The benefits of Sargassum
With so much seaweed washing up on our shores and floating around inshore areas,
The Green Pages spoke with Dr Nick Bates, interim director at Bermuda Institute of Ocean Sciences, to get a better understanding of what this important, although unsightly, weed has to offer both our ocean and land environment.
Green Pages: Please give a description about the type of seaweed that is found around Bermuda and where it comes from.
Dr Nick Bates: The Sargassum weed that typically washes ashore on Bermuda’s beaches is a brown seaweed (brown algae;
Sargassum natans) that has a lifecycle that is entirely free-floating (pelagic) and a lifespan of about a year. Surface patches of Sargassum weed can be observed from space using satellite imagery and it is typically found across the Sargasso Sea that surrounds Bermuda, and in the Gulf of Mexico and tropical waters of the Atlantic. It is thought that much of the Sargassum weed grows in the Gulf of Mexico during springtime and then is transported with the Gulf Stream into the Sargasso Sea with large patches observed off Bermuda in wintertime.
GP: Why is there so much Sargassum currently in the water around Bermuda? Is this more than usual for this time of year, and if so, what is causing there to be so much more than usual?
NB: There are probably a few causes right now. Typically we see more Sargassum weed during the winter months, but its occurrence on Bermuda’s beaches has been a little unusual compared to the last few decades. Patches of Sargassum weed accumulate in the waters surrounding Bermuda due to the ocean currents in the North Atlantic. If you go back to the 60s and 70s there are anecdotes of fishermen going through huge patches of Sargassum offshore that were difficult to navigate through. Over the last few decades, there seems to have been less Sargassum weed offshore but in the last few years large patches have washed ashore. There are a number of causes for this that are naturally occurring.
In the past couple of years, changes in the natural climate and weather patterns across the North Atlantic appear to have allowed Sargassum weed to grow more. Such changes are related to large atmosphere and ocean circulation changes that originate across the planet in the eastern tropical Pacific Ocean and in the Arctic Ocean.
In addition, turbulence in the ocean around Bermuda causes large eddies (that have a diameter of up to a 100 kilometers) of warmer or cooler water to form. These ocean eddies can cause sea level and tides to be abnormally high or low by as much as 30 centimetres.
This winter, an ocean eddy has been close to Bermuda. The circulation of surface waters around this ocean eddy has allowed patches of Sargassum weed to accumulate in greater amounts near Bermuda.
This winter, there have also been strong westerly winds that have blown mats of Sargassum weed onto the beaches. So we’ve had increased concentrations of Sargassum weed and strong northwesterly winds blowing it onto the beaches and into the Great Sound, conditions favourable for what we’ve been seeing.
But if you go back 50 years ago, anecdotally, there are reports that there was a lot more Sargassum weed. So we go through these natural cycles in the marine environment.
GP: Is there any benefit to the surrounding ocean for there to be this much Sargassum?
NB: Yes, it helps the ocean ecology. In these patches of Sargassum weed, found at the surface of the ocean, there is very interesting biology. It is a habitat for unique species and a nursery for juvenile fish. For example, the Sargassum fish is a unique and beautiful fish species that feeds off shrimp that also live within the weed. Patches of seaweed are a refuge for juvenile fish such as chubs, dolphin fish, triple tail fish, trigger fish, flying fish and pipefish. Juvenile turtles may use the floating mats as protection and feeding. Tuna and dolphin fish may also hide and hunt beneath mats of Sargassum weed.
Patches of Sargassum weed are also beneficial for pelagic seabirds that feed on the fish that hang out beneath the Sargassum patches. So it is important for the biology and ecology of the seas surrounding Bermuda.
It’s also a benefit to the beaches and the intertidal zone of Bermuda. When seaweed is washed up on to the beaches, it’s a new habitat for many organisms and a natural fertiliser.
Sargassum weed is broken down by myriad microscopic organisms that in turn feed larger organisms such as invertebrates and insects. Pick up a mat of Sargassum weed on the beach and numerous sand flies will scatter. Birds and many other invertebrates can take advantage of this new food source that the Sargassum mats provide. It’s a natural fertiliser that brings organic carbon and nutrients to the beaches of Bermuda. Seaweeds throughout the world are used as fertilisers, adding to the nitrogen and phosphorus contents in the fields.
GP: Is there any threat to there being too much Sargassum?
NB: No. There is no threat. It will break down. The storms will come along and push the Sargassum around and it will mix in with the sand which is important for the intertidal and beach ecology. The only issue, probably, is the collection of plastics and marine debris, but that would wash ashore anyway. It just doesn’t look pretty. Obviously it affects our visitors, but it is a natural occurrence and other beaches around the world have the same problem seasonally. It can be collected and composted.
The Sargassum weed is not changing the ocean chemistry surrounding it.
GP: Is it unusual for Sargassum to show up in places such as Foot of the Lane and Hamilton Harbour?
NB: In the last few decades it has been unusual. But with the increase in the production of Sargassum weed and the strong westerly winds this winter have caused it to be found in places like the Foot of the Lane and Hamilton Harbour. This also happened a couple of winters ago as well.