Algae Blooms

What is that in the water?


Summer is the time of year when algae “blooms” can turn the water interesting colors and be very noticeable. But what is an algae bloom, and what causes it?

Algae are photosynthetic organisms that are natural components of marine and freshwater ecosystems. Common bloom-forming algae found in Jefferson County lakes include green algae and blue-green algae (cyanobacteria), and in our saltwater bays dinoflagellates, diatoms and other plankton can be dominant. Blooms occur when a combination of warm temperatures, sunlight, and nutrient-rich waters cause algae to reproduce rapidly, often producing a layer of scum on the surface of the water. Within a few days, a clear lake, pond, ditch, lagoon or bay can become cloudy with algae growth. Blooms are most common in summer but can also occur at any time of year.

Some blooms may be harmful to other organisms, and are referred to as Harmful Algae Blooms (HABs). Some organisms, such as cyanobacteria, can produce biotoxins, or poisons, that can cause illness or death in humans and animals. Not all algae are toxic, and even known toxin-producing species may not produce detectable levels of toxins in every bloom. Toxicity cannot be determined visually, and can change from one day to the next. Laboratory testing is the only way to know if a bloom is toxic.

In freshwater, these biotoxins can affect humans and animals through water contact and ingestion. In marine water they can be consumed through marine shellfish. Shellfish are filter feeders that ingest particles from the water, including algae. When they consume toxin-producing algae, they accumulate these biotoxins in their tissue. These toxins do not affect the shellfish, but concentrations of these toxins can reach levels that can cause illness or death in humans and other animals when the shellfish are consumed. In spite of the common term “red tide”, blooms that cause shellfish to be toxic usually are not red and may not be visible at all without a microscope.

However, not all blooms are toxic. But non-toxic algae blooms can still block sunlight from underwater habitats, can clog fish gills, and bloom decomposition can consume oxygen in the water, creating an anoxic environment detrimental to aquatic life. Fish kills can occur when this happens.

In Jefferson County, freshwater and salwater algae blooms are a regular occurrence. Several of our lakes, including Anderson Lake, develop cyanobacteria (blue green algae) blooms. Go to our Lake Water Quality pages to find out more. Discovery Bay, Kilisut Harbor and Mystery Bay frequently experience harmful algae blooms that can make shellfish toxic to eat - other bays can also be affected - and these blooms can be invisible to the eye. See our Shellfish Monitoring page for more information. In Hood Canal blooms of coccolithophores, (phytoplankton that thrive in still, nutrient-poor water in mild temperatures), have been a recent phenomenon. They are not harmful to other organisms, though they do make the water appear turquoise in color. 


For more information, please see the following:

Jefferson County Lake Status (toxic algae)

Shellfish Safety Map     and Hotline 1-800-562-5632