Whilst it is commonly known that excess nitrogen entering the Earth's oceans is contributing to algal blooms, which can consume oxygen and kill off other aquatic life, threatening biodiversity in aquatic ecosystems, researchers in the United States have discovered that increased nitrogen also benefits two tiny but toxic phytoplankton species, harmful to marine life and human health

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Researchers from San Francisco State University, working as part of the National Science Foundation's Ecology and Oceanography of Harmful Algal Blooms program (ECOHAB), found that nitrogen entering the ocean either through natural processes or pollution boosts the growth and toxicity of a group of phytoplankton that can cause the human illness Amnesic Shellfish Poisoning.

The poisoning occurs when large algal blooms, which affect regions off the South Coast of the United States, produce a toxin called domoic acid. When blooms of certain types of plankton occur, as a result of excess nutrients entering waters, high concentrations of domoic acid can accumulate in shellfish and small plankton-eating fish. Animals further up the food chain, including marine mammals, seabirds and humans can become ill and even die as a result of eating these animals.

Marine ecologist William Cochlan commented, "Regardless of its source, nitrogen has a powerful impact on the growth of phytoplankton that are the foundation of the marine food web, irrespective of whether they are toxic or not. Scientists and regulators need to be aware of the implications of both natural and pollutant sources of nitrogen entering the sea."

Although the circumstances that lead to algal blooms can occur naturally, pollution, particularly agricultural runoff containing fertiliser and effluent from sewage plants and slurry lagoons, has led to increases in these blooms. Nevertheless, in laboratory tests, Cochlan revealed that, while both natural nitrogen and nitrogen resulting from pollution equally support the growth of harmful algae, in all cases the natural form of nitrogen caused the most toxic cells to develop.

He also found that the toxic plankton become more harmful under lower light levels, which forces them to grow slower. Cochlan and his researchers noted that, in low light levels, "the species P. cuspidata, underwent up to a 50 fold increase in toxicity".

Cochlan's research suggests that the toxicity of small toxigenic algae is affected by the type of nitrogen they consume. He found that under low light levels, and as a result slower growth, phytoplankton cells that were fed on naturally occurring nitrates were more toxic than cells that were fed on either urea or ammonium caused by pollution.

"Our results demonstrate that the reason for the growth of these specific harmful algal blooms off the coast of North America from British Columbia to California may in fact be due to totally natural causes," he concluded.image expired

Although he said that such toxic blooms could be the result of "natural upwelling of nitrogen," Cochlan cautioned that nitrogen from pollution could play an important role in sustaining a "seed population" of harmful algae, which would keep the bloom going until upwelling resumes and the bloom is able to grow again and perhaps increase its toxic effect on the marine ecosystem.

The phenomenon is not unique to North America either; in 2009, a recurring algal bloom in Brittany, Northern France though to have been caused by a high amount of nutrients discharged into the sea by intensive pig farming, led to the death of several animals and caused one veterinarian to become seriously ill.