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The objective of this study was to investigate long-term trends in the subsurface sum- mer phytoplankton community and their underlying mechanisms in relation to nutrient enrichment of the open Black Sea. We analysed parameters describing the summer chlorophyll a (chl a) and annual mean nitrate depth distribution in terms of their maximum concentration and its depth, the mean nitrate gradient for the layer between the 2 maxima, and Secchi depth. Investigating the entire period from 1964 to 1996 did not reveal any direct relationship between the nitrate and phytoplank- ton levels. The nitrate level in the open Black Sea increased until 1984/85, corresponding with gen- eral increases in anthropogenic riverine nutrient inputs to the sea. Thus, the average maximum nitrate concentration and the nitrate gradient for all depth profiles increased between 1969 and 1984 and then remained constant. Despite the early increase in nitrate characteristics, the maximum in the chl a profile remained unaltered from 1964 to 1984. After the mid-1980s, the magnitude of the chl a profile gradually increased to 1991 and peaked with an exceptionally high concentration in 1992. In this same period there were only moderate fluctuations in the nitrate profile characteristics. The change of the summer phytoplankton biomass levels since 1984/85 coincided with an intensification of the Rim Current and the associated transport of nitrate to the euphotic zone. Prior to 1984/85 strong stratification and a deep halocline resulted in nitrate accumulation only, whereas uplifting of the halocline, combined with intensified advective currents, increased the transport of nitrate to the euphotic zone during the summer. This resulted in an associated increase in phytoplankton produc- tion and biomass in the following period. While this study shows that the open Black Sea, like the coastal regions, has been impacted by eutrophication, the response is different from the coastal region due to the different physical characteristics of the open waters.

Long-term variability of vertical chlorophyll a and nitrate profiles in the open Black Sea: eutrophication and climate change