Dangerous disturbances were detected due to changes in hydrosulphuric layer depth; said layer is a major part of the Black sea being 1800 m thick. This hydrosulphuric zone leads to concentration of all life forms of the unique marine basin in a thin upper water layer (130 m), making the sea very sensitive to climate impact and various biologic interferences. The Black Sea’s ecosystem is even more sensitive to alien biologic species – invaders – due to quasi-closed character of this water basin. We shall need about 2100 years to fill the empty basin of the Black sea with Mediterranean water via the Bosporus – imagine how isolated is the Black sea from the Mediterranean sea, said Mikhail Flint, research fellow from the Institute of Oceanology.

Influx of vital oxygen from upper water layer to so-called cold intermediate water layer, which defines the border of hydrosulphuric zone, happens mainly in winter. That is why any climate change (temperature fluctuations are most evident in winter) causes shifts in hydrosulphuric zone positions. Current data of oceanologists, which have been observing temperature fluctuations of the Black sea for recent 56 years, show that climate of the Black sea coast has its temperature maximum, which can be compared with that of the sixties. However, current temperature peak has followed significant regional cooling, which was detected in the beginning of the 90s. Such contrast in climate caused almost total elimination of cold intermediate water layer and drop of its oxygen content. This change was followed by rise of hydrosulphuric zone border – 12 m in recent 14-15 years. Such changes mean that total of oxygen-containing zone decreased by about 10 %.

However, everything is not as bad as it may seem. Oceanologists consider such evolution of the Black sea parameters to be reversible. Several cold years of oxygen influx to deep-sea water layers will be enough for upper border of hydrosulphuric zone to get back to its “normal” level.

Another problem, which is considered to be even more serious, is phytoplankton blooming dynamics changes. Phytoplankton is the basis of marine food chain – it feeds almost all inhabitants of the Black sea ecosystem. Scientists have discovered that summer blooming in the Black sea is not caused by its usual inhabitants – diatomic algae, which now share only 2-5% of its algae population, but by Coccolithophora, small (microns in diameter) algae, which are also responsible for high turbidity of the Black sea waters. Scientists also mention that blooming of Coccolithophora, which nature is still not clear, is detected all over the World Ocean.

However, Coccolithophora algae are not the only reason of the Black sea water transparency drop. Two invaders were detected there – two new biologic species: comb jellies mnemiopsis and beroe ovata. Mnemiopsis comb jelly invaded the sea in eighties and caused catastrophic drop in zooplankton biomass by eating it out. Thus, water transparency has dropped significantly, because eliminated zooplankton stopped eating small algae, moreover, this comb jelly exudes tons of mucus during its vital processes. Plankton-eating fish have lost their food, thus their number dropped – both dolphins and predatory fish have suffered. Fish catches became much smaller – losses of 1989-1990 amounted to $180-350 million per year.

Beroe ovata comb jelly, which has invaded the sea in 1999 due to water temperature rise of 2 degrees eats only mnemiopsis. That is why food zooplankton started growing again, even exceeding its average amount, which was detected before mnemiopsis appeared. However cold winter of 2003 showed possible abrupt changes of the situation if another cooling happens: mnemiopsis becomes abundant, and we lose fish stocks and water transparency causing extinction of very important algae responsible for clearing coastal waters.

Source:
    http://www.nkj.ru/
    http://www.ibss.iuf.net
    http://marine.rutgers.edu/

Kizilova Anna