The Black Sea, a large inland sea between Europe and Asia, is a saltwater body with significantly lower salinity than open oceans. This unique characteristic creates a distinctive marine environment, influencing its unusual hydrographic properties and the types of life it can support.
Understanding Black Sea Salinity
The average surface salinity of the Black Sea ranges between 17 and 18 parts per thousand (‰), roughly half that of typical ocean water (33-37‰). The primary reason for this reduced salinity is a positive freshwater balance. Significant volumes of freshwater enter the Black Sea from large rivers, including the Danube, Dnieper, and Don, which collectively drain a vast area of Europe. High rainfall over the basin also contributes to this freshwater input. The combined inflow from these sources far exceeds the rate of evaporation from the sea’s surface, resulting in a net surplus of freshwater that continually dilutes its waters.
Distinct Layers and Water Flow
The Black Sea exhibits a distinct hydrographic structure, characterized by two water layers that resist mixing. The less saline, lighter freshwater-influenced layer remains at the surface, while a denser, saltier layer occupies the deeper regions. This stratification is maintained by a two-way flow of water through the Bosphorus Strait, which connects the Black Sea to the Mediterranean Sea via the Sea of Marmara.
Denser, more saline water from the Mediterranean Sea flows into the Black Sea along the bottom of the strait, sinking to form the deep layer. Simultaneously, the less saline surface water of the Black Sea flows out into the Mediterranean through the upper part of the strait. This density difference creates a permanent separation between the upper and lower water masses, severely limiting vertical circulation and mixing.
Life in the Depths
A direct consequence of these unmixed layers is that the deeper parts of the Black Sea are anoxic, meaning they are severely depleted of oxygen. Below a certain depth, around 150-200 meters, oxygen is almost entirely absent. This lack of oxygen means that most complex marine organisms, such as fish, dolphins, and many invertebrates, cannot survive in these oxygen-starved conditions. Instead, the deep anoxic layers are primarily inhabited by specialized anaerobic bacteria that can thrive in the absence of oxygen, often breaking down organic matter and producing hydrogen sulfide. This environment also contributes to the exceptional preservation of organic materials, including ancient shipwrecks, found in the Black Sea’s depths.