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The Science Behind When the Sea Turns Green

July 8, 2013
An algal bloom at the edge of the Homebush Bay brickpit in the state of New South Wales, Australia. Image Credit: by Ian Sanderson.

An algal bloom at the edge of the Homebush Bay brickpit in the state of New South Wales, Australia. Image Credit: by Ian Sanderson.

By Paige Brown, Museum Blogger-in-Residence.

The kids are laughing in the backseat, clashing together sand buckets and shovels, ready for a day on the white sandy beach. As you pull up to the beach, unload the car of brightly-colored umbrellas and lawn chairs, and climb the dunes to get your first glimpse of the open ocean, you are surprised by a blanket of green as far as the eye can see.

A beach covered by plush green algae might seem odd but fun, a bit of holiday spirit in July, especially for us Saint Patrick’s Day fans. But the bright color and lushness of this bloom belies the fact that it is a very visual indicator of extreme climate conditions and industrial pollution.

A massive bloom of Enteromorpha prolifera, more commonly called sea lettuce, recently hit the coast of Qingdao, China and the Yellow Sea, contributing to the largest algal bloom ever recorded in China.  According to a Chinese news agency, officials in the city of Qingdao had used bulldozers to remove 7,335 tonnes of sea lettuce from beaches as of early July 2013. That is approximately the equivalent of the weight of 150 adult right whales, like our own Museum’s Stumpy the Whale. While children swim in the overwhelming bloom, many scientists are pointing to the problematic source of this “green tide.”

“Algal blooms often follow a massive discharge of phosphates or nitrates into the water,” University of Cambridge and EnAlgae Project researcher Brenda Parker said in a recent article in The Guardian. “Whether it’s farming, untreated sewage or some kind of industrial plant that is discharging waste into the water.”

While blooms of sea lettuce don’t pose a direct threat to the health of beachgoers, the quickly propagating growth is difficult to clean up in a timely manner and, as it begins to decompose on the beach, it produces toxic fumes of hydrogen sulfide, a gas that smells like rotten eggs. Algal blooms can also create “dead zones,” areas of ocean that are depleted of oxygen, and choke off other marine species, including fish important to our commercial fishing industry and our diet. Other algal blooms, however, can create toxins that are harmful to humans.

Algal blooms are also an indication of bigger problems such as water runoff of farming industry chemicals and fertilizers containing phosphorus and nitrogen. These elements, along with warming ocean temperatures, are contributing to the re-occurrence of massive algal blooms year after year.

Global ocean warming and increased fertilizer use to supply a growing world population with sufficient crops can contribute to periodic growth in algal mass that, as it is consumed by marine bacteria, contributes to a depletion of oxygen in the water. This oxygen depletion, also called hypoxia, can devastate marine biodiversity and commercial fishing activities.

To help prevent detrimental algal blooms and other harmful impacts of nitrogen-containing nutrients including wetland degradation, agricultural industries can reduce fertilizer use, as most fertilizers are used in excess of their actual required threshold. Agricultural areas can also be surrounded by regions of vegetation dedicated to absorb excess nutrients before they follow water runoff into creeks and rivers.

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