Our trip to the Black Sea began on the Dnieper River in Ukraine and ended in the tiny Turkish fishing village of Sariyer at the mouth of the Bosphorus. The Dnieper is one of five major rivers that flow into the northern side of the Black Sea and the Bosphorus, a long narrow marine channel, is its only connection to the other oceans of the world.
Wisps of early morning mist were rising from the surface of the water when our van rolled into Sariyer. The passenger ferry back to Istanbul would not be leaving for another hour - time for a second cup of coffee. Ahmet, our driver, led us to a small outdoor café on the very edge of the quay where we settled down to watch the passing scene and reflect on what we had learned during our two-week sojourn on and around the Black Sea.
At Sariyer the Bosphorus, a narrow 31 km waterway linking the Black Sea to the Marmara Sea, is only about half a kilometre wide and a little more than 100 metres deep. It effectively throttles the flow of tidal water in and out of the Black Sea and limits the mixing of oxygen-rich surface water with stagnant bottom water. Looking at the clear blue surface it's difficult to believe that 90 per cent of the water in the Black Sea is a toxic brew of hydrogen sulfide gas. For hundreds of years it has yielded a bountiful harvest of fish - salmon, sturgeon, anchovies and sardines - but all of this life is confined to the upper few hundred feet of water. Below a depth of about 150 metres the Black Sea, 1,100 km long and two kilometres deep, is a zero-oxygen "dead zone," utterly devoid of life.
The cause of dead zones is fairly simple: microscopic plants, phytoplankton, in the sunlit surface water produce oxygen as a byproduct of photosynthesis. But when they die and sink to the bottom they are broken down by bacteria which use up the oxygen and release a variety of toxic gasses. When there is no mixing between the surface and the depths the toxins are trapped and continue to accumulate.
In the Black Sea the problem is exacerbated by the addition of pollutants, particularly nitrogen and potash, which stimulate the growth of more phytoplankton and result in more dead organic matter at the bottom.
Between them the Dnieper, Dniester, Danube, Don, and Southern Bug rivers drain most of Eastern Europe into the Black Sea, bringing with them the industrial pollutants and agricultural runoff from 16 nations. Over fishing, pollution and the introduction of noxious species have all contributed to a dramatic decline in its fish population, but the Black Sea fishery continues to limp along while measures are slowly being taken to mitigate the problem.