The Algae Problem

When people think of algae, they often picture a thick green scum floating on the surface of a pond — the kind you can see, smell, and avoid. But one of Canyon Lake’s most dangerous threats doesn’t look like that at all. Instead, it is nearly invisible, hiding in the water until it strikes. This microscopic organism is called Prymnesium parvum, more commonly known as golden algae, and it has caused some of the most damaging fish kills in the lake’s history.

Algae themselves are some of the oldest and most important organisms on Earth. They are not plants, bacteria, or animals, but a diverse group of simple, photosynthetic organisms that live in water. Unlike bacteria, algae have structures similar to plants, including chloroplasts that let them convert sunlight into energy. While algae range in size, in lakes like Canyon Lake, most algae are microscopic and form the base of the aquatic food web, feeding fish and zooplankton. Unlike bacteria, which can thrive in dark and oxygen-free environments, algae depend on light and nutrients. Most are not harmful; in fact, algae produce much of the oxygen in our atmosphere and support aquatic ecosystems. Problems arise when certain types grow too quickly, fueled by excess nutrients, leading to what we call algal blooms. Blooms may discolor the water, create foul odors, reduce oxygen as they decompose, and in some cases release toxins that harm fish, pets, or people.

Types of Algae

There are many types of algae, each with its own traits.

• Green algae are common in freshwater and generally harmless, thriving in sunny, nutrient-rich conditions.
• Cyanobacteria, often called blue-green algae, are not true algae but photosynthetic bacteria. They can produce toxins and usually appear as thick mats or brightly-colored scums in warm, stagnant water.
• Diatoms, with their glass-like shells, bloom in cooler months, while dinoflagellates are more common in oceans but can occasionally trouble lakes.
• Golden algae, Prymnesium parvum, is an unusual species that can photosynthesize like plants but also feed on other organisms when nutrients are scarce. Under the right conditions, golden algae have triggered devastating fish kills, including the one at Canyon Lake in 2025. They remain suspended in the water as single cells, don’t form large mat structures, and their golden-brown tint is often subtle, meaning a lake can look normal until fish begin dying.

What Makes Golden Algae Dangerous?

What makes golden algae especially dangerous is the toxin they produce. These toxins target the gills of fish, damaging the delicate cells that exchange oxygen. Fish essentially suffocate even though the water around them has plenty of oxygen. During the April 2025 event, golden algae first appeared in Canyon Lake’s East Bay, then spread to the Main Lake within weeks. Threadfin shad, largemouth bass, crappie, carp, and bluegill all died in large numbers. Toxicity testing showed that even filtered lake water with no visible algae could still kill fish within hours, proving just how potent the toxin can be.

The biology of golden algae helps explain why these blooms appear and why they can be so destructive. Unlike many algae that rely solely on photosynthesis, golden algae are mixotrophs, meaning they can photosynthesize when conditions are good but also feed on other microorganisms when nutrients are scarce. This adaptability makes them extremely resilient. Golden algae thrive in waters with moderate salinity and certain chemical imbalances, such as elevated sulfates, chlorides, and nutrient shifts caused by runoff. Canyon Lake’s water chemistry, influenced by decades of buildup, can sometimes create the right conditions for golden algae to flourish. They can also take advantage of seasonal changes: As the lake warms in spring, oxygen-rich surface layers separate from oxygen-poor deep water, creating an environment where golden algae can dominate. At their peak in 2025, golden algae densities reached nearly 30,000 cells per milliliter — levels strongly linked to fish mortality.

Golden algae are an “unseen killer” in the truest sense. Residents may not notice cloudy or discolored water, and the bloom may not look alarming. But when fish begin floating to the surface, the damage has already been done. Understanding the difference between golden algae and more familiar green blooms is essential for the Canyon Lake community. By tracking conditions that favor golden algae and supporting proactive lake management, we can better prepare for, and hopefully prevent, future die-offs.

Article contributed by the Lake Advocacy Committee.