Changes in the hydrochemistry of the Black Sea inferred from water density profiles

Abstract

DURING the past two decades, catastrophic changes have occurred in the Black Sea ecosystem: the influx of pollution from the major rivers has caused intense eutrophication at the northwest coastal margin¹, and fish stocks have collapsed throughout the sea². The hydrochemical details of these events are still poorly understood^3–7, and a way needs to be found to distinguish long-term variations from short-term natural fluctuations^3,4 if future management of the Black Sea ecosystem is to be successful. We show here that a coherent description may be achieved by analysing the hydro-chemical data as a function of water density rather than depth. Our findings suggest that, contrary to the suggestion of Murray et al.³, the upper boundary of the low-lying anoxic waters has remained stationary since 1969, whereas the intermediate suboxic zone has enlarged, reducing the overall depth of the oxygenated upper waters by ∼20 m. Moreover, a long-term increase in the nitrate concentration and a concomitant decrease in the silicate and ammonia concentrations in this upper layer are indicative of the considerable changes taking place in the biochemical regime of the Black Sea.