Sentinel-2 imagery helps Bonaire identify influx of Sargassum

By Willy Tjiong and Sander Mücher

An abnormal amount of Sargassum brown seaweed invaded the Caribbean islands in the summer of 2011 [1]. Masses of Sargassum mats piled up on beaches trapping sea turtles and releasing harmful hydrogen sulfide gas when it decays [2]. The rotting seaweed repels tourists and have led to temporarily closure of hotel resorts and high-cleaning costs of beaches [1,3]. Sargassum was before a rare occurrence in the Caribbean Sea, yet multiple Sargassum brown tides have been reported every year since then. Climate change in combination with nutrient pollution of the Caribbean Sea might have fueled this phenomenon [4]. 

The highest record of Sargassum bloom, with approximately 20 million ton of biomass, occurred in the year of 2018 [4]. Consequently, a large amount of Sargassum brown tides flooded the Caribbean including Bonaire. An extensive area of Bonaire’s seagrass fields and mangrove forests that lies on the windward side of the island took part of the hit. When thick mats of Sargassum end up between the roots of mangrove trees they ‘suffocate’ and will lose their leaves [5,6]. Therefore, if current Sargassum trend persist, it could not only have an impact on tourism, but  it will have a direct impact on the coastal ecosystems.

Satellite Imagery

Nowadays, governments can use satellite images to map the Earth from space in a continuous manner and see high-level trends like mangrove’s health. More importantly, having ‘eyes at sea’ through satellites are useful for detecting Sargassum brown tides in (near) real-time and possibly alerting authorities before they reach the shore. Using satellite images is not ‘rocket science’ but it does require some experience to interpret them well.  

To put it simply, satellite systems take snapshots of the Earth’s surface in a sun-synchronous orbit. This allows us to view the Earth from space like photographs in daylight. In practical terms, satellite images capture the reflected energy from the sun at different regions or ‘bands’ of the electromagnetic spectrum (EM). Each object reflects a different amount of energy, and thus can be distinguished accordingly. Most satellites can capture reflected energy in the visible (i.e. red, green and blue-light) and near-infrared bands. For example, plant-like organisms like mangroves and Sargassum seaweed, strongly reflects near-infrared and absorbs red-light energy. Deep water absorbs most of the energy from the sun and therefore appears dark on satellite images [7]. 

Practical Applications

Using this information we can monitor coastal areas and sea regions for possible floating Sargassum mats. Besides, changes in mangrove areas and variation in its reflected energy can be derived from satellite images for further analysis. However, it should be noted that satellite images are limited by the information they measure. Mangrove trees and Sargassum brown seaweed are two different objects that may share similar characteristics in reflected energy. Therefore, proper validation on the field is necessary to test the reliability of the images. Moreover, cloud and weather conditions can hamper the detection of Sargassum on the sea. In this regard, researchers and imagery experts develop algorithms to fully harness the information that satellite images provide for monitoring coastal ecosystems and Sargassum accurately and to be used in decision-making systems. 

More details can also be found in MSc thesis of Willy Tjiong [8] who investigated the opportunities of Sentinel-2 satellite imagery [9] to identify sargassum. The freely available high resolution Sentinel-2 data provide weekly snapshots of the Earth’ surface, which makes it excellent for monitoring Sargassum and assessing the impact of it on the coastal ecosystem. The study shows that Sentinel-2 images are excellent at detecting floating Sargassum mats on open sea and coastal waters (see example figures 1 and 2). Further research is needed to improve the detection of Sargassum on the coast and between the mangrove stands. 

Figure 1. The left and centre picture depicts a false colour composite (NIR, green and blue bands) and potentially detected Sargassum on water (Sf) and on land (Sl), respectively. The right picture shows where these Sargassum mats were observed on 18 May 2019 near Lagun, Bonaire ⁸.

Figure 2. The left and centre picture depicts a false colour composite (NIR, green and blue bands) and potentially detected Sargassum on water (Sf), respectively. The right picture shows where these Sargassum mats were observed on 4 April 2018  a few ten kilometres east of Bonaire ⁸.