Black Sea

The Black Sea is like a two-layer drink.

Light, less salty water floats on top. Heavy, salty water stays at the bottom. The two layers hardly mix.

Fresh river water is lighter than salty sea water. In the Black Sea, the river water stays on top, in a layer about 150 meters thick. Heavier, saltier water from the Mediterranean sinks below it.

Because the layers hardly mix, oxygen from the air cannot reach the deep water. Down there, the dissolved oxygen drops to around zero. Bacteria in the deep water make a gas called hydrogen sulfide, which smells like rotten eggs.

The Black Sea's density stratification is the key: low-salinity river water forms the upper 150 m, while denser saline water from the Mediterranean, entering over the Bosporus, fills the basin below. This stable layering has persisted for about 7,000 years and largely prevents vertical mixing.

Below 150 m the dissolved oxygen concentration falls to around zero, so higher life forms such as plants and animals cannot exist there. In the oxygen-free water, bacteria produce hydrogen sulfide; Jutta Niggemann of the University of Oldenburg says that the smell of rotten eggs from a deep-water sample almost knocks you over.

Sea water holds tiny bits left over from living things.

Deep in the Black Sea, a smelly gas joins onto these bits and makes them last longer. So the Black Sea stores lots of carbon.

Seawater everywhere contains tiny amounts of dissolved organic matter, bits left over from living things. The Black Sea holds more than twice as much of this carbon as seas that are full of oxygen.

In 2021 scientists suggested a reason. Hydrogen sulfide in the deep water reacts with the organic matter and makes new compounds that last longer, so they build up. Nearly a fifth of the organic molecules in the oxygen-free water contain sulfur.

A team led by Gonzalo Gomez-Saez (AWI) and Jutta Niggemann (ICBM, University of Oldenburg) proposed in Science Advances that reactions with hydrogen sulfide stabilize dissolved organic matter (DOM) in anoxic waters. The reaction products are longer-lived than the reactants and accumulate, which helps explain why the Black Sea holds more than twice as much organic carbon as oxygen-rich marine areas.

Nearly a fifth of all organic molecules in the Black Sea's anoxic waters contained sulfur, far more than elsewhere, and many compounds were unique to the Black Sea. Niggemann describes this as a negative feedback that can counteract global warming over geological periods; the lead author, Gomez-Saez, adds that under current conditions the effect is too weak to noticeably affect climate change.

Plants and animals live only in the top layer, where there is oxygen.

Where the Danube River meets the sea, there are marshes and lakes. Hundreds of kinds of birds live there.

All the Black Sea's plants and animals live in its upper layer, because the deep water has no oxygen.

In Romania, the Danube River reaches the Black Sea through a delta of lakes and marshes. UNESCO calls it the largest and best preserved delta in Europe. More than 300 kinds of birds and 45 kinds of freshwater fish live there.

Because dissolved oxygen falls to near zero below 150 m, higher life forms are confined to the upper layer; the anoxic depths are the realm of bacteria.

The Danube Delta, where the Danube's waters flow into the Black Sea, was inscribed as a World Heritage Site in 1991 under criteria (vii) and (x) and covers 312,440 ha in Romania's Tulcea County. UNESCO describes it as the largest and best preserved of Europe's deltas, hosting over 300 bird species and 45 freshwater fish species in its many lakes and marshes; it is also a biosphere reserve and a Ramsar wetland.

Black Sea beaches are fun in summer.

Swim only where lifeguards watch, with a grown-up. If the water pulls you, swim along the beach, not against it.

General beach advice from the US National Weather Service is to check the local forecast, ask the lifeguards and swim only where lifeguards are on duty.

Rip currents are narrow channels of fast water that can form even on sunny, calm days. Don't try to swim straight back against one. Swim parallel to the beach until you are out of it, then head back to shore at an angle.

Beach-safety guidance from the US National Weather Service is general enough for any coast: check the local forecast, talk to lifeguards, swim at guarded beaches and never turn your back on the water, since rip currents often form on calm, sunny days.

NOAA's National Ocean Service notes that rip currents can reach 8 feet (about 2.4 m) per second, faster than an Olympic swimmer, and that swimmers who fight them tire. The advice is to swim parallel to the shore and return at an angle.