Assessing Black Sea Water Transparency Using Satellite and In Situ Measurements

Water transparency is one of the main parameters used to to asses the status of aquatic ecosystems. Despite its non-objective nature, the Secchi depth is one of the most common methods to quantify the clarity of waters in the upper water column.

The study area is part of the Black Sea, a unique and one of the most isolated components of the Planetary Ocean. The optical regime in the northwestern shallow waters is highly influenced by the main rivers discharge (dominated by suspended particles), while the upper layer of deep waters is dominated by chlorophyll concentration. Dissolved organic matter plays also a significant role in shaping up the optical characteristics of the Black Sea waters, throughout the entire basin.

Long-time archives of in situ measurements of Sechi depth are available for the Black Sea region. This study takes advantage of the wealth of such public dataset, distributed through SeaDataNet marine data infrastructure (https://www.seadatanet.org), together with new in situ Secchi depth measurements, collected by the authors. Compared to field observation, which shows a parameter value in a specific location, satellite sensors cover wider areas providing useful spectral information at various wavelengths. An approach based on both in situ and satellite measurements is necessary to synoptically assess the water transparency variability in the Black Sea.

This study links the in situ measurements and satellite estimations of Sechi depth with governing environmental factors, such as Danube hydrological regime and algal bloom events, in order to create a holistic image of the Black Sea waters optical regime in the last decades, as well as its spatial variability.