SAN DIEGO COUNTY, Calif. — Red tide returned to San Diego over the weekend. 

During the day, the algae bloom can cause the ocean to appear with a red or brown tint, but at night, when agitated, it can create a blue bioluminescence.

“That term, red tide, can be conflated with harmful algae. But really it just means that the water is colored red because a lot of the phytoplankton in the water,” said Clarissa Anderson, Executive Director of Southern California Coastal Ocean Observing System.

Loading ...

Several videos showing particularly bright blue bioluminescence were posted online. One from Newport Beach showed dolphins swimming in neon-colored water near a boat. 

Another showed bright blue waters off Pacific Beach.

“When you go to the beach, you see it in the waves or you see that amazing video of dolphins cutting through the water and stimulating the bioluminescence. That's another example of how that shear creates that chemical reaction,” explained Anderson. “We see it in the water quite frequently, but we don't always see these major bioluminescent events.”

The red tide in Southern California is not considered harmful to humans and its presence was not affected by the recent stay-at-home orders that closed beaches.

Beaches in the City of San Diego were reopened Monday at sunrise. 

Many surfers found the red tide made it difficult to see underwater and made the water gritty.

“Hard to see beyond a few inches in the water,” said Daniel Holst after surfing. “It’s kind of reminded me like river water after heavy rain. It was murky. I walked to the beach and thought it’d be smelly, but it smelled fine.” 

RELATED: Mayor Faulconer, Port District, police, lifeguards give report on first day of limited beach re-openings

RELATED: Officials report 3,141 cases of COVID-19 in San Diego County, 113 reported deaths

RELATED: Coronavirus in San Diego and California: Latest updates and news