Algae is an important part of the ecosystem but, when it gets out of control, it can cause chaos for both animals and humans
Algae are plant-like organisms that grow in or near water and generate energy through photosynthesis. They play an important role in keeping oceans healthy but, when things tip out of balance, it can wreak havoc.
When the ecosystem is in balance, algae can have many benefits. For example, phytoplankton (plant-like plankton) produce oxygen and provide food for lots of other animals in the ocean.
- Algae: What it is, why algae blooms can be toxic and how it differs to plants
- How do plants breathe underwater?
The reason we have vibrant coral reefs is because of tiny microalgae known as zooxanthellae that live inside the coral’s bodies. Without the algae living inside, the coral would look white – the algae provide that splash of colour as well as generating food for the corals through photosynthesis. When corals bleach (and sometimes die), it’s an indication that they’ve lost these important houseguests.
But algae isn’t always a good thing. When algal blooms become too big and get out of control, or contain dangerous toxins, they can cause serious problems.
Here are some of the issues harmful algal blooms (HABs) can cause.
Welcome to the dead zone
Sometimes, conditions cause algae to grow out of control. This can happen when water temperatures increase or when additional nutrients flow into the water; for example, when phosphorous and nitrogen from agriculture and wastewater are washed from the land into the sea and fertilise the algae, causing huge growth spurts.
The algal bloom can grow so much that it blocks the sunlight from reaching habitats below. Then, when it dies, the decomposition process uses up all the oxygen in the water, creating what is known as a dead zone. Without enough oxygen, the animals in these waters perish.
Produce toxins
Certain types of algae – such as cyanobacteria, which is often called blue-green algae – contain dangerous toxins. These can work their way up the food chain as animals eat the algae and then are eaten by predators, which are, in turn, eaten by larger predators.
These can kill many animals and cause strandings. In June 2026, researchers at UC Santa Cruz confirmed that two young humpback whales found dead on the coastline had been affected by domoic acid: a neurotoxin that comes from harmful algal blooms.
“Domoic acid is a naturally occurring marine biotoxin produced by certain harmful algal blooms and can accumulate in marine food webs,” writes Mike Peña on the UC Santa Cruz website. “Exposure can cause neurological impairment, seizures, and death in marine mammals.”
The disturbing phenomenon might sound familiar if you’ve ever seen Alfred Hitchcock’s movie The Birds, which was inspired by real-life bird attacks in California in 1961.
“Sooty shearwaters, which are sea birds, seemingly started attacking people in the city, dive-bombing people, buildings and cars, and regurgitating and vomiting half-digested fish,” says Professor Anne Goodenough on the University of Gloucestershire’s website.
The birds had consumed domoic acid and the neurological impacts caused them to go wild. “The birds didn’t know where they were or what they were doing, so they weren’t attacking, they were completely disorientated,” she says.
Another neurotoxin produced by marine algae – brevetoxin – was behind the mass deaths of bottlenose dolphins and manatees in Florida in 2007 and 2008.
And the mystery of the 'spinning' sawfish in Florida – when the fish started acting strangely and spinning in circles before dying – was solved when researchers confirmed that a cocktail of different toxic algae was behind the deaths.
Illness
As you might imagine, eating poisonous seafood doesn’t just have catastrophic effects for animals. If humans eat the affected fish or shellfish, they can become seriously ill.
When some shellfish filter-feed in water containing algal toxins, they can accumulate these and people can get sick if they eat them. “Algal toxins can cause diarrhoea, vomiting, tingling, paralysis and other effects in humans, other mammals or fish,” writes the World Health Organisation on its website. And, even more worryingly, cooking or freezing the fish does not destroy the toxins.
And when a type of plankton called dinoflagellates that produce ciguatoxins are eaten by fish, this contamination can also be passed onto humans. “Symptoms of ciguatera poisoning include nausea, vomiting, and neurologic symptoms, such as tingling sensation on fingers and toes,” writes the WHO. “There is currently no specific treatment for ciguatera poisoning.”
Swimmers, water users and pets can get sick just from touching or swallowing water affected by harmful algal blooms.
To minimise your risk of getting sick from contaminated seafood, check for local announcements about algal blooms in your area, do not swim in or collect seafood from affected areas and get medical help at once if you notice any symptoms after possible exposure.
The smell of death surrounds you
There are several other inconveniences. When large swathes of algae bloom and die, their rotting remains can wash up onshore en masse.
It’s not pretty, choking once beautiful tropical beaches with unsightly brown mats that stink of rotting eggs. Businesses that rely on tourism can be hit hard as travellers avoid their ugly, eggy coastlines in favour of somewhere more appealing.
Sargassum seaweed is a particular culprit. This brown seaweed spends its life bobbing along in the open ocean, providing a floating habitat for many different creatures. If small amounts wash up, it’s not much of an issue but when sargassum blooms uncontrollably, the consequences can be awful.
“On land, when washed ashore, decaying seaweed releases toxic gases such as hydrogen sulphide and ammonia, posing health risks and contaminating groundwater,” writes Plymouth Marine Laboratory on its website. “Communities across the Caribbean and West Africa are also facing economic hardship, as the seaweed clogs fishing gear, damages boat engines, and deters tourists with its strong odour and unsightly build-up.”
Once harmful algal blooms are underway, there’s not much that can be done to nip them in the bud, although experts are trying to find solutions.
“Scientists are researching ways to control HABs, like using tiny bubbles to aerate the water and dissolve the algae; using chemicals or bacteria to halt the algal growth; and binding the algae with clay, causing it to sink and collect excess nutrients,” writes NOAA’s National Ocean Service on its website. “Scientists can also detect and forecast the location of HABs to warn communities of these events.”
But we can prevent these events by reducing our carbon emissions and making choices that minimise pollution that can cause HABs, such as choosing soaps and fertilisers that don’t contain phosphates, minimising your water use to prevent runoff and picking up dog poop (which contain lots of nitrogen and phosphorous).
