07/14/2025 | News release | Distributed by Public on 07/15/2025 10:56
Algae are nutritious organisms that lie at the base of many marine food chains. But there seem to be more stories about harmful algal blooms (HABs) sickening both people and animals in recent years. We asked Dr. Raphael Kudela of UC Santa Cruz to explain what's going on.
First, what is a harmful algal bloom, and what causes one?
"Harmful algal bloom" is really a societal term. Algae are not trying to cause problems. Generally, we like algae because they produce oxygen, take up carbon dioxide, and feed the food webs that produce seafood.
HABs occur anywhere there's algae, which includes freshwater and oceans. What makes algae "harmful" really is anything we don't like. Some algae produce toxins that are harmful to humans, fish, water birds, whales, and other marine mammals. These algae can have both ecological and economic impacts (such as shutting down fisheries).
Are HABs happening more often than they used to? Why?
A nice meta-analysis a few years ago took global datasets and looked for an increase in HABs. It showed that globally, there has not been a consistent increase. But if you break it down by region and organism, some regions are seeing increases, some are flat, and some are decreasing. We're interested in where HABs are happening more.
We see problems where we're putting extra nutrients into the water, for instance through wastewater discharge. Urea, ammonia, and other forms of nitrogen pose particular problems. We also suspect that climate change is having an effect. Algae also like a certain temperature: the warmer it is, the faster they grow.
Earlier this year, an "unprecedented, multi-toxin event" sickened and killed hundreds of seabirds, sea lions, and dolphins off the coast of Southern California. What happened?
In the last several years, we've seen problems with algae that create domoic acid, which is often concentrated in shellfish. Domoic acid attacks your nervous system, creating short-term memory loss or serious brain damage, and it can kill you. Human fatalities have gone way down because shellfish are well regulated in the US and the rest of world-but you can't tell a pelican to not eat fish.
We think the bloom started offshore because sick animals started showing up before the algae appeared at the coast. In California, diatoms are at the base of the food chain. They are eaten by squid, anchovy, and sardines, which aren't bothered by domoic acid and concentrate it in their bodies. When a pelican, a dolphin, or sea lion eats those fish, it gets a much bigger dose and may be poisoned. Pelicans have fallen out of the sky while flying, and a poisoned sea lion once swam up the Sacramento River. You'll see sea lions walking on freeways in Los Angeles: they don't know where they are. If they keep getting exposed, they'll die. Then the algae and domoic acid made it to the coast, and the state shut down the mussel harvest.
At the same time-and this is the really unusual part-another toxic algae called Alexandrium appeared. You don't need much Alexandrium to create problems: it creates saxitoxin, which shuts downs the nerves that control your lungs. There is no known antidote-they put you in an iron lung to keep you breathing until you expel the toxin. This toxin does the same thing in birds, whales, and other marine mammals-it shuts down their ability to breathe. In Santa Barbara, the toxin abundance was screaming high: 10 times the dangerous level. If you'd eaten shellfish, you'd probably just fall over dead.
It's unusual to have both toxins at the same time. This is the first time in recent history we've seen both. It suggests that something is shifting in Southern California that's making both organisms happy, creating more toxins. There's concern about whether this will continue. We need to think about why it's getting to the point where it's causing problems.
What actions can prevent HABs?
First, we need to reduce nutrient runoff from wastewater discharge. A lot of agricultural runoff used to end up in Monterey Bay. Ten years ago, farmers were told that they should not let nutrients leave their land as runoff above a certain level. Farmers are now using less fertilizer and paying less for it, and thus there are fewer nutrients in the bay. This is a win-win.
We can do more of this, including with golf courses. It's not terribly expensive to select plants that thrive on high nutrients and place them so they capture and filter runoff. They will also take up CO2. There are a lot of benefits, including better water quality. As an individual, don't overfertilize your lawn. Whatever's not taken up gets into wastewater and rivers.
Second, if we can reduce the warming associated with climate change, we'll see fewer harmful algal blooms. That is a global issue that is not easy to fix, but the longer we take to reduce warming, the more difficult it will be to reverse the changes.
We also need to continue bringing wild animals that are poisoned by HABs into rehab centers to keep them from dying. We have a good success rate at saving many different species. We can put them on an uncontaminated diet while their bodies clear the toxins. If they don't have brain damage, they're released again into the wild.