Post uploaded on: 8th September 2021
SECTION HYPERLINKS
INTRODUCTION
If you are of a philosophical disposition, you are likely to concur with my opinion 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 large-scale disasters which capture our attention and evoke our sympathies for those affected by it.
Then there are those adverse phenomena that do not occur all of a sudden, rathery they build up gradually, over time escaping the attention of TRP-driven media outlets and by extension, escaping public scrutiny. Only as the murmur manifests into a spiteful demon does one realize the scale of the issue and devastation that Nature 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 infestation 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 and microscopic Microalgae
(My firm, helps rejuvenate open water bodies with nature-based nano solutions, countering harmful algae, organic pollutants, disease-spreading insects, sludge and foul odor. Read more here).
Most species of these sea-dwelling algae 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 surface.
Free-floating as they are, these Seaweeds accumulate in large blooms of brownish 'raft / mat'-like structures as seen in Figure 2.
SPIKE IN SARGASSUM BLOOMS AND UNDERLYING CAUSES
A confounding Geography question for you - Which is the only Sea in the world which has no Land boundaries?
The answer is Sargasso Sea - as it is bounded by ocean currents on all four sides.
This region is a floating golden rainforest - a nourishing habitat for several aquatic creatures such as fish, eel and turtle which rely on the abundantly present Sargassum - from which the sea also derives its name. The mythical Bermuda Triangle is also located within. A web search on Sargassum will lead you to pages about the health benefits from consuming it, efforts to farm it globally and its various economic applications - cosmetics, medicines and even biogas. The presence of Sargassum on shores also helps reduce wind erosion.
Therefore, you may wonder - 'Why is the growth of Sargassum an environmental issue?'
For starters, the scale of Sargassum growth and spread has been massive since 2011 as depicted in the Map+Chart depiction in Figure 4 below-
Satellite Imagery from July 2018 revealed the largest marine algae bloom in the world - over 22 million tons of Sargassum not just restricted to the Sargasso sea but stretching from the Americas to Africa (~9000 kms wide).
This gigantic marine invasion has harmed the existing aquatic ecosystem and is posing a significant threat to the coastal economy as well. This very vegetation which, in small quantities, nourished aquatic life now strangles the same species wherever it spreads and 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, preventing wind erosion. Post 2011 however, enormous quantities land on shores right from April to September and as they decompose, pungent gases are released which become hotbeds for insect infestation.
The coastal economy, especially the Beach tourism industry, suffers greatly as a result. The scale of beach-side Sargassum infestation can be observed from images posted online.
'What has led to this spike in Sargassum Blooms?'
Simply put, the conditions required for Sargassum to grow have become more favorable - two factors in particular -
a) An increase in nitrogen-concentration in the Amazon river runoff which empties into the Atlantic Ocean from the north of South America nourishes Sargassum blooms, and
b) 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.
The common thread between both these factors is, unsurprisingly, Anthropogenic influences i.e. irresponsible Human activity. The rapid conversion of Amazonian forests into farmlands have resulted in large increases in nitrogen-rich fertilizer runoff, which contributes towards climate change increasing coastal upwelling. 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, however, one can counter with conviction that preventive measures are the need of the hour.
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 because of loss of habitat but has wreaked havoc in the Everglades in Florida, USA, where it has become an invasive species (due to human intervention, unsurprisingly). An apex predator, it has wiped out birds, mammalian and crocodile species and is now subjected to state-sponsored hunting interventions.
SARGASSUM DETECTION USING REMOTELY-SENSED MULTISPECTRAL IMAGERY
Remote Sensing / Satellite Imagery is useful for several Earth Observation workflows including mapping the scale of Sargassum invasion and monitoring its growth. Floating Marine Vegetation can be conveniently distinguished from Water as electromagnetic waves in the Near-Infrared (NIR) range (abundantly present in Solar Radiation), react differently upon interaction with both these types of surfaces. The presence of chlorophyll in Vegetation leads to the Red edge effect - heavy absorption of Visible Light but heavy reflection of NIR, thereby enabling delineation from Water - which heavily absorbs Visible Light and completely absorbs NIR. Thus, Optical Satellite Imagery, which captures Solar Radiation reflectances from the surface features on Earth is well-suited for monitoring Sargassum at Sea.
For this post, I've chosen the Caribbean Sea as the study area. It lies to the south of the Sargasso Sea. Home to several picturesque islands, this region has been facing the brunt of Sargassum invasion.
(Much thanks to RUS Copernicus for providing the training which helped to perform this analysis)
The Sargassum Detection maps (Figure 5 and Figure 6) have been prepared from two Optical Imagery datasets acquired in April and May 2021 respectively by Sentinel-3 Satellite (suitable for sea-surface topography workflows).
You can click on the maps to to enlarge it for better viewing-
While Remote Sensing data from the year 2018 captured a record breaking Algae Bloom, 2021 is experiencing a similar predicament as per a recent National Geographic article. This piqued my curiosity and I decided to extract and compare detections from both these years-
Slider 1: Sargassum Detection - April 2021 vs July 2018
The scale of Sargassum invasion is very similar, isn't it?
DRAWBACKS OF USING MULTISPECTRAL IMAGERY FOR EARTH OBSERVATION
The use of Optical Imagery for Sargassum Detection workflow is not always completely reliable. As it is, Optical Imagery Datasets can be acquired only during daytime due to the need for a passive light source - Sunlight. Even during daytime, Clouds, Haze and Solar Glint can distort the reflectance readings considerably - as you can see in the map output in Figure 8 below. Moreover, one cannot detect completely submerged Sargassum rafts using Optical Satellite Imagery either and there is a degree of uncertainty too - whether the detected floating Marine Vegetation is Sargassum or not (Ground Truthing can help validate remotely-sensed data).
Nonetheless, Optical Satellite Imagery is commonly-used for this workflow and for other large-scale detection / classification workflows as it is very cost-effective, has a high revisit frequency, and can capture data across large surface areas compared to drone-based or at-sea monitoring alternatives.
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