Nutrient and phytoplankton trends on the western Black Sea shelf in response to cultural eutrophication and climate changes

Abstract

Long-term trends (1960s–1990s) in surface winter nutrients and summer oxygen concentration of the euphotic zone, as well as seasonal and interannual variability in surface chlorophyll a (chl a) and Secchi depth ( Z d) were investigated for different shelf regions (depth <50 m) of the western Black Sea. Increasing phosphate and nitrate levels and changes in the summer oxygen concentration on the shelf before the mid-1980s corresponded well with the increase in riverine nutrient inputs. At the same time, decreasing silicate levels resulted almost equally from enhanced diatom stripping and trapping of silicate in the numerous dams constructed on the Danube River. The associated decrease in the Si:N ratio caused a shift towards more non-siliceous phytoplankton blooms. A decoupling of winter nutrient levels and summer oxygen concentration on the shelf after the mid-1980s suggests that other sources of inputs, such as regenerated nutrients from shelf sediments and/or upwelling, may have increased substantially. Large variations in the regional climate during the 1980s and 1990s could potentially account for increases from either or both of these sources and the resulting high summer primary production despite decreasing winter nutrients. The seasonal pattern in chl a within the Ukrainian NW shelf is similar to the open Black Sea, with low chl a in summer and high concentrations in cold months. The seasonal chl a variations on the Romanian and Bulgarian shelves also show high concentrations in May/June, most likely related to the Danube River maximum discharge during spring. As a result, chl a annual means in these two regions are significantly higher than – in the Ukrainian NW shelf.