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The Sargassum Phenomenon – WS Mexico

- Mexico 2015
Last night was a windy one here at UNAM field station. This weather itself did not surprise anybody – but what had come with the wind and the waves surely did. When the sun rose above the horizon, the beaches between Puerto Morelos and Cancun were covered with a fresh thick layer of sargassum seaweed that had washed ashore during the night.

Half of this green-brownish mass still floated in front of the beach and formed a carpet, which was up to 40 meters wide. “This sargassum bloom started eight months ago and has been causing a problem for water quality since then”, says SECORE science board member and UNAM field station director Dr. Anastazia T. Banaszak (See the differences in water quality in our photo of the water samples.)

[20150728 Day 2 Set 1]

[20150728 Day 2 set 2]

Nobody really knows what is causing this phenomenal sargassum growth. Usually, this vine­like, floating algae, circulates throughout the Gulf of Mexico and North Atlantic, where it forms the nearly 2 million­ square­ mile Sargasso Sea. The seaweed often washes up on beaches in the Gulf, the southern U.S. Atlantic coast and the northern Caribbean in spring and summer. But in 2011, it began showing up in unprecedented amounts, and in areas where it had been seen rarely or not at all before. „Satellite data suggest the amount of sargassum in the Gulf of Mexico, Caribbean and Atlantic may hit an all­time high in 2015,” reports Melissa Askill in a Newsweek web article.

But what is causing this phenomenon? One possible explanation could be found in the Amazon rainforest. „In recent years, the Amazon basin has experienced some of the world’s highest rates of deforestation. And without vegetation to hold soil in place, rain washes that soil and whatever it contains into streams and rivers. So when the Amazon basin saw greater than normal amounts of rain in 2011 and 2012, unusually high levels of nutrients such as nitrogen and phosphorus washed into Atlantic waters around the mouth of the Amazon River off the north coast of Brazil. Sargassum passed through this nutrient ­rich water and responded by growing like, well, a weed. Ocean currents carried it from there to the Lesser Antilles and western Caribbean“, writes Melissa Askill. Other experts name the El Nino as another possible cause.

However, here at the beach of UNAM field station the decaying sargassum carpet has turned the former crystal clear seawater into a murky soup. Our workshop participants had to swim further out to visibly inspect the gamete collection nets they were supposed to test for their buoyancy. „Close to the pier the water is ‚craptastic’. You can not even see your fins,“ one of the students reported after the snorkeling. 

[20150728 Day 2 set 3]

30 meters behind the pier the water gets clearer and one can actually see the sea bottom again. It is covered by corals and sea grass. Does the rotting seaweed cause a threat for Porto Morelos coral reefs? “We do not know that,” says coral expert Prof. Daniel F. Gleason from Georgia Southern University, who is one of the teachers in this year’s Mexico workshop. “Decaying sargassum is causing a release of nutrients that could lead to eutrophication. Furthermore, it is changing the water clarity, as we could see while snorkeling today. Reduced water clarity decreases the amount of light available for corals, like the elkhorn coral we are studying, from functioning at an optimal level. That is because light is critical for the symbiotic algae in the coral tissues to carry out photosynthesis.”

The day after tomorrow we are going to head out to the reefs for another snorkeling practice. So we will get to observe the water quality there and well let you know about it in one of our next blog entries.

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