"We propose here that the Sargassum-seaweed bloom problem in the Caribbean-Gulf of Mexico-Atlantic can be greatly decreased, particularly in its peak years, if less nutrients carried by the seawater reach the Sargassum, thereby inhibiting their growth.
This can be done in an eco-friendly manner by broadly cultivating seaweeds in key locations, coasts and offshore—these locations will be selected based on waters with high nutrient content and other criteria.
These cultivated seaweeds will uptake for their own growth the nutrients that otherwise would go on to promote the growth of Sargassum mats, thus inhibiting the formation of blooms, while providing in a profitable manner controlled-production of seaweed biomass. This seaweed biomass stays in the place where the seaweeds are grown on or right below the sea surface, attached to ropes and nets, and has many uses (and will have more uses as the seaweed revolution advances).
Moreover, seaweed cultivation provides a variety of ecosystem and socio-cultural services (besides the bio-remediation role considered here), including mitigating climate change and enriching biodiversity, and may eventually come to represent the needed complement to growing global agricultural limitations and food security."

Ricardo Radulovich, PhD
Francisco Baradín
José Andrés Sánchez

Department of Biosystems Engineering

University of Costa Rica

Many, including us, are searching for uses of the Sargassum blooms’ biomass as the means to deal with the problem, perhaps even turning it into an opportunity.

The other way to deal with the problem is to have no problem. This is the solution we advocate here, to decrease Sargassum blooms by intercepting nutrients along the pathway in the water before nutrients reach Sargassum, or vice-versa.

The nutrients that cultivated seaweeds will uptake from the water to produce a ton of their biomass will now not produce a ton of Sargassum biomass.

The problem

Many, including us, are searching for uses of the Sargassum blooms’ biomass as the means to deal with the problem, perhaps even turning it into an opportunity.

However, notwithstanding advances based on the many applications of the seaweed biomass—from food/feed to a variety of other uses (which are most of them commonplace among people used to seaweeds, though new uses are arising), the major complication is the one-two punch of the unpredictability of blooms and the fact that oftentimes the amounts to collect and process are extraordinarily large (or, more often, nonexistent). This extreme and as-yet unpredictable variability precludes investment on processing facilities and the establishment of markets based on Sargassum biomass. On the other hand, the cost to collect or harvest the biomass, before or after beach-arrival (including specialized equipment—which moreover needs to be designed and tested), is very high, as is the cost of not collecting it.

Efforts, nonetheless, must continue. However, the other and far less explored focus to the problem is to stop or decrease the frequency and intensity of the Sargassum blooms. Yet, how can this be done? This is a question particularly relevant from an economic feasibility perspective. Wouldn’t it be nice if the solution to the blooms is not only inexpensive or even pays for itself, but it moreover represents a very profitable and timely opportunity?

Perhaps as it happens with every new problem when our understanding is not complete or thorough enough, several causes are considered without priority to establish the reason(s) for or to intervene the Sargassum blooms. Among these, of course and different other causes notwithstanding, nutrients play a critical role, and the bottom line is that without abundant nutrients in the water Sargassum blooms of any significant scale cannot occur.

This is a matter of simple stoichiometry: each ton of seaweed biomass needs so many kg of nitrogen, phosphorus, potassium and all the other nutrients necessary for the structure, maintenance and growth of a living organism. Without that, even without adequate amounts of a single nutrient of all the ones that are needed, growth will not happen—no matter other factors like currents or water temperature.

What happened to the Sargasso Sea?

The figure below (taken as a photo from Wang et al. 2019) is magnificent for several reasons. Not only does it show the extent of the 2018 Sargassum bloom and land locations more affected by it (by July 2018), but it also shows a minimum role of the Sargasso Sea (in the mid-Atlantic). Evidently, as these authors and others have discussed recently, now there seem to be or we are aware now that there are several permanent or semi-permanent sources of Sargassum or locations at sea where they live permanently, besides and beyond the Sargasso Sea, which was considered the only permanent source. This situation is seen as an expansion from the original habitat of these aggressive Sargassum species (blooms consist mostly S. natans, with S. fluitans and other Sargassum spp. recruited from different locations), or the invasion by them of different habitats, complicating the issue. Different causes may play a role besides the fact that seed is now found in different locations, not only in the Sargasso Sea. Among causes cited there are changes in currents and increases in water temperature, and, of course, increases in nutrients in the water.
(Wang et al., 2019. The great Atlantic Sargassum belt. Science, 365:83-87)

Presence of Sargassum, as determined using satellite imagery, for parts of its expanse during the extreme 2018 Sargassum bloom–which covered 8850 kilometers from the Gulf of Mexico to the coasts of Africa (in what has been termed the great Atlantic Sargassum belt).

Satellite images showing presence of nutrients as measured by chlorophyll content of water. Nutrients are just about everywhere while coasts are very much loaded as they come from land.

The Solution: Starve Sargassum

Of course, several limitations remain before implementing this solution of starving Sargassum by removing nutrients from the water before they reach Sargassum or before Sargassum reaches them (which by the way we found out that this solution has been already proposed in China for the control of eutrophication and algal blooms 1).

