Monitoring Sargassum Seaweed Invasion using Remote Sensing
- Arpit Shah
- Sep 8, 2021
- 6 min read
Updated: Mar 27
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INTRODUCTION
If you are of a philosophical disposition, you are likely to concur with me when I say that of late, Mother Nature is making its displeasure very well known. Every passing day, one hears about some grave misfortune or the other - earthquakes, hurricanes, landslides, wildfires and other disasters which capture our attention and evoke our sympathies for those affected by it.
Then there are those types of adverse phenomena which do not occur all of a sudden, rather they accumulate gradually over time, escaping the attention of TRP-hungry media outlets and by extension, evading public scrutiny. Only as the murmur manifests into a spiteful demon does one realize the scale of the issue and the devastation it would wreak (or more unfavorably, has already).
Do read the latest IPCC report on Climate Change - a Code Red for Humanity.
The topic covered in this post falls under the 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 hence, are called Macroalgae - in contrast to the the unicellular, microscopic Microalgae
(My firm helps rejuvenate open water bodies and accelerate the effectiveness of Wastewater Treatment plants with nature-based nanotechnology, countering harmful algae, organic pollutants, sludge, foul odor and more).
Most species of these sea-dwelling macroalgae are rootless and do not need the support of the Seabed at any stage of its life-cycle. The gas-filled berries called pneumatocysts help the Sargassum wrack to remain buoyant and rapidly reproduce on the water surface. Free-floating as they are, these Seaweeds accumulate in large blooms of brownish 'raft/mat'-like structures as seen in Figure 2 below-
SPIKE IN SARGASSUM BLOOMS AND UNDERLYING CAUSES
Which is the only Sea in the world which has no Land boundaries?
Answer: It is the Sargasso Sea as it is bounded by ocean currents on all four sides.
This region is a floating golden rainforest - a nourishing habitat for numerous aquatic creatures such as fishes, eels and turtles which rely on the abundantly present Sargassum Seaweed for food and refuge and from which the sea itself derives its name. The mythical Bermuda Triangle also lies within.
The health benefits of consuming Sargassum are cited in research and it is farmed for its various economic applications - be it cosmetics, medicines or biogas, in various parts of the world. Besides, its presence on shores acts as a natural defence to against wind erosion.
Therefore, you may wonder - Why is the growth and spread of Sargassum deemed to be a disaster?
For starters, the scale of the invasion has been exponential, particularly from 2011 onwards-
Imagery Analytics from July 2018 revealed the largest recorded marine algae bloom - over 22 million tons of Sargassum not just restricted to the Sargasso Sea but stretching from America to Africa (~9000 kms). This very species which, in small quantities, nourished aquatic life now strangles it wherever it spreads - it depletes the water of oxygen resulting in fish kills and damage to coral reefs. Prior to 2011, small rafts of Sargassum used to land on the Caribbean and Mexican coasts for two to three weeks in a year and played a positive role in nourishing dune plants which stabilized the shoreline. Post 2011 however, enormous quantities began to land on shores between April and September and upon decomposition, released pungent gases which became hotbeds for insects to breed. The Coastal economy, especially the Beach Tourism sector, continues to suffer greatly as a result. The scale of beach-side Sargassum inundation can be observed online.
What has led to this spike in Sargassum Blooms?
Simply put, the nourishment sources for Sargassum have become plentiful. Two factors stand out-
an increase in nitrogen-concentration in the runoff from Amazon river which empties into the Atlantic Ocean from South America
an increase in coastal upwelling - a complex phenomenon where the nutrient-rich waters at the depth of the ocean rise to the top due to the wind-led displacement of the warmer waters above
Anthropogenic influences i.e. (irresponsible) human activity is a common thread contributing to both these factors - the rapid conversion of Amazonian forests into farmlands have resulted in an exponential use of nitrogen-rich fertilizers which transfers into the water body as runoff, whereas global warming and climate change have led to ocean warming, acidification and deoxygenation which jointly serve to increase the coastal upwelling rates. As a result, the increase in availability of nutrients on the surface of the ocean has spurred the growth of Sargassum. While much of the remediation efforts are directed towards controlling the spread of Seaweed, one can argue that the root cause is being deliberately overlooked.
In a way, one can draw an equivalence to the tragedy involving the Burmese Python - a magnificent reptile which is listed as a vulnerable species in its native South East Asia due to loss of habitat but has wreaked havoc in the Everglades (Florida, USA) where it has become an invasive species - no surprises that the root cause was irresponsible human behaviour. An apex predator, it has found the wetlands of Florida to be an ideal source of food and refuge. Breeding in numbers, the python population has wiped out the native birds, mammals and reptiles from the territory and is now subjected to state-sponsored hunting initiatives in a desperate bid to control its population.
SARGASSUM DETECTION USING MULTISPECTRAL SATELLITE IMAGERY
Satellite Imagery is utilized for several Earth Observation workflows including monitoring the spread of Sargassum Seaweed. Floating marine vegetation can be conveniently distinguished from water as both these surfaces respond differently to near-infrared electromagnetic waves (present in solar radiation, the mode of acquiring Multispectral Imagery). The presence of Chlorophyll in vegetation contributes to significant reflection of near-infrared wavelengths, also known as Red edge effect. This is in sharp contrast to water which completely absorbs near-infrared wavelengths-
I've chosen Caribbean Sea as my study area to perform the Imagery Analytics. Home to several picturesque islands, this region lies to the south of the Sargasso Sea and is facing the brunt of Sargassum invasion.
While the method of performing the analysis can be viewed here, the outputs derived from processing Sentinel-3 Multispectral Imagery datasets (suitable for sea-surface topography workflows) have been enclosed below-
Clicking on the image will enlarge it
While 2018 was the year of a record-breaking algae bloom, 2021 is experiencing a similar predicament going by a recent National Geographic article. This piqued my curiosity and I decided to extract and compare Sargassum detections from both these years-
Slider 1: Sargassum Detection - April 2021 v/s July 2018
The extent of Marine Vegetation is so similar!
DRAWBACKS OF USING MULTISPECTRAL IMAGERY FOR EARTH OBSERVATION
The use of Multispectral Imagery for Sargassum Detection is not always completely reliable. These Imagery Datasets can be acquired only during daytime due to the need of a passive light source - Sunlight. Even during daytime, clouds, haze and solar glint can distort the reflection considerably - as you can see in Figure 8 below. Moreover, one can only detect surface features and not completely submerged Sargassum rafts. Besides, there is a degree of uncertainty too - whether the detected floating Marine Vegetation is Sargassum or not (Ground Truthing can help in validation).
Nonetheless, Satellite Imagery is commonly-used for this Earth Observation workflow as it is cost-effective, has a high temporal resolution i.e. high revisit frequency, and can capture data across large surface areas compared to aerial equipment such as drones or fixed equipment installed at sea.
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