How did Stepan Osipovich Makarov study water exchange through the Bosporus?

In 1881–1882, the Russian naval commander and oceanographer Stepan Osipovich Makarov conducted a detailed investigation into the Bosporus to study its water exchange.1

His research identified that the water exchange through the strait is characterized by a two-layer system. This complex flow involves surface water from the Black Sea exiting toward the Sea of Marmara, while deeper, saltier water from the Sea of Marmara flows in the opposite direction into the Black Sea.12