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Friday 8th March 2013
When regimes shift. The algal invasions of the Caribbean Sea
Biodiversity Contributor: Will Bugler

White sandy beaches, crystal clear seas, coral reefs and guaranteed sunshine. These are the natural assets that ensure that tourists continue to flock to the Islands of the Caribbean. But over the last few years there has been an extraordinary invasion that threatens to seriously damage the economies and ecosystems of the pristine coastlines. Island authorities from Anguilla to Tobago have been trying to rid seas and beaches of huge quantities of algal seaweed that has been, quite literally, piling up along the coasts. But as the authorities struggle to identify the cause of the algal blooms, the answer may lie in scientific research undertaken over 15 years ago.

The intruder is a species of algae called Sargassum that has tiny air bubbles inside it which allow it to float on the surface of the water. Though harmless in small quantities, the algae have been washing up on Caribbean beaches in huge numbers. In Antigua, the St. James’s Club & Villas had to close for 5 weeks in order to remove 10,000 tons of Sargassum from its beaches.  Swimmers have been warned away from some beaches because of fears that they could get tangled in particularly thick flows of the seaweed, which has been creating piles of up to 5 feet high.  

Authorities struggle to deal with such large quantities of the algae. In Barbados, the government has used booms of the type designed to contain oil spills to try and prevent the Sargassum from coming too close to the beaches. The algae normally originate from the Sargasso Sea, an area of very weak, circular ocean currents to the east of the Caribbean Sea.While authorities continue to try and prevent the Sargassum from coming too close to the coast, there has also been some innovative adaptation measures put in place by some island states. In Tobago, for example, the government has been encouraging farmers to use the seaweed as fertilizer for their crops. If treated properly, the seaweed can be mulched and spread on farmland to create an effective organic fertilizer. But the scale of the problem that has caught authorities by surprise; David Freestone, executive director of the Sargasso Sea Alliance, was unequivocal, “This is completely unprecedented… [it] has never happened [on this scale] in living memory”.

If left unchecked the phenomenon could have serious implications for the region’s tourist industry that makes up 80% of GDP for some of the islands in the Caribbean archipelago. The blooms, although less intense than a month ago, have lingered on into the November high season. While they are not directly dangerous to human health, in large quantities the algae omits a strong odor as organisms that get caught in the mass decay on the shorelines.

 

So what is causing these algal super-blooms? And why have they only recently appeared?

There have been many theories as to what has caused the Sargassum invasion, ranging from shifts in ocean currents to the gulf oil spill. Some, including Jerry Ault, professor of marine biology and fisheries at the University of Miami, suggests that climate change may be a factor. Cooler autumn weather slows algae growth but climate change may have caused, warmer autumn temperatures and slight shifts in ocean circulation patterns and nutrient systems which could have stimulate Sargassum growth and cause it to drift westward.

Possibly. But, in my opinion, the most robust explanation for the phenomenon can be found by looking at resilience theory. In short the ecosystem changed from one stable state to another, from one that was not algae dominated to one that was. There was a ‘regime shift’.

This theory is supported by a study in the mid 1990s, published in the journal Science, which studied the reasons for large Sargassum blooms around the coast of Jamaica. Terence Hughes, the study’s author, showed how ‘regime shifts’ in complex marine ecosystems had allowed the algae to flourish. His findings clearly highlight the importance of analyzing and understanding the significance of every part of complex socio-ecological systems.

Hughes and his team found that over-fishing in the region reduced the number of herbivorous and carnivorous fish. Initially this had little impact on algal growth as it had the effect of dramatically increasing the number of Diadema sea urchins (the fish were their main predator).

Diadema urchins graze heavily on algae keeping the Sargassum at bay. At this point the reef had less fish and more urchins, but recovery was still possible. If fishing was reduced then the stocks could recover and the urchins would be eaten back. But with less fish, the system had become less resilient. It was more vulnerable to other shocks.

Over time ocean acidity increased, principally due to climate change, and there were several hurricanes and storm surges, in quick succession. These shocks destroyed some of the coral and with it the Diadema urchins. In the normal way, storm damage to reefs is not catastrophic, the reefs recover, and, in the meantime, there are enough herbivorous fish to keep Sargassum blooms at bay.  

In this case of course, there were too few fish, and so when the urchins lost much of their habitat, there was neither enough fish, nor enough urchins to control the algal blooms, and the ecosystem had entered into a fundamentally different state. One where there was nothing left to control the algae.

The relationships that govern complex systems can influence not only the actors within that system, but can also affect the vulnerability of each actor and of the system as a whole. In human systems, complexity reduces our ability to understand the relationships that make them work (see the global financial system before the 2008 crash and, worryingly, see the global financial system as it remains today).

The emergence of algal blooms in the Caribbean Sea shows that in order to understand the implications of our actions that affect such systems, we must first understand what gives the system its resilience. 

 

Will Bugler is Editor at Get Resilient, he has worked within the ‘Adapting to Climate Change’ department at Defra, Friends of the Earth, and for the UK government's advisory body on climate change issues; The Energy and Climate Change Committee.