-The paper discusses spatial distribution of suspended matter in the southwestern Crimean coast of the Black Sea according to high-resolution satellite data from Sentinel-2 MSI scanner and medium-resolution satellite data from MODIS Aqua and Terra in 2017-2019. Spatial and temporal evolution and wind transport of suspended matter are studied along with their contribution to the anthropogenic load of the coastal areas. Satellite observation data are compared with long-term annual mean data on the concentration of suspended matter from 1998 to 2018. The analysis of remote sensing data helped identify the areas with the highest values of bio-optical indices, which are consistent with the location of the river mouths and municipal sewage pipes.
catchment area of 2 million km2 including the territories of 20
states of Europe and Asia Minor. The effect of the continental
waters is the most noticeable in the northwestern shelf of the
Black Sea, where the Danube, Dniester, Dnieper and Southern
Bug flow into the sea with a total catchment area
of 1.46 million km2 [
and includes the southwestern part of the Crimean coast adjacent to Sevastopol – constituent territory of the Russian Federation. This area follows the coastline along the west, southwest, and south of the Crimea, including successively Kalamitskiy Bay, the northern side of Sevastopol, the Heracleian Peninsula, and the Southern coast of the Crimea up to Foros (Fig. 1). To the north of the Heracleian Peninsula, the Kacha, Belbek, Alma, and Chernaya rivers flow into the Black Sea bringing in the pollutants. In addition, along the entire coast, there are pipes of centralized municipal waste water discharge. The coastal area under study also gets the anthropogenic pressure from numerous unauthorized waste water discharges of illegal tourist sites. The recreational importance of the region emphasizes the relevance of the choice of the study area.
To implement the approach based on the analysis of satellite images, we used the data of Sentinel-2 MSI optical scanner from the open archives of the European Space Agency available on the Copernicus Open Access Hub portal (https://scihub.copernicus.eu/dhus/#/home). These data include visible channels (490–665 nm) with a 10-meter spatial resolution and near infrared channels (705 nm) with a 20meter resolution. First, among the images for 2017–2019, those with no or minimal cloud cover above the coastal zone were selected, then the data were decrypted using SNAP Desktop software. In the selected satellite archives, the suspended matter was detected by a combination of channels: B02 (Blue) – 490 nm, B03 (Green) – 560 nm and B05 (Vegetation red edge) – 705 nm. Then, the values of remote sensing reflectance (Rrs(λ) at a wavelength λ) were extracted for each pixel and transferred to a model grid using the Panoply application.
NASA Ocean Color (http://oceancolor.gsfc.nasa.gov) were
also used for analysis. These data are standard MODIS L2
products with a spatial resolution of kilometer. Part of them
were previously rejected according to the criteria listed in [
Local wind situation plays an important role in the
transport of anthropogenic pollution. In the framework of this
study, statistical analysis of the time series of wind speed and
direction for the period 2017–2019 was carried out, based on
the data from the SKIRON regional atmospheric reanalysis
(http://forecast.uoa.gr). The version of the atmospheric
model used is a 72-hour forecast of meteorological
parameters for the Azov-Black Sea and Mediterranean basins
with a time step of 2 hours and a horizontal resolution of 0.1°,
which was created at the University of Athens based on the
assimilation of meteorological observations [
The prevailing wind directions in the study area were determined as well as the frequency of their wind speed in different years and seasons during the study period. For the entire region under study and separate points corresponding to river mouths and municipal effluents, mean wind speed and direction were determined. Vectors in Fig. 3–5 show the results of the wind speed averaging over the two days before the date of the shot. These values were also averaged over space in three latitudinal ranges: (1) 44°–4.5°N; (2) 44.5°–45°N; (3) 45°–45.7°N, – and summarize the data on wind direction and speed for the sections of the coast near major cities: Saki, Sevastopol and Yalta.
Satellite observation data were converted into spatial
distribution maps of the bio-optical indices index and bbp on
a model grid with a resolution of ~ 300 m at the shelf of the
southwestern part of the Crimean coast
(http://www.gebco.net). The dimensionless index
characterizes total concentration of the living and nonliving
components of organic matter in sea water [
Consider the criterion for determining the boundary of the area using the satellite data. A total of 268 images were processed, of which the most regular data were from May to September (116 images). The boundary of the polluted area from the satellite data was determined by the minimum values of the color indices index and bbp. The criterion for determining the boundary of the area includes several stages. At the first stage, spatial maps of color indices for the entire study period 2017–2019 were constructed on a model grid. At the second stage, the upper and lower boundaries of the color indices were determined, since the distribution of index and bbp in each case lies in different ranges. When comparing with the satellite images which correspond to the cases of the most powerful sewage and river flows, the upper bounds for the indices were obtained: index23 = 2, index34 = 9, and bbp(560) = bbp (555) = 1 m-1. The upper boundaries correspond to the real maximum values from the ranges of the index variability and are driven by the need for correct data presentation on the maps. The lower boundaries are the same for all cases and are defined as 2.7% and 2.5% relative to the upper bounds of the bio-optical indices (index23 = 0.05; index34 = 0.25 and bbp(560) = bbp(555) = 0.025 m-1).
