Anastasia Marquis Bioluminescent Algae


Why Are Parts of the York River Glowing in the Dark?

After images like the one above from Ana Marquis Photography started popping up on social media showing the York River literally glowing in the dark—we wanted to see it for ourselves. Sure enough, as the wind kicked up a few waves by the fishing pier, several of the whitecaps became bluecaps—as far as the eye could see. The all-natural phenomenon has a scientific explanation, and while it sadly doesn't involve Aquaman or The Deep, it's fascinating nonetheless.

We reached out to the Virginia Institute of Marine Science. Here are some of our takeaways after connecting with Dr. Kim Reece who graciously answered some of our/your questions.

Bioluminescent Algae York River

What exactly is causing that eerie blue glow?

It’s the result of a large algae bloom. More specifically, Alexandrium monilatum (Alex for short). In late summer, the salinity, temperatures, and nutrient levels tend to be optimal for the algae and in 2020 – they seem to have all come together to create the perfect conditions for a bloom formation.

But why does it light up?

There are a couple of possible reasons, but still no definitive scientific explanation. The cells could use bioluminescence to signal partners for sexual reproduction, but it could also be a way to tell predators to back off, that they are toxic. Sort of like a warning: “Eat at your own risk. This could be your last supper, buddy!”

Where’s the best place to see it in Yorktown?

Below, are satellite images taken by the National Oceanic and Atmospheric Administration. The one on the right is from Sept. 7. Look closely. You can see the very dense (red) bloom patch in the lower York River, below the Coleman Bridge. There is also a patch out in Hampton Roads and a patch in the James River. There are blooms up in the upper part of the Rivers, but we're told those are probably different species.The picture on the left removes the background from the upper parts of the rivers. These views come courtesy of Dr. Richard Stumpf/NOAA, using Copernicus Sentinel-3 data.

Bioluminescent Algae York River Satellite Images

Just how big is this bloom?

The bloom has dense patches that move with the winds, tides, and currents, which makes it difficult to follow.

When’s the last time this has happened in the York River?

There was a large bloom in 2007 and there have been many since then. However, there weren’t any the past two years, which helps give scientists a baseline for comparison.

How long will it stick around?

The blooms can last anywhere from a little more than a week to longer than a month, but it will depend on conditions. Temperature and salinity are important factors as well as light, nutrients, and wind patterns. Researchers still don’t fully understand all of the environmental parameters needed to form and to maintain the algae bloom. That’s why they are studying samples and comparing conditions to data from previous years to try to hopefully identify some direct correlations to conditions in the York River at the time.

Below image courtesy of Photographer Michael Snell.

York River glowing blue shot by photographer Michael Snell

Is marine life at risk?

“Alex” does produce some toxic compounds that can be harmful to aquatic life. In prior years, there were reports of dead shellfish in the area of the bloom. It could be the toxicity or the fact that the algae sucks out a lot of the oxygen in the water. VIMS has both field and laboratory studies taking place right now to keep an eye on things. They are paying particular attention to larval fish and oysters.

Is it safe for people?

Dr. Reece says VIMS partners with the VDH to report algae cell counts from samples they take. There's actually a map of HAB (harmful algae bloom) cell counts on the health department's website. It's a useful tool to monitor as it also shows where you may be able to see the bioluminescent effects at night. 

Interactive HAB Map

Beneath the map is the key for each color-coded dot you see. It will let you know if humans and pets are at risk. The last sample in the York River was taken a few days ago and there is no current HAB Advisory in effect. This means the algae is not known to be harmful to people or dogs but, as mentioned, could be toxic to aquatic life. If you suspect you are experiencing dermal or respiratory irritation after swimming in the vicinity of this bloom, please contact the Virginia HAB Hotline at 1.888.238.6154. The best advice? When it doubt, stay out!

On that page, you can also report blooms as well as any dead fish, crabs, or aquatic animals you might see. 


We noticed as we were walking along the shoreline - our footprints were lighting up as our shoes hit the sand. It was in an area of the beach where we actually couldn't visibly see any of the algae in the water. Dr. Reece explained that the bioluminescence occurs when the cells are disturbed, causing a chemical reaction. We loved her equally scientific and fun response she gave. "A compound is combined with oxygen by an enzyme and then sparkles."

While the blue seems to burn brightly in person, it's surprisingly very difficult to capture on a cell phone camera (as you can tell in our posts at the top of the page). For all you photographers out there - if you have any cool images you'd like to share, please be sure to tag us on Facebook and use #visityorktown with your posts on Instagram.