To begin with, there is the question of where do nutrients come from? Besides the standard answer: nutrients come from anywhere there are land and people (mainly in the form of run-off from agriculture and treated and untreated sewage waters—see chlorophyll content images in this section), there is also the matter of some specific sources of nutrients that are being identified besides what we normally associate with nutrients directly derived from land into the sea from agricultural runoff and sewage, like upwellings and Saharan dust, plus increases in the Amazon’s input of nutrients perhaps due to increased agriculture.

The issue gets more complex when we consider that Sargassum masses can travel hundreds of kilometers and grow while doing it. For example, for one year of growth with poor nutrients one ton of floating Sargassum can grow to approx. 158 tons, yet in higher nutrient waters the same ton can grow in a year to about 10 billion tons (Franks, J.S. et al. 2016)2.

Once we know about nutrients in water, or sufficiently about that, particularly about where they come from and the pathway for nutrients to reach Sargassum or for Sargassum to reach where nutrients are, the other question is how these nutrients can be stopped from reaching Sargassum or not be there when Sargassum arrives or passes by. Of course, we are talking about Sargassum that will become blooms and now, lacking nutrients, they will not become blooms or will become blooms of far lesser severity.

Of course, to stop adding nutrients from land to the sea (mainly runoff from agriculture and treated and untreated sewage and other waters) would be the first choice to solve the problem. Yet this seems very difficult to implement because it is a very extensive situation with multiple, perhaps millions of sources contributing (a typical non-point pollution situation). Though efforts to decrease the voluntary and involuntary addition of nutrients from land into the sea must continue, it will take a long time to change agriculture’s fertilizer use practices and the management of sewage waters in so many countries involved.

The other way to deal with nutrients, which is the one we advocate here (analogous to what the Chinese colleagues cited above advocate), is to intercept them and remove them from the water before they reach Sargassum or Sargassum reaches them. This is at the core of this proposal and the key issue is that it can be done without cost because it can be done through the cultivation of seaweeds and the use of their biomass in an economic manner.


1 Fei, X. 2004. Solving the coastal eutrophication problem by large scale seaweed cultivation. Hydrobiologia, 512:145-151.

Xiao, X., et al. 2017. Nutrient removal from Chinese coastal waters by large-scale seaweed aquaculture. Nature Scientific Reports, DOI: 10.1038/srep46613.

2 Franks, J.S. et al. 2016. Pelagic Sargassum in the Tropical North Atlantic. Gulf and Caribbean Research, 27: DOI: 10.18785/gcr.2701.08

The proposal: Cultivate seaweeds to starve Sargassum

Back to stoichiometry:

The nutrients that cultivated seaweeds will uptake from the water to produce a ton of their biomass will now not produce a ton of Sargassum biomass.

An example of what is proposed. Instead of the naturally occurring Sargassum (but, of course, aided by human addition of nutrients to the sea), its controlled cultivation is a solution to intercept nutrients that would otherwise go on to feed the Sargassum blooms. Shown here is a cultivated Sargassum line, but many species can be cultivated, for different applications of their biomass (e.g. Radulovich et al. 2015, World Bank 2016)3.

Seaweed cultivation in our continent has been shown to be technically doable and besides sizable commercial cultivation in Chile, it is practiced at least in pilot—though growing–manner in the USA, Canada, Mexico, Costa Rica, Panama, Colombia and some Caribbean islands.

While there are several seaweed species that are and can be cultivated, the different applications for their biomass–like hydrocolloids (mainly carrageenan and agar, others as well), food and feed, the famous Caribbean beverage, bioactives, biofuel, and a growing list–have a market value adequate enough to support their farming.

Expanding seaweed cultivation, besides being a solution to the Sargassum bloom problem, represents many other advantages, beginning with being a new form of—aquatic—agriculture that can come to solve the growing limitations of land-based agriculture in the face of depletion of water resources and increased rainfall variability/shortages, and continued population growth and consumption. Also being an economic activity, it can bring prosperity to many coastal regions and less-developed countries with coasts. There are several other benefits derived from cultivating seaweeds, which will be treated more extensively at a later date, yet they can be found in the references just cited and in the aquatic farming proposal presented in

For this solution to be successful at the scale required (which will have to be determined), the use of seaweed biomass will have to be expanded and the proper areas for seaweed farming need to be determined. Yet, as seen by the chlorophyll images shown above, coastal farming is the indicated choice where to begin. Other criteria will have to include specific major sources of nutrients, and the direction of the Sargassum bloom path towards coastal regions (something that may mean that some regions should be targeted first and others later according to their contribution to the growth of blooms).


3 Radulovich, R. et al. 2015. Tropical seaweeds for human food, their cultivation and biodiversity enrichment. J. Aquaculture, 436:40-46.

Radulovich, R. et al. 2015. Farming of seaweeds. Chp. 3 in Seaweed Sustainability 1st. Ed: Food and Non Food Applications. Elsevier, pp. 27-59.

World Bank. 2016. Seaweed Aquaculture for Food Security, Income Generation and Environmental Health in Tropical Developing Countries. World Bank/GEF, Washington, D.C.

Two additional elements: promote consumption of seaweeds to expand markets and subsidize farming, maybe even as payment for services.

Selecting the right areas… chlorophyll maps… Around populated coasts is first choice both because closeness/easier to farm and because nutrients originate from land.

Other areas may be selected for large scale deployment, with or without harvest. Many other benefits besides bioremediation…