constructed based on the satellite data sufficiently covering the water area under consideration. In order to study the cases of increased content of suspended matter at the sea surface, all satellite images for 2017–2019 from Sentinel-2 MSI and MODIS Aqua/Terra were analyzed. Due to the common problems of remote sensing (cloud cover), for each year, only three periods (1–3 days) were selected when the area adjacent to the southwestern part of the Crimean Black Sea coast was clearly visible in the optical images. Typical examples of these events demonstrating the spread of suspended matter in the study area in 2017–2019, are demonstrated in Fig. 3–5. They show the optical images obtained from Sentinel-2 for each date (Fig. 3–5, left panels) and the corresponding time samples of the index distributions from high and medium resolution satellite data (Fig. 3–5, central and right panels).
recognized in both the constructed maps and in the corresponding satellite images (Fig. 3–5). In particular, there are cases with a relatively high content of suspended matter without sharp frontal changes off the western coast of the Crimea (the Kalamitskiy Bay and part of the area off the Heracleian Peninsula), and the cases where suspended matter transports along the southern coast of the Crimea are less common. As a rule, the narrow southern part of the area 5–20 m above the isobaths, stretches along the southern side of Sevastopol up to Foros. Note that within the shallow Kalamitskiy Bay, suspended matter can spread both along the coast and into the open sea. At the areas adjacent to the Heracleian Peninsula and the Southern coast of the Crimea, coastal transport (with clear boundaries) predominates, which is associated with the presence of temporary wind currents and the passage of sub-mesoscale eddies.
Comparing the wind fields obtained using SKIRON reanalysis data with the maps of bio-optical indices and satellite images confirms the influence of wind on the direction of suspended matter transport. Fig. 3–5 show the wind direction (clockwise from the north), which has the highest repeatability within the three subregions adjacent to major cities (Saki, Sevastopol and Yalta). They also show the average values of wind speed. All the observed suspended matter transfer cases along the southern coast found in the satellite imagery occurred either during strong southeast winds or shortly after them at an average wind speed of 7 m/s. The prevailing wind direction on the day of satellite sounding or on the previous day was between 120° and 170°.
At the same time, high amounts of suspended matter in the Kalamitskiy Bay were noted under weak wind with a predominant direction between 135° and 225°.
The values of bio-optical indices, which increase by the
beginning of summer and remain within the limits of their
upper bounds until the beginning of autumn, in many cases
fit well into the existing ideas [
A joint analysis of the area with elevated amount of suspended matter from the satellite images and the corresponding SKIRON atmospheric reanalysis data (Fig. 3– 5) demonstrates that the cases when the area is the largest detected on April 27, 2017 (708 km2), September 19, 2018 (1144 km2) and June 18, 2019 (1788 km2), occur under the prevailing south and southeast winds at a speed of nearly 5 m/s. Satellite images obtained on April 27, 2017 correspond to the largest area with elevated values of suspended matter in response to a strong southeast wind (6–8 m/s), which dominated in the study area from April 22, 2017. Note that due to the shallow waters of Kalamitskiy Bay, suspended matter here is also connected with the presence of turbid seawater. Thus, with the prevailing southeast wind, the region with a high suspended matter content (1144 km2) in the Kalamitskiy Bay occupies nearly twice its water area (504 km2) limited by Cape Yevpatoriysky in the north and by Cape Lukull in the south.
The paper shows the possibility of using satellite observations to detect the cases of suspended matter distribution in the sea that occur regularly in the coastal areas of the southwestern Coast of Crimea. Based on the meteorological and satellite data, the spatial characteristics of the areas with high suspended matter content in 2017–2019 were studied in relation to the wind action variability. The analysis of the area in the northwestern shelf of the Black Sea with high suspended matter content was carried out, the directions of suspended matter transport were established depending on the wind regime. Based on the comparison of atmospheric reanalysis and remote sensing data, it was revealed that the distribution of turbid waters is directed mainly along the coast and depends on the strength and direction of the acting wind. It was found that at the southwestern coast of the Crimea in 2017 and 2019, the predominant direction of the coastal suspended matter transport was east, and in 2018, north-west. To the east of the Heracleian Peninsula, coastal transport prevails, which is associated not only with the wind disturbances, but also with the passage of submesoscale eddies. As a result of the study, it was found that the greatest anthropogenic load in the summer of 2017–2019 affected the areas of the river Belbek mouth and the outfall of the waste water treatment facility "South" (Sevastopol). The southeast winds in the area of the Kalamitskiy Bay lead to an increase in the area with a high suspended matter content up to 1144 km2.
A detailed study of the influence of wind effects on the structure and dynamics of pollution in the surface layers of the Black Sea coast of Crimea requires specific field measurements, as well as numerical modeling and is included in the scope of the future work. In further studies, it is supposed to estimate the magnitude of the displacement of the boundaries of the pollution and to use, along with satellite images, the results of three-dimensional hydrodynamic modeling and in situ measurements data.