The Drifting Ecosystem: Sargassum

Anjani Ganase, marine biologist, talks about Sargassum and signs of the changing ocean

In recent years, regional news networks have reported on repeated inundation events occurring on many beaches on southern and eastern Caribbean islands from massive amounts of Sargassum seaweed during the summer months. Sargassum on beaches has been seen in Tobago, Barbados and Antigua; the events were first reported in 2011 – 2012, 2014 – 2015 and again in 2016. Furthermore, during 2014, the amount of Sargassum that was washed up appeared to be greater in comparison to 2011, and now, six years later the events seem to have become a common scenario. Coastlines along Brazil and in West Africa have also experienced deluge by the Sargassum. Previous records in the news and scientific reports on these Sargassum inundation events in the southern Caribbean and West Africa are rare. Scientists have begun to investigate whether these events are part of a natural long-term cycle or the result of changes to the natural system.

Let us take a step back to first understand Sargassum seaweed, the ecosystem they house and the functional natural role in the ocean. The Sargasso Sea is located in the North Atlantic Ocean, aptly named after its most noticeable resident the Sargassum species of brown algae. The Sargassum seaweed is the only known algae that drifts in the open seas (pelagic). This is made possible through the use of air-filled bladders that keep them afloat and close to the light to photosynthesise and grow. Although many species of Sargassum are born attached to the benthos then carried out to the open ocean, there are two species, common to the Sargasso Sea, that are purely pelagic; they grow and propagate in open water without ever being attached to substrate.

The floating mats of Sargassum seaweed in the open water are a place of refuge for critters in the vast open sea. Sargassum is known to house a high diversity of marine life. Some of which are endemic, relying solely on the Sargassum as their home and source of food. Some of these endemic species include fish, such as the Sargassum fish – a species of frogfish known for camouflaging and pouncing on prey – as well as invertebrates such as crab and shrimp. Apart from the permanent residents, other organisms take temporary shelter, including juvenile sea turtles and marine fish larvae that seek refuge until they have grown. As a result, predators have also learned to hang around floating mass of Sargassum for an easy meal. 

Sargassum fish, Histrio histrio, sourced from "The Bahama Islands" by The Geographical Society of Baltimore 1905.

It was initially thought that the Sargassum that flooded our beaches came from the north, in the Sargasso Sea, carried by stronger winds and shifting currents. However, further investigations revealed that the Sargassum on Caribbean beaches was not that found in the Sargasso Sea. Through the use of satellite information of the ocean’s surface, it was found that the Sargassum came from the south from the circulating currents near the equator – the North Equatorial Recirculation Region (Gower et al. 2013), a place less known for Sargassum.

There are many theories about the unusual drift patterns of the Sargassum mats, and many of these studies are on going. One theory states that the conditions for algal bloom in the equatorial region were related to the nutrient outflows of the Amazon river; and possibly related to rising ocean temperatures and changes in circulation related to climate change (Johnson et al. 2012, United Nations 2016).

In the past, other “tides” of algae have become a source of nuisance and are even hazardous. Some naturally occur, for example along Florida’s gulf coast, where the red tides are named for the harmful algae (dinoflagellates) blooms (HABs). The algae release brevetoxins that affects the nervous system of marine organisms and humans. These tides have been recorded on ships’ logs in the region for centuries, but there is some debate about whether the tides have become more and more common. In England and the US, increasing events of green tides have been related to the blooms of the green algae, Ulva, which correlated with the increase in coastal eutrophication (a term which refers to excess nutrients due to runoff from the land, which causes plant growth and death of aquatic life from lack of oxygen).

Other golden (Sargassum) tides in the Gulf of Mexico have been linked to the increase in nutrient runoff from the Mississippi river into the Gulf (Smetacek and Zingone 2013). Ecological management plans were put into effect to reduce the amount of nutrients that run off into the ocean; and on-going research on the life cycles of the algae that lead to mass propagation.

Fortunately, there are no obvious toxic effects from Sargassum; but the smelly decaying seaweed is a nuisance especially on tourist beaches. The inundation may also affect local fisheries by clogging up nets. Mitigation efforts and resources to clean up the algae are quite expensive. Further investigations by scientists will allow us to plan for future, potentially permanent, changes to our ocean ecosystems and our connected livelihoods.

Sargassum natans (brown algae), San Salvador Island, Bahamas, taken in 2008. Photo by James St. Johns (https://www.flickr.com/photos/jsjgeology/), Creative Commons attribution 2.0

References:

Johnson, D. R., Ko, D. S., Franks, J. S., Moreno, P., Sanchez-Rubio, G. (2012) The Sargassum invasion of the Eastern Caribbean and Dynamics of the Equatorial North Atlantic, Proceedings of the 65th Gulf and Caribbean Fisheries Institute.

Smetacek, V., Zingone, A. (2013) Green and golden seaweed tides on the rise, Nature, 504, 84-88.