Using Sentinel-3 to monitor chlorophyll concentrations in the Black Sea

By Elena Nikolaeva (Georgian National Environmental Agency Hydrometeorological Department )

High concentrations of phytoplankton can be observed as coloured patches on the surface of water due to the presence of chlorophyll in their cells and sea currents transporting and aggregating these phytoplankton. These two principles have been used here to investigate sea surface currents using the chlorophyll concentration observation.

The Georgian Black Sea coastline currents were monitored using the OLCI Level 2 Chlorophyll concentration product from Sentinel-3, using data from 07:00 to 08:00 UTC on various dates. The products present a spatial distribution of chlorophyll concentrations across the region.

Georgian coastal currents are mainly determined by rivers flowing into the Black Sea, Coriolis forces from the Earth’s rotation, and the position of underwater features that interact with the currents. However, the patterns of the sea surface currents can also be influenced by wind direction. Surface winds can also generate upwelling currents, which interact with coastal and underwater features to pull water up from below.

Does a light breeze have an influence on the spatial distribution of chlorophyll concentrations?

Figure 1 shows several distributions of chlorophyll concentration in the Black Sea. These images are chosen as they provide data coverage over the whole Georgian coastline and one satellite frame covers the whole target area. Due to cloud cover most of images from July 2017 to February 2018 are not suitable for the aim of observing the whole Georgian coastline. However, it was possible to use a combination of some images to explore local physical and biological dynamics.

As can be seen from all the examples presented in Figure 1, the chlorophyll concentration distribution varies across the different dates. Ground observations of the wind for the time (from 07:00 to 08:00 UTC) and date of the satellite data acquisitions were used to explore the influence of a light breeze on the spatial distribution of the chlorophyll concentration. The red arrows indicate an average of the wind direction. The base of the arrow is located at the coordinates of the two ground stations near to the coastline. The length of the arrows indicates the average wind speed (m/s) during the period. The black arrows indicate the unit vector, with a length of 1 m/s.

Research of the Black Sea has shown that the main currents follow the shape of the Georgian coastline, influenced by the bathymetry. In the examples from 10 July, 3 September, 19 October, 22 October and 26 November 2017 (upper arrows) it appears that the chlorophyll concentration distribution is driven towards the shore. This is especially clear in the examples from 10 July (Figure 2, left) and 19 October (Figure 2, middle), where high concentrations of chlorophyll are visible. However, the variability in the patterns is high, possibly due to the influence of the wind directions.

Stronger southeasterly winds (~3 m/s) may contribute to the compacting of the chlorophyll in the north of the image, example from 10 July (Figure 2, left). Here it seems that the separate streams of flowing rivers have merged.

The opposite situation is the shown in the example from 19 October (Figure 2, middle). In this case the wind affects the direction of the surface current which, in turn, changes the pattern of the chlorophyll distribution. A similar effect is seen in the example from 26 November (Figure 2, right), but to a lesser degree.

In the examples from 7 August, 3 September, 19 October and 22 October (Figure 3) patterns of the chlorophyll distribution follow the direction of the arrow (wind).

There is no high concentration of chlorophyll in the cases from 6 August and 18 November (Figure 4, left and middle), nor any clear pattern of the chlorophyll distribution in the case from 15 December (Figure 4, right).

Despite the fact that the wind speed in most cases does not exceed 3 m/s (average for 1 hour), the patterns of the chlorophyll concentration is influenced by the wind direction in most of the examples. However, there are many factors to consider in both how currents are influenced, and what drives changes in chlorophyll concentration. The question is still open, but using different types of satellite data we can begin to find answers.

I would like thank to EUMETSAT for the invitation to the course “Using the Copernicus Marine Data Stream for Ocean Applications”. I was able to prepare this report based on the knowledge which I have accumulated during this course.