximately May to September. Chlalevels within this interval were moderate, with a mean of 0.15 ± 0.04 mg m–3during the first ‘quiet’ period (1964 to 1986), but increased steadily at a rate of 0.06 mg m–3yr–1during 1988 to 1991 and sharply in 1992 (mainly due to high July values) to 0.99 ± 0.7 mg m–3. In contrast, negative trends were characteristic of the third period (1993 to 1996): an abrupt decrease in chlato 0.26 ± 0.08 mg m–3in 1993 and a negative trend (–0.02 mg m–3yr–1) during 1993 to 1996. Low concentrations of PO₄ (0.015 to 0.138 μM) in the upper 25 m layer throughout the year and the absence of statistically reliable interannual trends in distribution suggest that this nutrient was limiting the level of primary production during the second period. This means that phosphate concentration in the upper 25 m layer cannot be considered a reliable indicator of the presence or absence of anthropogenic eutrophication in the open Black Sea. This long-term variability in phytoplankton characteristics agrees well with the data on interannual changes in other ecological variables of the open Black Sea characterized by a collapse of the Black Sea ecosystem during the second period and its recovery after 1992. Comparison of changes in the open and shelf areas of the Black Sea between these 3