21 March 2023

Florida, behind the invasion of seaweed two actions of man on land

For years, the amount of sargasso algae present in Caribbean and US waters and beaches has been increasing. And it's no longer relegated to the months when the water is warmer.

The coasts of the US state and other Caribbean states are invaded by abnormal quantities of sargasso: it is estimated that the seaweed are almost 9,000 kilometers long and weigh 20,000 tons. However, the abuse of agricultural fertilizers and deforestation are at the basis of the phenomenon. A confirmation of the bond that unites the health of the soils with that of the oceans

by Emanuele Isonio


We certainly do not discover today that the portion of the North Atlantic Ocean between the Azores to the east and the Antilles to the west was characterized by the presence of large banks of seaweed. The Sargasso Sea, which refers to the fantastic stories of pirates and explorers, owes its name to that type of vegetable. So widespread in certain periods of the year as to slow down the navigation of sailing ships. Cristoforo Colombo already complained about it: it seems – the Genoese navigator recounted in his logbook – that they were stranded on immense sandbanks.

A floating “blob”

Centuries later, however, the presence of these legendary algae appears to be out of control: much greater in quantity (the sargasso tide extends for about 8850 kilometres, weighing approximately 22 million tons of matter). And above all it is multiplying in unusual periods. Too soon, in months that should still be late winter. So much so that in recent days and weeks it has invaded many beaches in Florida and the northern Caribbean.

From resource to pathology

And so, the presence of sargassum – which in “normal” quantities are essential for the biodiversity of that portion of the Atlantic, producing oxygen and ensuring an ideal habitat for marine fauna. – becomes pathological. Putting at risk the health of the corals, the beaches, the precious mangroves, but also human health and the economic induced activities ensured in those fishing and tourism areas. The owners of the accommodation facilities on the coasts concerned are well aware of this, as they have noticed a decrease in tourists and bookings as the phenomenon has grown. Along the beaches between Miami and Jacksonville fear that this year will be exceeded the record (22 million tons) recorded last year.

L'immagine satellitare mostra l'estensione dell'area coinvolta dalla crescita anormale delle alghe nell'Oceano Atlantico a gennaio 2023. I colori più caldi indicano una presenza più massiccia di sargassi. FOTO: South Florida University.

The satellite image shows the extent of the area affected by abnormal algae growth in the Atlantic Ocean in January 2023. Warmer colors indicate a more massive presence of sargasso. PHOTO: South Florida University.

But what is behind this unusual proliferation? Researchers from the University of South Florida’s Department of Marine Biology have been studying the phenomenon for several years. Also using satellite data provided by NASA, they have observed an increase in sargassum blooms since at least 2011. According to the team led by professors Mengqiu Wang and Chuanmin Hu, the phenomenon is connected with what is happening in the Amazon.

The main suspects are in fact the increase in deforestation (resumed at full speed in Brazil with President Bolsonaro) to make way for large expanses of arable land and the massive use of chemical fertilizers. In fact, the latter are designed to favor the rapid growth of agricultural products. But those same substances then end up arriving in waterways up to the great Amazon River. And from there to the sea. Ocean currents have done the rest, causing an uncontrolled increase in sargasso algae. Factors that make Professor Hu say that “the recurrent blooms of sea bream in the tropical Atlantic and in the Caribbean Sea may become the new norm”.

I principali indiziati sono infatti l’aumento della deforestazione (ripresa a pieno regime in Brasile con il presidente Bolsonaro) per far posto ad ampie distese di suolo coltivabile e l’uso massiccio di fertilizzanti chimici. Questi ultimi infatti, sono progettati per favorire la crescita rapida dei prodotti agricoli. Ma quelle stesse sostanze finiscono poi per arrivare nei corsi d’acqua fino al grande Rio delle Amazzoni. E da lì fino al mare. Le correnti oceaniche hanno fatto il resto, provocando un aumento incontrollato delle alghe sargasso. Fattori che fanno dire al professor Hu che “le fioriture ricorrenti dei saraggi nell’Atlantico tropicale e nel Mar dei Caraibi possono diventare la nuova norma”.

The links with African slash and burn

Another Florida university – that of Miami – has added a further element to the search for the main causes of the proliferation of sargasso. And he would have identified it in Africa, where the practice of slash and burn is still widely used: a forest area is subjected to cutting and clearing of the undergrowth; the branches are arranged in strips and burned, while coal is produced with the wood. The open surfaces are cultivated for one/two years by exploiting the fertility of the soil inherited during the forestry phase, to then be abandoned to the recolonization of the natural vegetation until the levels of fertility suitable for a new cultivation are restored.

But this practice, in addition to stressing the soils (population growth has reduced the time between one felling and another, making forest recovery difficult), would cause the release of nutrients into the atmosphere, starting with phosphorus. The hypothesis was advanced by Cassandra Gaston, an associate professor at the University of Miami and an expert in atmospheric chemistry. “The smoke given off by those fires is certainly bringing in nutrients that are readily available after they are carried by the winds and deposited in the ocean,” Gaston explains. And in doing so they become an involuntary accelerator of the multiplication of sargasso algae.