Why Are Algal Blooms Making Marine Mammals Sick?

In April 2025, thousands of marine animals along the California coast were negatively affected by harmful algal blooms.

At first, I thought algal blooms were simply large amounts of algae growing in the ocean.

But the more I researched them, the more I realized they can affect entire marine food webs.

Certain algal blooms produce a toxin called Domoic Acid, which can spread through ecosystems and seriously harm marine animals like sea lions, dolphins, whales, and seabirds.

What surprised me most is how something microscopic can eventually affect huge marine mammals higher up the food chain.

Credits: pacificherring.org

Credits: pacificherring.org

Credits: pacificherring.org

Credits: pacificherring.org

Harmful Algal Blooms Spread Through Food Webs

One major cause of algal blooms is fertilizer runoff.

Fertilizers contain nutrients like nitrogen and phosphorus. When runoff carries these nutrients into rivers and drains, they eventually reach the ocean and help algae grow rapidly.

Plankton feed on the algae even when toxins like Domoic Acid are present. Then fish and filter feeders eat the plankton, and larger predators eventually eat those fish.

As the toxin moves higher through the food chain, marine mammals like sea lions, dolphins, whales, and birds can become poisoned. The more I researched algal blooms, the more I realized how connected marine food webs really are.

In another post, I explored how overfishing affects marine ecosystems and how disruptions in food webs can impact biodiversity across the ocean.

Domoic Acid Can Change Animal Behavior

Domoic Acid is produced by a type of algae called Pseudo-nitzschia australis.

When the toxin enters an animal’s body, it can affect the nervous system and brain. Scientists observed symptoms like seizures, confusion, abnormal behavior, and in severe cases, death.

Some surfers along the California coast even reported sea lions acting aggressively or unusually during algal bloom events.

Researchers believe many of these abnormal behaviors were linked to Domoic Acid poisoning.

The more I learn about animal behavior, the more I realize environmental conditions can heavily affect how animals experience the world around them.

In another post, I explored how marine animals process information and interact with their environments in very different ways from humans.

Climate and Ocean Conditions Affect Algal Blooms

One thing I found interesting is that harmful algal blooms are strongly connected to ocean conditions.

A process called upwelling brings cold, nutrient-rich water upward from deeper parts of the ocean. Upwelling helps support productive ecosystems along the Pacific Coast, but it can also provide nutrients that allow harmful algae to grow rapidly.

According to NOAA Fisheries, warm ocean conditions combined with nutrient-rich cold water can accelerate harmful algal blooms.

The same ocean systems that help ecosystems thrive can also unintentionally support toxic algae growth. The more I study ocean ecosystems, the more I realize nature is built on balance.

In another post, I explored how the Pacific Ocean supports interconnected habitats, biodiversity, and environmental systems across enormous distances.

Tracking Wind Patterns Can Allow Scientists to Predict Upwelling Credit to: Researchgate.net and UC Santa Cruz

Look Closer ...

At first, harmful algal blooms may seem like a problem affecting only algae.

But the more I researched them, the more I realized entire ecosystems can become affected through food webs, ocean conditions, and biodiversity.

Small organisms in the ocean can sometimes create massive effects higher up the food chain.

The More I Thought About It ...

The more I learned about algal blooms, the more I realized how fragile ecosystem balance can become when environmental conditions change.

Something microscopic can eventually affect whales, sea lions, dolphins, birds, and entire marine food webs.

The ocean often hides these connections beneath the surface until the effects suddenly become visible.

Wild World Question

Do you think harmful algal blooms will become more common as ocean conditions continue changing?

If You’re Into This:

• Marine Biology

• Oceanography

• Marine Ecology

• Environmental Science

In the ocean, even microscopic changes can affect entire ecosystems.