Sentinel-3 spots Sargassum seaweed

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In early July 2019, Sentinel-3 captured images showing blooms of Sargassum seaweed, which made the headlines due to their rapid growth and how far they spread.

Sentinel-3 spots Sargassum seaweed
Date & Time
3 July 2019 14:00 UTC
Ocean Colour

By Ben Loveday (PML) and Hayley Evers-King (EUMETSAT)

Sargassum seaweed occurs naturally in its namesake - the Sargasso Sea (Figure 1). However, recent studies using time series of satellite data (Wang et al., 2019) have found an increase in both spatial extent and density of this seaweed — stretching across the Atlantic Ocean.

Sargassum plays an important role in ocean ecosystems as a habitat for pelagic (open ocean) animals, including various species of fish and turtles.

However, when large amounts of this seaweed reach the shore, they can cause numerous problems for both people and animals. Beaches full of rotting seaweed can affect tourism, and pose a potential health issue for beachgoers (Figure 2).

Figure 2
Figure 2: Sargassum rotting on a beach in Tobago. Credit: Jeff Mayers

Satellite ocean colour data provides images at suitable temporal and spatial scales to track Sargassum from the open ocean, to the coastal zone. Figures 3 and 4 show how Sargassum can be detected using ocean colour data from the OLCI sensor aboard Sentinel-3, in both true colour imager (Figure 3) and using specific mathematical formulae (‘algorithms’) such as those used in the aforementioned study (Figure 4).

Figure 3
Figure 3: An enhanced colour (Red-Green-Blue) Rayleigh corrected OLCI RGB image, showing Sargassum accumulations (green string-like features) near Tobago and Grenada, 3 July 14:00 UTC.
Figure 4
Figure 4: Sargassum can be identified from ocean colour data using the Floating Algal Index algorithm of Chuanmin Hu.

In a project by CLS, funded by the European Space Agency, satellite data from various sensors including OLCI is being used to track the Sargassum and help local managers prepare for its impacts.

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