The Black Sea is a semi-enclosed body of water at Europe’s southeastern edge, bordered by six countries: Ukraine, Romania, Bulgaria, Turkey, Georgia, and Russia. It covers approximately 422,000 square kilometers, with a maximum depth reaching over 2,200 meters. This sea connects to the Mediterranean Sea through the Bosporus and Dardanelles straits, which influence its distinct characteristics.
The Black Sea’s Distinctive Layers
The Black Sea exhibits a highly stratified water column, where its deep waters do not mix with the upper layers. This stratification is primarily due to a significant difference in salinity and density between the surface and deep waters, forming a permanent boundary. Less dense, fresher surface water, influenced by substantial riverine input, floats atop saltier, denser water from the Mediterranean Sea.
This density difference prevents vertical mixing, confining oxygen to the upper layer, which extends down to about 100-200 meters. Below this depth, the waters are permanently anoxic, meaning they are devoid of dissolved oxygen. This anoxic zone, constituting over 90% of the Black Sea’s total volume, is characterized by high concentrations of hydrogen sulfide. This layering shapes the distribution and types of life found throughout the Black Sea.
Marine Life in the Upper Waters
The oxygenated upper layer of the Black Sea supports diverse marine life adapted to its brackish conditions. This includes fish species such as the commercially important anchovy and sprat, which form large schools. Other fish include turbot, red mullet, and gobies, found closer to the seafloor in shallower, oxygenated shelf areas. Sturgeons, including beluga and starry sturgeon, also inhabit these waters.
The Black Sea is home to three marine mammal species: the common dolphin, bottlenose dolphin, and the Black Sea harbour porpoise. These cetaceans inhabit the oxygen-rich surface waters, feeding on abundant fish populations. The Black Sea harbour porpoise, a distinct subspecies, is smaller than its oceanic relatives and found in coastal zones.
Invertebrates are prevalent in the upper layers and coastal regions. Examples include jellyfish, such as Aurelia aurita and Rhizostomae, which drift through the water column. Mussels, oysters, and crabs are common inhabitants of the seafloor in oxygenated shelf areas, with species like the predatory clam Rapana venosa also present.
Life (or Lack Thereof) in the Anoxic Depths
Below the oxygenated surface layer, the Black Sea’s deep waters present an environment largely inhospitable to complex multicellular life. The absence of oxygen and presence of hydrogen sulfide make it a “dead zone” for most fish, invertebrates, and marine plants. Higher organisms cannot survive in these depths.
The anoxic depths are far from lifeless. They teem with microbial communities, including anaerobic bacteria and archaea. These microorganisms survive without oxygen, utilizing sulfate to break down organic matter, which produces hydrogen sulfide. Some bacteria also metabolize compounds like manganese and sulfur. This microbial activity plays a role in the deep-sea biogeochemistry of the Black Sea.
How Geography Shapes Black Sea Species
The Black Sea’s species composition results from its geological history and restricted connection to the global ocean. Historically, the Black Sea was a freshwater lake, isolated from the Mediterranean. Around 7,500 years ago, rising sea levels allowed Mediterranean saltwater to breach the Bosporus sill, forming the stratified, brackish sea.
The Bosporus and Dardanelles straits limit water exchange with the Mediterranean, contributing to the Black Sea’s lower salinity. This restricted connection limits species influx from the Mediterranean, resulting in lower biodiversity. However, it has fostered the evolution of endemic species. Freshwater input from major rivers like the Danube, Dnieper, and Dniester also influences the salinity balance and supports adapted species.