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A poisonous algae that could sicken humans if they consume infected seafood is spreading in the Pacific Ocean, stretching from Southern California north to Alaska.

The massive algae bloom was first spotted in May and has been growing steadily since.

"This is the largest and one of the longest-running blooms we've seen in the last fifteen years," Raphael Kudela, a professor of ocean sciences at the University of California Santa Cruz (UCSC), told VICE News. "The toxin levels are also the highest we've measured. Because it has lasted so long, the toxins have worked their way into the food web, resulting in closures of various sport and commercial fisheries along the West Coast."

While algae blooms in the Pacific Ocean are not unusual, the unprecedented extent of the brownish bloom this year is due to a complex set of factors. Scientists believe that the warmer waters of the Pacific Ocean and increased use of fertilizers in farms due to the ongoing drought might be playing a role.

Joaquim Goes, research professor at Columbia University's Lamont-Doherty Earth Observatory, connected the appearance of the blooms to sudden and erratic rain showers along the West Coast in April and May that washed nutrient-rich fertilizer the ocean.

"I think that might have been the initial trigger for this bloom," he said.

The fertilizer runoff provided the necessary nutrients for algae to grow, while the warm waters of the Pacific created a conducive environment, said Goes.

Cells of Pseudo-nitzschia, a genus of microalgae that includes several species that make the neurotoxin domoic acid. (Image via Vera Trainer/NOAA)

According to researchers from the University of Washington, the National Oceanic and Atmospheric Administration, and the Institute of Ocean Sciences in British Columbia, sea surface temperatures in the northeast Pacific were remarkably warm during the winter of 2013-2014. By May 2014 the region of unusually warm waters, popularly referred to as "the blob," extended into coastal areas along the Pacific.

Last week, scientists from several government agencies raised concerns about the spread of the blob into Washington's Puget Sound. They believe the warm waters — and the toxic blooms that result — could be devastating to the sound's marine life and potentially harm humans who consume the region's shellfish.

"When the ocean is really warm like this, it also forces fish and marine mammals closer to shore because there is not as much food available farther offshore, increasing the chance that animals are exposed to toxin and allowing the toxin to work its way into the food web," UCSC's Kudela said.

A strong El Niño formation, a naturally occurring spike in Pacific Ocean temperatures that can lead to heavy rainfall on the West Coast, persists in the eastern Pacific and is widely expected to last until next spring. Scientists say the current warm conditions could develop into one of the strongest ever — and climate change might have something to do with it.

"The connection with climate change is that these are the sorts of conditions we would expect with a warming ocean," Kudela said. "It's always difficult to attribute any one event to climate change, but it is certainly consistent with what we expect to happen."

The Pacific Ocean bloom has been caused by high concentrations of Pseudo-nitzschia, a type of phytoplankton that produces a harmful substance called domoic acid. The acid is a neurotoxin that can destroy nerve tissue and lead to vomiting, diarrhea, abdominal cramps, headache, and dizziness in humans. In severe cases it can also result in breathing trouble, confusion, cardiovascular instability, seizures, permanent loss of short-term memory, coma, or death.

California officials have warned consumers about seafood caught in Monterey, Santa Cruz, and Santa Barbara counties after detecting dangerous levels of the toxin.

"These organisms can accumulate a lot of biotoxins without harming themselves. When humans and other marine animals consume the shellfish, that is when health problems start," Goes said.

While only a few mild cases of poisoning in humans have been reported, sea lions have been much more greatly impacted. In the latter half of May alone, right after the bloom was detected, more than fifty California sea lions along the Monterey coast fell ill.

"The California Department of Public Health (CDPH) and volunteers along the coast continue to monitor for domoic acid levels in seafood," Matt Conens, a spokesman for the CDPH, told VICE News. "Although phytoplankton and shellfish monitoring have indicated a decline in bloom activity in recent weeks, a recent report from UC Santa Cruz indicated that Pseudo-nitzschia was increasing in cell numbers again."

Yet, worryingly, the Pacific Ocean bloom is not without its peers.

This year's Lake Erie algae bloom is worse than last year's, which forced Toledo, Ohio to shut off its drinking water supply due to high levels microcystin. The toxin can cause liver damage when ingested — a serious issue not just for Toledo, but for millions of Americans that really upon the lake for drinking water.

The Chesapeake Bay also comes under annual attack from algal blooms due to agricultural and industrial runoff, as well as warming water temperatures. The infamous "red tides" off the New England coast are also vast algae blooms.

As sea surface temperatures increase, algae blooms are becoming more common across the globe.

A blue-green bloom of Noctiluca scintillans now spreads across the Arabian Sea each winter, from Oman on the west to India and Pakistan on the east. While it is non-toxic, the bloom has begun to alter the region's food chain, fed by sewage flowing into the sea.

Managing and predicting these blooms is difficult, but scientists say reducing the amount of chemical runoff may reduce the severity of algae blooms.

As for the Pacific Ocean bloom, it doesn't seem to be going away anytime soon.

"As long as there are nutrients in the water, it will continue to grow," Columbia University's Goes said. "As it moves along the coast, it gets whatever nutrients it needs from the rivers draining into the coastal waters. This will be a very prolonged bloom."

Watch the VICE News documentary "California's Sea Lion Die-Off" here: