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  • Writer's pictureArpit Shah

Mapping Sargassum Seaweed Invasion using Remote Sensing

Updated: Jul 3, 2023

If you are of a philosophical disposition, you are likely to concur when I conclude that of late, Mother Nature is making its displeasure very well known. Every passing day, we hear of some misfortune or the other - earthquakes, hurricanes, landslides, fires and other large scale disasters which capture our attention and evoke our sympathies for those affected.


Then there are those disasters which build-up gradually over time - those which escape the daily attention of the news media and by extension, escapes our attention as well. Only as it manifests into a spiteful demon do we realize the trouble we have gotten ourselves into. For example, the latest IPCC report on Climate Change - a Code Red for Humanity.


Sargassum fluitans brown algae on rubbly aragonite sand beach (San Salvador Island, Bahamas)
Figure 1: "Sargassum fluitans brown algae on rubbly aragonite sand beach (San Salvador Island, Bahamas)" by James St. John is licensed under CC BY 2.0

The topic which I will cover today may well fall under the latter category - the disconcerting growth of Sargassum, a particular type of seaweed which is commonly found in the waters of the Atlantic Ocean.


Seaweed, unlike its unicellular algae counterparts, can be seen with the naked eye, hence it is also called 'macro'algae.



 
Sargassum seaweed inundating a beach in Oistins, Barbados
Figure 2: Sargassum seaweed inundating a beach in Oistins, Barbados "P6250014-by Kaspar C is licensed under CC BY-NC 2.0

The defining feature of most Sargassum species is that they do not need the support of the sea-floor at any stage of its life-cycle, especially reproduction, which happens on the water surface itself.


The gas-filled berries (pneumatocysts) on this seaweed helps it to stay afloat and they bunch together in large swathes of brownish 'raft/mat' like structures.


 

Before we move ahead, a confounding geography question for you - Which is the only sea in the world which has no land boundaries?

Figure 3: Sargasso Sea depiction.
Figure 3: Sargasso Sea depiction. ShaneGross.com

A: Sargasso Sea - Because it is surrounded by four Atlantic Ocean currents. This region is also known as the 'floating golden rainforest' due to the abundant presence of Sargassum (from which it derives its name) and the thriving marine ecosystem which it nurtures.


Seafarers used to navigate here with caution and superstition as the mythical Bermuda Triangle envelops this region.


A basic Google search on Seaweed or Sargassum will lead you to pages related to health benefits of consuming this plant, efforts to farm it across the world and its various economic uses - from shampoo and cosmetics to medicines and even extracting biogas. Additionally, not only does Sargassum provide a nourishing habitat for several aquatic creatures (fishes, eels, turtles etc.) but also its presence on the shores helps prevent wind erosion. Therefore, you may wonder as to why this supposed growth of this innocuous type of seaweed is a distressing phenomenon...

 

For starters, the scale of Sargassum's growth has been massive since 2011. Consider the map based depiction below and observe its increase from the graph right at the bottom of the visual-

Sargassum Growth Map
Sargassum Growth Chart
Figure 4: Sargassum Growth Map & Chart: Source: Island Innovation - https://islandinnovation.co/caribbean-seaweed-problem-costing-millions/

In terms of measurement, the July 2018 readings represented 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). This marine invasion of gigantic proportion has harmed both the existing aquatic ecosystem as well as posed a significant threat to the human ecosystem along the coasts.


This very plant which, in small quantities, nourished aquatic life now strangles the same species wherever it spreads and makes the water deplete of oxygen resulting in fish kills and damage to coral reefs. Prior to 2011, small lumps of Sargassum used to land on the coasts for two to three weeks in a year and played a positive role in nourishing dune plants and stabilizing the shoreline, preventing erosion. Post 2011 however, vast quantities land on the shores right from April to September (spring to summer) and upon decomposition, release pungent gases and become hotbeds for insect infestation.


The coastal economy, especially the popular Caribbean and Mexican beach tourism industry, suffers greatly as a result. The scale of Sargassum inundation can be observed from the photos here.

 

What has led to this spike in growth?


Simply put, the conditions required for Sargassum growth have become more favorable. Two factors contribute predominantly - a) An increase in nitrogen rich fertilizer runoff from the Amazon river which empties into the Atlantic Ocean from the northern part of South America and b) An increase in coastal upwelling - a complex phenomenon involving the nutrient rich waters at the depth of the ocean rising to the top due to the wind-led displacement of the warmer waters above.


The common thread between both these factors is, of course, unrelenting and unrepentant human activity. While the rapid conversion of Amazonian forests to agricultural fields have resulted in the increase in fertilizer runoff, climate change is to be blamed for the increase in coastal upwelling.


Both these factors have led to an increase in nutrients on the surface of the ocean resulting in a flourishing environment for the Sargassum seaweed. While much of the remediation efforts are directed towards controlling the spread, however, one can argue with considerable conviction that preventive measures are the need of the hour rather than corrective actions.


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 because of loss of habitat in its native region i.e. South East Asia but has wreaked havoc in the Everglades - Florida, USA, where it became an invasive species (due to human intervention, unsurprisingly) and nearly wiped out all other native wildlife and is, therefore, subjected to considerable hunting now.

 

Analyzing satellite imagery has played an important role in understanding the scale of the Sargassum problem and observing its growth patterns. Floating vegetation can be distinguished from water as light, especially in the near-infrared spectrum, interacts differently with both these types of surfaces. The main surface property difference lies in the presence of chlorophyll in vegetation which leads to the Red edge effect thereby enabling mappers to extract this information from optical imagery.


For this exercise, the area of study is the Caribbean Sea which lies to the south of the Sargasso Sea. Home to several picturesque islands, this region has been facing the brunt of Sargassum inundation.


(Much thanks to RUS Copernicus for the training provided to perform this analysis)


Sargassum detections have been made on two days in April and May 2021 respectively. You can click on the two maps below to enlarge and view it.

Mapping Sargassum Seaweed Invasion in the Caribbean Sea - 21st April 2021
Figure 5: Mapping Sargassum Seaweed Invasion in the Caribbean Sea - 21st April 2021
Mapping Sargassum Seaweed Invasion in the Caribbean Sea - 18th May 2021
Figure 6: Mapping Sargassum Seaweed Invasion in the Caribbean Sea - 18th May 2021
 

While 2018 was a record breaking algae bloom year, 2021 is experiencing a similar predicament as per a recent National Geographic article. We can compare the imagery outputs of these two years and verify the situation for ourselves-

Very evident, isn't it?

 

The continuous stream of imagery captured by Earth observation satellites facilitates the monitoring of such critical events. However, the extracted output is not completely reliable for an accurate assessment. Optical imagery works only when there is a passive light source available i.e. sunlight. Even during the day - clouds, haze and solar glint distort the imagery considerably.

This imagery output from July 2021 at another location in the Caribbean sea is heavily distorted because of presence of clouds (where grey basemap is visible) and presence of solar glint (black section on the right).
Figure 7: This imagery output from July 2021 at another location in the Caribbean sea is heavily distorted because of presence of clouds (where grey basemap is visible) and presence of solar glint (black section on the right).

Additionally, Sargassum rafts which are lying completely below the water surface are likely to go undetected using this method. There is a degree of uncertainty too - whether it is truly Sargassum or some other type of floating vegetation (This can be addressed by having in-situ data for validation purposes).


Nonetheless, satellite imagery analytics is a commonly used monitoring mechanism for such large scale phenomenon due to it being more cost-effective than, for example, accurate but expensive drone-based monitoring.

 

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Regards,

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