Hood Canal blooms again, as biologists assess role of armored plankton

In what is becoming an annual event, portions of Hood Canal have changed colors in recent days, the result of a large bloom of armored plankton called coccolithophores.

Coccolithophore from Hood Canal’s Dabob Bay viewed with scanning electron microscope.
Image: Brian Bill, Northwest Fisheries Science Center

Teri King, a plankton expert with Washington Sea Grant, has been among the first to take notice of the turquoise blooms each year they occur.

“Guess who is back?” Teri wrote in the blog Bivalves for Clean Water. “She showed up June 24 in Dabob Bay and has been shining her Caribbean blueness throughout the bay and spreading south toward Quilcene Bay.”

Yesterday, I noticed a turquoise tinge in Southern Hood Canal from Union up to Belfair, although the color was not as intense as I’ve seen in past years.

The color is the result of light reflecting off elaborate platelets of calcium carbonate, called coccoliths, which form around the single-celled coccolithophores. The species in Hood Canal is typically Emiliania huxleyi.

Seth Book of the Skokomish Tribe lowers an instrument to measure light levels during a coccolithophore bloom this week in Dabob Bay.
Photo: Tiffany Royal, Northwest Indian Fisheries Commission

In the past, coccolithophore blooms seem to appear when the waters of Hood Canal are calm and sunny. The organisms are said to out-compete other types of plankton when nitrogen diminishes in surface waters. Nitrogen, a key nutrient for phytoplankton, can be used up in Hood Canal during periods of calm, dry weather. It will be interesting to see how the plankton population changes after recent rains may have infused a bit more nitrogen.

Meanwhile, biologists with the Skokomish Tribe have begun to investigate how the coccolithophore blooms could be affecting shellfish in Hood Canal. In recent years, shellfish growers have reported higher-then-usual oyster mortalities around the time of these blooms.

In 2017, Blair Paul, the tribe’s lead shellfish biologist, conducted a dive survey of the vast underwater geoduck beds in the midst of a coccolithophore bloom. Blair said he noticed that the geoducks weren’t eating, and the light levels appeared to be reduced.

Tiffany Royal, a public information officer for the Northwest Indian Fisheries Commission, wrote about his finding, quoting Blair in a news release: “Now we want to know two things: if there is a correlation between low crab and shrimp abundance when there is a coccolithophore bloom, and if there is reduction in food production in the water column for all shellfish nutrition.”

Tribal biologists are taking samples of water for concentrations of plankton while also looking at water chemistry. They are also testing for light levels inside and outside the plankton blooms.

Since the coccolithophores seem to dominate the waters after other major plankton species have declined, it is important to know whether shellfish will eat the coccolithophores, Blair said. They aren’t toxic, but their shells may be too abrasive for the shellfish to consume, he noted.

Seth Book, a tribal biologist who coordinates with the federal Environmental Protection Agency, told me that he is interested in the ecological role that coccolithophores play in Hood Canal, which is known for its low-oxygen conditions and occasional fish kills.

“We are concerned with potential reduction in primary productivity due to reflection and light attenuation, which means less food for shellfish,” he wrote in an email. “We have started to call it an ecosystem-disrupting harmful algal boom. Not toxic that we know of, but it appears to have impacts other than pretty water.”

Increasing levels of carbon dioxide in the atmosphere also complicates the picture. Since coccoliths are made of calcium carbonate, they might play a significant role in the carbon chemistry of Hood Canal — given their sheer number during a major plankton bloom.

The investigation of coccolithophores in Hood Canal is funded by a grant from the Bureau of Indian Affairs. A report is expected in the fall, and the tribe will follow with a mitigation plan that considers how to reduce damage to shellfish resources.

“The tribes have been here thousands of years and will continue to be here,” Seth said in the news release. “It could be a natural cycle, but what we’re seeing is having implications to shellfish and treaty resources. It could possibly spread to other parts of Puget Sound as well.

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