What are the environmental impacts of Sargassum inundations?

When searching for impacts of Sargassum, it is very easy to find papers and articles that highlight the importance of pelagic Sargassum as a habitat and nursery ground particularly for fish such as juvenile swordfish, dolphinfish and flying fish¹². Juvenile loggerhead³ and green turtles⁴ use Sargassum rafts as refuges to help them on their journey to the nursery waters in the Sargasso Sea, the sea that gets its name from the two holo-pelagic species of Sargassum: Sargassum fluitans and S. natans⁵⁶.

The Sargasso Sea is also where American and European eels migrate to breed, both these species are considered endangered⁵¹. The Sargassum from the Sargasso Sea is also home to ten endemic species including a worm, snail, shrimp, crab and fish (the Sargassum frogfish)⁵¹².

What seems to have happened is a slight shift in 2009/2010, in a weather (particularly air pressure) system that stretches between Iceland and the Azores called the North Atlantic Oscillation. This wobble (which could be attributed to climate change) led to some Sargassum being carried over to the coasts of Spain, Portugal and Morocco. From there it was carried by currents along the coast of west Africa where it was then picked up by the North Equatorial current⁷.

At the same time, changes in land use - particularly deforestation and the associated increase in agriculture - and lack of water treatment has led to large flows of nutrients into the tropical Atlantic via the Amazon, Congo and Orinoco rivers. Sargassum that had survived its long pan-Atlantic trip, bloomed as it was carried into these excessively nutrient rich coastal waters⁷⁸.

This bloom then started to hit the coasts of the Caribbean sea and then as it was carried into the Gulf of Mexico, it met more nutrient rich waters and continued to bloom. This is in addition to the Sargassum that still originates from the Sargasso sea and is carried in by the Caribbean current into the Gulf of Mexico. And this process has continued since 2011, when the great Sargassum belt was established in the northern equatorial Atlantic⁶⁹.

This new Sargassum belt is distinct from the Sargasso Sea Sargassum, to put it simply they are composed of different morphological types of Sargassum⁹. The Sargassum belt has been found to possess lower biodiversity than the Sargasso Sea¹⁰ (certainly not all endemic species found in the Sargassum belt) and there are no reports that it acts as an important nursery as the Sargasso Sea does. Instead the Sargassum belt breaks apart with rafts being carried by the Caribbean and Lesser Antilles currents into the Caribbean sea and Gulf of Mexico, growing and expanding as it travels in these nutrient enriched waters⁸⁶.

It is hard to know exactly what happens, at what point these mats go from habitat to death trap. Sampling in the Gulf of Guinea showed dissolved oxygen to be at a low level of 3mg/L¹¹(below 5mg/L is considered low oxygenation and potentially harmful¹²).

Studies of Sargassum in lagoons and shallow water bays off Quintana Roo in Mexico showed decreased oxygen concentrations even below 1mg/L, also known as hypoxic conditions. Anything that is caught in the water under the mat will suffocate¹². Similar results have been found in seagrass meadows elsewhere on the Yucatan peninsula, with sustained low dissolved oxygen levels (<5mg/L) being recorded.

In addition the water temperature has been found to be up to 6°C higher¹³ within the Sargassum mats than the surrounding water (quite possibly contributing to the hypoxia as warm water does not hold oxygen as well as cold water)¹⁴.

By the time the mats are crossing over coral reefs and seagrass beds, these changes are likely happening¹⁴. Photos from recent (as in 22nd July 2022) inundations show a large number of coral reef and seagrass associated species included in mass mortality caused by Sargassum beaching.

These are reef and shallow water dwellers, not pelagic, ocean going animals. This is a huge negative - Sargassum it is gradually killing off reef and seagrass animals [14]. It is also bringing in and killing off pelagic animals. Dead Hawksbill, green and loggerhead turtles have all been washed in with Sargassum mats, adult and hatchlings alike¹⁵. The impact on loggerhead and green turtles may not be so clear in all places, with some indication that adults and hatchlings can make their way through the mats¹⁶. However in Florida there is some indication the inundations are having a negative impact on hatchlings¹⁷.

Coral reefs are crucial habitats with a rich biodiversity, they are important in protecting islands from storm surges, and protect other carbon rich biodiverse habitats such as mangroves and seagrass meadows.

Sargassum puts these habitats at risk as well¹⁸. Not so much has been documented about mangroves, though the hypoxic conditions and build up of hydrogen sulphide gases very likely damages mangrove root systems. Negative impacts on seagrass are better documented¹⁴. Temperature changes and shading inhibits growth in seagrass, even causing biomass loss¹⁴.

The leachate (which contains high amounts of arsenic) from slowly decaying Sargassum impacts on major coral species larvae settlement success¹⁹, directly impacting the ability of coral reefs to breed and to grow. Light penetration is reduced by up to 70% (corals need light to survive)¹³. Along with elevated temperatures, coral reefs could be further endangered by the continuous influxes of Sargassum.

You can read about the impact that Sargassum has on human health in another blog post here.

Intercepting Sargassum before it reaches coastal habitats could alleviate some of these terrible impacts, it should be done thoughtfully and researched well, but the horrifying impacts it already has are abundantly clear.