One of the distinguishing features of the seas and oceans is their salinity. The average salinity of the world's oceans is approx. 35-36 ‰ (promille - one-thousandth of something). This means that in one litre of seawater 35-36 g is the salts dissolved in it. It is estimated that if these salts crystallized, they would cover the entire surface of the earth with a layer thicker than 100 m. But it is not only our familiar sodium chloride (table salt) but salts of various chemical elements - magnesium, potassium, calcium, copper, manganese, iron, etc. It is safe to say that seawater contains the full range of the earth's chemical diversity.
But where do the salts in seawater come from?
Most of the salts come from the land. Rainwater and groundwater, river and lake waters are in constant contact with land rocks. They interact with them and in the process, various chemical compounds are formed, including a large number of salts. And by virtue of the eternal water cycle, sooner or later they find themselves in the seas and oceans.
The seas and oceans also "produce" their own salts. Land-like reactions occur on the seabed, especially where seawater comes in contact with a rock base. But also waters springing from the bowels of the earth, which are in contact with the rocks that make up the earth's crust.
But then why doesn't the salinity increase?
The main reason is that the reverse process takes place - the salts of seawater interact with the rocks and crystallize or turn into insoluble minerals and compounds that settle to the bottom. And so an equilibrium is reached between the formation of new amounts of salts and their mineralization.
But still, there are seas with different salinity?
Yes - the Black Sea is one of them - it has an average salinity of 17.3 ‰, which defines it as moderately salty. The reasons for this are both in his historical past and in the present. The Black Sea has formed about 8000-10 000 years ago, as a result of the formation of a connection between the existing remains of the ancient Sarmatian Sea (a vast freshwater body stretching from Central Asia to Central Europe). Then, probably due to a large earthquake and heavy rainfall, the present-day Bosphorus Strait was formed, connecting the Black Sea with the Sea of Marmara (Part of the Mediterranean Sea). The waters of the latter began to invade the territory of the present Black Sea and shape its modern appearance. And so gradually its salinity increases. But not enough to be equal to that in the Mediterranean - a very small volume of water comes from the Mediterranean to the Black Sea, due to currents and the profile of the Bosphorus. Apart from that, three of Europe's largest rivers, the Danube, the Dnieper and the Don, flow into the Black Sea, carrying large volumes of freshwater, and evaporation is too low to allow salinity to increase.
Other seas are much saltier, for example, the legendary Dead Sea has a salinity of up to 350 ‰ (one-third of the volume of waterfalls on the salts in it). This is because the Jordan River flows into the Dead Sea, carrying large amounts of sediments of clay, sand, rock salt and gypsum, which settle to the bottom. The Dead Sea is a stagnant lake - its water regime is due only to evaporation, which is remarkable here - in summer temperatures exceed 50oC.
And Atanasovsko Lake?
Although Atanasovsko Lake is a coastal reservoir, its existence is inextricably linked to the sea. It is known that in the past it was a slightly salty lake - estuary, into which flowed three main tributaries - Azmakdere, Kurbardere and Dermen Dere. As a solution to eradicate the then deadly malaria in 1906, its transformation into salt pans began. The inflow of the three rivers into the lake was stopped, and their waters were directed to flow into the Black Sea. On the other hand, an artificial connection with the Black Sea has been established through a dug canal. Fresh sea waters enter the lake through it, which is the main reason to know Atanasovsko Lake as it is today - a coastal lagoon. And part of Atanasovsko Lake has salinity close to that in the Dead Sea - over 300 ‰! The life of Atanasovsko Lake depends on the sea waters - on the one hand for the production of sea salt - an indispensable natural product for man, and on the other - for its unique ecosystem to function.
But is there life in such extreme salinity?
Yes! As amazing as it is, there are a number of organisms that have adapted and successfully live in this terrifying salinity. They have developed a number of impressive mechanisms to deal with the harmful effects of salts in water - they produce specific substances to maintain their homeostasis, accumulate large concentrations of salts in their body or can be inactive adverse periods. In Atanasovsko Lake such emblematic species are the microscopic red alga - Dunaliella, which gives the water its red colour, the salt shrimp - the favourite food of the flamingo and other feathered inhabitants of the lake and the glasswort - the only plant surviving in the scorching conditions of the high.