Mapping Sargassum Seaweed invasion using Remote Sensing
- Arpit Shah
- Sep 8, 2021
- 6 min read
Updated: Dec 13
SECTION HYPERLINKS
INTRODUCTION
If you are of a philosophical disposition, you are likely to concur with me when I say that, of late, Mother Nature has been making her displeasure very well known. Almost every day, one hears about some grave misfortune or the other—earthquakes, hurricanes, landslides, wildfires and other disasters—which capture our attention and evoke sympathy for those affected.
Then there are adverse phenomena that do not occur suddenly. Instead, they accumulate gradually over time, escaping the attention of TRP-hungry media outlets and, by extension, evading public scrutiny. Only when this murmur manifests into a spiteful demon does one realize the scale of the issue and the devastation it would wreak—or, more unfavourably, has already wreaked.
Do read the latest IPCC report on Climate Change - a Code Red for Humanity.
The topic covered in this post falls squarely under this latter category: the alarming growth and spread of Sargassum, a particular type of seaweed commonly found in the Atlantic Ocean. Seaweeds are visible to the naked eye and are therefore classified as macroalgae, in contrast to unicellular, microscopic microalgae. (My firm helps rejuvenate open water bodies and accelerate the effectiveness of wastewater treatment plants using nature-based nanotechnology, countering harmful algae, organic pollutants, sludge, odour, and more.)
Most species of these sea-dwelling macroalgae are rootless and do not require support from the seabed at any stage of their life cycle. Gas-filled berries called pneumatocysts help the Sargassum wrack remain buoyant and reproduce rapidly on the water surface. Free-floating as they are, these seaweeds accumulate in large blooms of brownish raft- or mat-like structures (Figure 2):
SPIKE IN SARGASSUM BLOOMS AND UNDERLYING CAUSES
Which is the only sea in the world with no land boundaries?
It is the Sargasso Sea, as it is bounded by ocean currents on all four sides. This region is often described as a floating golden rainforest—a nourishing habitat for numerous aquatic creatures such as fish, eels, and turtles, which rely on the abundantly present Sargassum for food and refuge.
The sea itself derives its name from this seaweed. The mythical Bermuda Triangle also lies within this region.
The health benefits of consuming Sargassum have been cited in research, and it is cultivated for various economic applications—ranging from cosmetics and medicines to biogas,—in different parts of the world. Its presence on shorelines also acts as a natural defence against wind erosion.
Given this, one may wonder: why is the growth and spread of Sargassum now deemed a disaster?
For starters, the scale of the invasion has been exponential, particularly since 2011.
Satellite Imagery analytics from July 2018 revealed the largest recorded marine algae bloom—over 22 million tonnes of Sargassum, stretching not just across the Sargasso Sea but from the Americas to Africa (~9,000 km). A species that once nourished aquatic life in small quantities now strangles it wherever it spreads. Dense blooms deplete oxygen levels in water, leading to fish kills and damage to coral reefs.
Prior to 2011, small rafts of Sargassum would land on Caribbean and Mexican coasts for two to three weeks a year, playing a positive role in nourishing dune plants and stabilizing shorelines. Post-2011, however, enormous quantities began arriving between April and September. Upon decomposition, they released pungent gases and became breeding grounds for insects. Coastal economies—especially beach tourism—continue to suffer significantly as a result.
See the scale of beach-side sargassum inundation here.
So what has led to this spike in Sargassum blooms?
Simply put, its nourishment sources have become abundant. Two factors stand out:
Increased nitrogen-concentration in runoff from the Amazon River, which empties into the Atlantic Ocean
Increased coastal upwelling—a complex phenomenon where nutrient-rich deep waters rise to the surface due to wind-driven displacement of warmer surface waters
Anthropogenic influences—irresponsible human activity—form a common thread linking both factors. Large-scale conversion of Amazonian forests into farmlands has resulted in excessive use of nitrogen-rich fertilizers, much of which enters the ocean through runoff. Simultaneously, global warming have led to ocean warming, acidification, and deoxygenation, collectively increasing upwelling rates.
The resulting surplus of nutrients at the ocean surface has accelerated Sargassum growth. While most remediation efforts focus on controlling the spread of the seaweed, one could argue that the root cause is being deliberately overlooked.
A parallel can be drawn with the tragedy of the Burmese python—a magnificent reptile listed as vulnerable in its native Southeast Asia due to habitat loss, yet an invasive species in Florida’s Everglades. Introduced through irresponsible human behaviour, it found ideal conditions to thrive, devastating native bird, mammal, and reptile populations. It is now subject to state-sponsored hunting in a desperate attempt at population control.
SARGASSUM DETECTION USING MULTISPECTRAL SATELLITE IMAGERY
Satellite imagery is widely used for Earth Observation, including monitoring the spread of Sargassum seaweed. Floating marine vegetation can be reliably distinguished from water because these surfaces respond differently to near-infrared (NIR) wavelengths present in solar radiation—the energy source for multispectral imagery.
Chlorophyll in vegetation reflects NIR wavelengths strongly (the Red edge effect), whereas water absorbs them almost completely.
For this analysis, I selected the Caribbean Sea as the study area. Home to several picturesque islands, it lies south of the Sargasso Sea and has been severely impacted by Sargassum invasion.
While the methodology can be viewed separately, the outputs derived from processing Sentinel-3 multispectral imagery—well suited for sea-surface monitoring—are shown below.
Mapping Sargassum Seaweed Invasion in the Caribbean Sea on:
21 April 2021
18 May 2021
Clicking on the image will enlarge it
While 2018 witnessed a record-breaking algae bloom, 2021 appears to be facing a similar predicament, as noted in a recent National Geographic article. This prompted me to extract and compare Sargassum detections from both years.
The similarity in spatial extent is striking.
Slider 1: Sargassum Detection output: April 2021 vs July 2018
The similarity in spatial extent is striking.
DRAWBACKS OF USING MULTISPECTRAL IMAGERY FOR EARTH OBSERVATION
These datasets can only be acquired during daylight hours because they depend on sunlight as a passive illumination source. Even then, clouds, haze, and solar glint can significantly distort reflectance values. Additionally, multispectral sensors can detect only surface-floating vegetation, not submerged Sargassum rafts.
There is also an inherent uncertainty in classification—whether the detected marine vegetation is indeed Sargassum. Ground Truthing is often required for validation.
Nonetheless, satellite imagery remains widely used for this workflow. It is cost-effective, offers high temporal resolution (frequent revisit cycles), and can capture vast oceanic extents—advantages that aerial platforms such as drones or fixed marine sensors cannot match at scale.
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