You can float in the Black Sea, but it is not as effortless as floating in some other highly saline bodies of water. The ability to float depends directly on the density of the water, which in turn is determined by its salt content. The Black Sea presents a unique environment because its surface water, where people swim, has a relatively low salt concentration. This results in buoyancy that is greater than in freshwater but noticeably less than in the open ocean. Its distinct characteristics, driven by a balance of river inflow and limited connection to the global ocean, influence floating.
The Physics of Floating
Floating is governed by the principle of buoyancy, which states that an object immersed in a fluid experiences an upward force equal to the weight of the fluid it displaces. For a person to float easily, the water must be dense enough so that the weight of a small volume of displaced water is equal to the person’s body weight. Water density is primarily increased by dissolved salts, which add mass to the water without significantly increasing its volume.
The greater the density of the water, the less volume a body needs to submerge to displace a weight of water equal to its own mass. This is why floating in a swimming pool requires some effort, but floating in the ocean is much easier. The dissolved salt acts as a natural buoyancy aid.
Salinity Levels and Comparison to Ocean Water
The Black Sea’s surface salinity is significantly lower than that of the world’s major oceans. Its average surface salinity measures around 17 to 18 parts per thousand (ppt). This is a contrast to the average open ocean salinity, which is approximately 35 ppt.
Because the Black Sea’s surface is only about half as salty as the open ocean, it offers less natural buoyancy, meaning a person will sink lower into the water while floating. This difference highlights why floating requires more effort compared to a typical ocean beach. In extreme comparison, the Dead Sea has a salinity that can exceed 300 ppt, making floating an absolute certainty due to the water’s immense density.
The Black Sea’s Unique Layering Structure
The Black Sea is the world’s largest body of water classified as meromictic, meaning its water is permanently stratified into layers that do not mix. This stratification is maintained by a dense, highly saline bottom layer overlaid by a less dense, fresher surface layer. The constant influx of freshwater from major rivers, such as the Danube, Dnieper, and Dniester, keeps the surface water density low.
This less dense surface water, which is rich in oxygen and where all marine life and human activity occur, extends to a depth of roughly 100 to 200 meters. Below this level, a sharp boundary called the pycnocline prevents the layers from mixing. The deeper layer consists of older, much saltier water that flows in from the Mediterranean Sea through the Bosphorus Strait.
The dense, deep water is completely anoxic, meaning it lacks dissolved oxygen, and instead contains high concentrations of hydrogen sulfide. This permanent deep layer covers over 90% of the Black Sea’s volume. The stability of this layering is maintained because dense, salty Mediterranean water flows in at the bottom of the basin, while lighter Black Sea water flows out near the surface.