In August 1961, a bizarre scene unfolded in the skies above the coastal town of Capitola, Calif., on the northern end of Monterey Bay.

Residents watched in terror as seabirds dive-bombed into the ground at kamikaze speed and violently vomited fish. The carcasses of hundreds of birds were strewn in the streets.

The strange incident partly inspired Alfred Hitchcock's 1963 horror flick The Birds, but exactly what happened remained a mystery for five decades.

In 2011, a research group revealed that the birds were victims of poisoning by domoic acid, a potent toxin produced by algae that targets the nervous system, inducing severe seizures and killing wildlife. 

A bloom thought to be the largest ever observed on the West Coast of North America is currently menacing Monterey Bay with unprecedented levels of domoic acid. Wild and farmed shellfish operations from California to B.C. have been forced to stop harvesting until the bloom clears. 

While toxic algae are common in waters across the planet, there is mounting evidence that the frequency and severity of these events are on the rise and that global climate change may exacerbate the problem.

Here's some important background on toxic algal blooms and how they could affect you. 

What are toxic algal blooms?

The massive growth of algae on the West Coast falls under a category of natural phenomena that scientists call harmful algal blooms, or HABs. They are often called red tides because they sometimes render the water a rusty-crimson colour. This is a misnomer, however, as algal blooms can be many colours and aren't necessarily connected to tides. 

Lake Erie Algae

Ontario, Michigan and Ohio recently agreed to cut phosphorus run-off from nearby farms in Lake Erie by 40 per cent to prevent toxic blooms in the lake. (Haraz N. Ghanbari/Associated Press)

Under the right conditions, a bloom can blanket surface waters for weeks on end, sometimes becoming a floating toxic glob that stretches thousands of kilometres in length and tens of metres deep. Some get so large they can be seen clearly from space.

While there are many kinds of harmful blooms that have varying effects on aquatic ecosystems, only a small percentage of algae species actually produce biological toxins. Those that do, however, can create some of the most harmful natural toxins known to science.

These toxins make their way up the food chain and can accumulate in dangerously high concentrations in aquatic animals that people eat, especially shellfish like oysters and mussels. Some shellfish can store toxins for weeks after a bloom passes. 

How can toxic algae affect humans?

There are generally four widely recognized conditions associated with toxins produced by algae, as well as several lesser-known illnesses. 

Perhaps the most well-known in Canada is Paralytic Shellfish Poisoning (PSP), which leads to symptoms from slight numbness in the extremities to full-on paralysis and even death in humans.

PSP is caused by saxitoxins, which generally do not break down under high heat conditions. In other words, cooking your mussels won't save you. The U.S. National Oceanic and Atmospheric Administration has recorded algal blooms that produced so much of the toxin that one or two small contaminated mussels could kill a healthy adult human.

Blue-green algae Lake Erie Toledo Water Problems

Cyanobacteria can produce toxins that cause respiratory and liver failure. (Haraz N. Ghanbari/Associated Press)

Saxitoxins were among the first shellfish-derived poisons to be isolated and studied, including by the U.S. military, who tried to develop chemical weapons intended to disrupt the neurological function of enemy soldiers on the battlefield. During the Cold War, some American spy plane pilots carried a dose of pure saxitoxin to take their own lives if faced with the prospect of capture. 

Domoic acid is responsible for amnesic shellfish poisoning and can cause severe short and long-term memory loss in humans. Similarly, the potent neurotoxin has been implicated in whale deaths and has been proposed as an explanation for mysterious behaviour occasionally observed in manatees in Florida. 

Algae are similarly responsible for diarrhetic shellfish poisoning — you can probably guess the primary symptom — and neurotoxic shellfish poisoning, which tends to be less severe than other algal toxin-related conditions. 

There is also an unusual group of blue-green algae (which are actually bacteria) called cyanobacteria, which live in both oceans and lakes. These species can produce potent toxins that cause liver and respiratory failure in humans and animals.

What causes toxic algal blooms? 

The short answer is that no one really knows.

The thousands of species of phytoplankton are in a constant, complex dance with numerous factors such as sunlight, water temperature, currents and the presence of essential nutrients like nitrogen, phosphorous and carbon. 

Algae bloom

The precise factors leading to toxic blooms largely remain a mystery. (Washington State Department of Ecology Marine Monitoring Unit)

What combination of these factors causes a bloom to explode is not entirely clear. In some instances, a sudden increase in nutrient levels seems to be the primary cause. For example, in the western basin of Lake Erie, farm run-off high in phosphorous has been blamed for an increasingly persistent bloom that forms each summer and fouls drinking water for hundreds of thousands of people. 

Interestingly, some algae species capable of producing toxins are not always toxic. Consider the enigmatic fish-killing species Heterosigma akashiwo, which decimated farmed salmon populations in B.C. in the 1980s. Heterosigma is always present in some concentration, but only becomes toxic under certain conditions.

It's as if some combination of environmental factors simply flips a switch, but just what those conditions are remains elusive (it's not even known what type of toxin Heterosigma produces).

Or in 2011, when an algae species that has likely persisted in B.C.'s waters for millennia without producing a toxin made 62 people across Canada sick with diarrhetic shellfish poisoning after eating contaminated oysters harvested in the Gulf Islands. Once again, what induced toxicity in that particular year remains a mystery.