Ask the Naturalist

Why Is Lake Merritt Turning Blue at Night?

October 2, 2018

What’s causing the water in Lake Merritt to glow blue after dark?

Every fall I get reports from rowers in Oakland’s Lake Merritt of a soft blue glow around their oars at night. Fear not a strange chemical leaching into the jewel of Oakland (as many may imagine). The blue light has a biological origin and points to a diversity of life thriving in this old tidal lagoon. This was the first year I was able to get out on the water and see the bioluminescence myself, which stretched from near green bridge all the way to the boat house, where it was markedly the brightest.

Lake Merritt is a brackish water system, a human-shaped but natural lagoon that’s fed by both the Bay tides and creeks running down from the Oakland hills. During the summer months when the creeks run dry, the salinity of the lake’s water rises to become much more like that of the Bay. Coinciding with this change in salinity, a bloom in marine organisms begins in the shallow warm waters. By late summer and early fall the microscopic organisms responsible for the blue glow, called dinoflagellates, erupt.

Dinoflagellates are a phylum of single-celled organisms found mainly in marine environments. The name comes from the Greek for “whirling whip” — two long whip-like “flagella” emanate from their armored-looking bodies and help them navigate. Approximately 70 species of dinoflagellates have demonstrated bioluminescence, and they are responsible for the majority of glowing waterways seen around the world.

These organisms operate at many different locations within in a food web. Some are photosynthetic, some eat other microorganisms, and some will do both. Some of the photosynthetic dinoflagellates are famous for their symbiosis with corals, sea anemones and other jellyfish. The free swimming ones tend to increase as waters seasonally warm and can sometimes bloom in such numbers that they cloud the water a muddy red color, also dubbed a red tide.

Bioluminescence, the ability of organisms to produce light, has evolved independently at least 40 different times. The bioluminescence dinoflagellates have evolved is in the blue-green spectrum, the visible spectrum of light that travels the furthest in water, which allows dinoflagellates to light up predators swimming around them and in turn make it easier for a predator of their predator to eat the threat and keep the dinoflagellates safe.

Glowing waves along the California coast and in Tomales Bay are usually attributed to one of the larger dinoflagellates such as Pyrocystis or Noctiluca, but Lake Merritt appears to have a different player involved since plankton tows of the glowing bloom didn’t capture either of these. I went out and captured a few as well, and after many hours looking down the barrel of a microscope I believe the most likely culprit is something in the suborder Gonyaulaceae. Lake Merritt’s proximity to the Oakland port also opens up the possibility that like many of the other organisms in the lake, the dinoflagellate may not originally be from California.

bioluminescent dinoflagellates
A microscope view of bioluminescent dinoflagellates captured in Lake Merritt. (Photo by Damon Tighe)

The best time to see Lake Merritt’s bioluminescence is in September or October around a new moon. Producing light is very energy intensive so dinoflagellate bioluminescence oftentimes operates on a circadian clock, with viewing at its best a few hours after dark. The glow is usually strongest away from shore, but going out to the end of docks in the lake will usually suffice to get far enough out in the bloom that putting something in the water and stirring a bit will elicit a ghostly blue trace. If you scoop water up into a jar, you’ll notice that it can be made to glow by shaking, but will only do so a few times.

Bellevue Tower with glowing bioluminescent Lake Merritt. (Photo by Damon Tighe)

Ask the Naturalist is a reader-funded bimonthly column with the California Center for Natural History that answers your questions about the natural world of the San Francisco Bay Area. Have a question for the naturalist? Fill out our question form or email us at atn at!

About the Author

Born in Klamath Falls Oregon and raised in Calaveras County of California, Damon Tighe attended Saint Mary’s college of Moraga California where he worked on local newspapers while earning a Biology/Chemistry degree. He taught High School in Portland, Oregon and moved back to the Bay Area to work on the Human Genome Project at the National Lab’s Joint Genome Institute. He spent a bit of time pursing a MFA in Natural History and Science Filmmaking in Montana, but returned to the Oakland to work on biofuels and single cell genomics.

He currently manages an apartment complex in downtown Oakland, works for Bio-Rad laboratories designing curriculum and training educators from Colorado to the coast in biotechnology, and is well versed in local fungi, plants and slowly but surely critters of Lake Merritt.