Long-term variations of surface chlorophyll a and primary production in the open Black Sea

Abstract

Extensive data sets on surface chlorophyll a (chl a), depth-integrated primary production (DIPP) and phosphate (PO4, μM) averaged for the upper 25 in 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 (chl a and DIPP), long-term trends were obtained only for chl a since only these data covered the entire open sea and all periods investiged. A positive correlation was found between DIPP (using 2 different 14C methods: actual in situ and simulated in situ) and the more numerous chl a data 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 chl a may be valid also for DIPP. The results show that interannual fluctuations in chl a are more pronounced during the warm months, from approximately May to September. Chl a levels within this interval were moderate, with a mean of 0.15 ± 0.04 mgm-3 during the first 'quiet' period (1964 to 1986), but increased steadily at a rate of 0.06 mg m-3 yr-1 during 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 chi a to 0.26 ± 0.08 mg m-3 in 1993 and a negative trend (-0.02 mgm-3 yr-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 periods, and similar changes in the plankton community and in pelagic fish stocks in the second half of the 1980s to the beginning of the 1990s in other regions of the northern hemisphere, connected with changes in the climatic regime during this period, have led us to conclude that global climatic processes have played an important role in changing the phytoplankton characteristics of the open Black Sea and affecting the whole pelagic ecosystem, especially over the last 2 decades.