Green Swirls on Clear Lake

In the middle of May 2024, bright green swirls covered most of Clear Lake in California. A satellite called Landsat 9 took a photo of the lake on May 15, and the green color was easy to see from space.

The color came from an algal bloom, which happens when tiny living things called algae grow very quickly. This bloom may contain blue-green algae, also known as cyanobacteria. Like plants, they use sunlight to make their food. Some of them can also produce a poison that is harmful to people, but only a water sample can show exactly what is in a bloom.

Clear Lake, about 100 kilometers north of San Francisco Bay, is naturally rich in nutrients, the natural food that helps algae grow. Scientists think it may have had a lot of algae since the end of the last ice age, about 10,000 years ago.

In recent years, however, people have added more nutrients to the water. Rivers and water running off nearby farms bring nutrients such as phosphorus into the lake, and fish called carp stir up nutrients from the bottom. As a result, harmful blooms have become more common. Even without poison, too many algae can hurt the lake. When they die, bacteria use up oxygen as they break them down, which can create areas with very little oxygen.