Marine Water photo

Marine Water Quality Monitoring

Marine Algae Blooms

Sunshine grows more plankton

Spring and summer are the time of year when sunshine and warm temperatures aren’t just making our lawns and garden plants grow, but are also contributing to algae blooms in our marine waters. An algae bloom is the visible appearance of millions of tiny plant-like organisms in the water. These tiny algae, or phytoplankton are present all year. The recipe for blooms is abundant sunlight, nutrients and the right water conditions.

Noctiluca algae bloom

The word phytoplankton comes from phyto, meaning plant-like, and plankton, which means drifting. Both phytoplankton (plant plankton) and zooplankton (animal plankton) are so tiny that their movements don’t match the movement of seawater driven by tides and wind, and thus they drift with the currents. Phytoplankton need sun and nutrients to grow. In winter, stronger winds and storms mix the plankton out of the surface waters and into the darker water below. In spring, growing conditions improve as longer days with stronger sunlight provide more light and warm the surface waters, making the water less dense and hence more stratified (layered) near the surface. This warmer, less dense water retains the phytoplankton near the sunlit surface where nutrients are also plentiful – and voilà – the conditions are perfect for phytoplankton to grow vigorously and form visible blooms at the water’s surface.

Algae blooms come in several colors

Blooms can appear in different colors ranging from green to red, orange, yellow or brown. Often these colors are from the common pigments in phytoplankton that allow them to photosynthesize. However, blooms can also be from zooplankton, such as Noctiluca, which feed on the phytoplankton. Noctiluca visibly aggregate at the sea surface as part of their life cycle and feeding strategy. The nontoxic Noctiluca blooms usually appear as a rusty reddish color like tomato soup and are common in Puget Sound. A Noctiluca bloom is in the photo above.

Algae can produce toxins

Green blooms

Most blooms are harmless, but some types of blooms can produce toxins that can make people sick if they are exposed to high enough levels of the toxins. Exposure can come from inhaling or swallowing water with toxins or from eating contaminated shellfish (that fed on the toxic phytoplankton).

Algae blooms effect oxygen levels in the water

Phytoplankton blooms are a natural occurrence in spring. Blooms can also occur in summer and fall when there is an increase in nutrients from natural sources such as wind-driven mixing of surface waters with deeper waters, or human sources, such as wastewater treatment plants. As phytoplankton use up the nutrients in the surface waters, their growth slows and cells eventually die. Dying blooms can be an environmental concern because as the cells sink and decay, bacteria decompose the organic material, which in turn strips oxygen from the water. This microbial oxygen demand at times leads to very low oxygen conditions in the bottom waters, harming aquatic life.

Ecology is taking steps in Puget Sound to determine how human activities and natural factors affect nutrient and low dissolved oxygen levels.


South Puget Sound Dissolved Oxygen Study is helping determine how humans affect this natural process. The study will inform actions to improve water quality.

Nitrogen in the Puget Sound Ecosystem describes what we know about the sources and contributions of this nutrient to Puget Sound.

Report bloom sightings

Ecology has been conducting monthly monitoring of Puget Sound and Washington’s coastal estuaries for phytoplankton abundance and nutrients (as well as other parameters) for decades through our marine flight program.  We are very interested to hear reports of bloom sightings so that we can direct our oceanographers to those locations to collect and analyze samples, and keep statistical information on their occurrence. Please note the date, time, location, and color of the bloom, and contact Dr. Christopher Krembs at with this information.

Could it be a pollution spill?

Sometimes algae blooms look like spilled paint, oil, or sewage. The reverse may also be true. Report an Environmental Problem tells what to do if you suspect that you are seeing pollution.

Related information

Eyes Over Puget Sound, from Ecology's Marine Flights team, shows surface conditions from a float plane once a month throughout the year.

Washington State Toxic Algae is the freshwater algae bloom-monitoring program for the state. Check to see what lakes are swimmable and which may not be.

SoundToxins is a site managed by the Northwest Fisheries Science Center at the National Oceanographic and Atmospheric Administration to inform the public about harmful algal blooms and to keep seafood safe from algal toxins.

Harmful Algal Blooms calls itself  "The nation's resource for understanding Harmful Algal Blooms and Marine Toxins in the Pacific..." and is also at the Northwest Fisheries Science Center.