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Pink Dye Experiment to Reveal Mysteries of Coastal Ocean Dynamics

Scripps Oceanography-led PiNC experiment uses non-toxic pink dye and a suite of instruments to study the coastal zone where a river meets the ocean

A curling pink wave and pink seawater in the foreground, with the San Diego coastline visible in the background.
Pink waves at Torrey Pines State Beach during the first dye release for the PiNC experiment, Jan. 20, 2023. Photo: Erik Jepsen/UC San Diego

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An experiment at a San Diego beach is turning the seawater pink, temporarily, in the name of science. 

This winter, researchers at UC San Diego’s Scripps Institution of Oceanography and the University of Washington are leading a pink-hued dye experiment, titled Plumes in Nearshore Conditions, or PiNC, to study how small freshwater outflows interact with the surfzone. 

Funded by the National Science Foundation (NSF), the project is focused on the estuary and surrounding coastline at Los Peñasquitos Lagoon, located within Torrey Pines State Beach and Natural Reserve in San Diego, Calif. The first of three dye releases began on January 20, with the remaining releases taking place in early February 2023.

A researcher oversees a pink dye release at a lagoon mouth.
Scripps coastal oceanographer Sarah Giddings oversees the first dye release at Los Peñasquitos Lagoon. Photo: Erik Jepsen/UC San Diego

Rivers and estuaries play an important role in delivering freshwater and materials such as sediments and contaminants to the coastal ocean, but little is known about how these plumes of more buoyant, fresher water interact with the denser, saltier and often colder nearshore ocean environment, particularly as the plumes encounter breaking waves. 

By releasing an environmentally safe pink dye in the mouth of the estuary, the PiNC research team can track the processes that take place when small-scale plumes of freshwater meet the surfzone. 

“I'm excited because this research hasn't been done before and it's a really unique experiment,” said Scripps coastal oceanographer Sarah Giddings, who is leading the PiNC study. “We're bringing together a lot of different people with different expertise, such that I think it's going to have some really great results and impacts. We will combine results from this experiment with an older field study and computer models that will allow us to make progress on understanding how these plumes spread.”

Researchers are tracking the fluorescent pink dye from land, sea, and sky using a variety of instruments including drones, sensors affixed to poles in the sand in the river mouth and surfzone, and a jet ski outfitted with a fluorometer—a device that measures the fluorescence or light emitted from the dye. Beyond the breaking waves, several moorings and sensors deployed along the seafloor are measuring ocean currents and other conditions including wave heights, tides, salinity, and temperature.

The researchers said the PiNC experiment will provide a first-ever view of the buoyant plume/wave mixing dynamics at play in this particular location, and will lead to an improved understanding of how other small-to-moderate outflows of freshwater interact with the waves at locations around the globe. The results of this study will provide crucial data for quantifying the spread of sediment, pollutants, larvae, and other important material in the nearshore environment.

In this video, you can watch the scientists in action as they release the pink dye and deploy drifters, drones, and other instruments to track the movement of freshwater plumes into the coastal ocean.
A group of oceanographers prepare to deploy drifter instuments into the water.
Suited up in pink vests, members of the PiNC research team prepare to deploy drifters in the water. These drifters, along with the pink dye and other oceanographic instruments, track how freshwater plumes move into the surfzone and beyond. Photo: Erik Jepsen/UC San Diego

Giddings noted that many previous oceanographic studies have focused on large-scale freshwater plumes with high outflow, but there's very little research that has been done on smaller-scale plumes that directly interact with the surfzone. The Los Peñasquitos Lagoon was chosen as the study site because it is a “prime example” of a small river plume discharging into the surfzone along a relatively uniform stretch of coastline. The lagoon is a small intermittently closed estuary nestled between the cities of San Diego and Del Mar. 

“Los Peñasquitos Lagoon is a very dynamic system, with different elements changing each day, often even over the course of one day,” said Scripps postdoctoral scholar Alex Simpson, a member of the PiNC research team. “I am looking forward to seeing how the balance of physical forces—ocean waves competing against river outflow—determine the fate of the estuary water as it enters the coastal ocean on the days that we conduct our field experiment. I'm also very excited to fly a new type of drone system with a camera that has nearly 100 times the capability to distinguish the visible light spectrum than our phone camera.”

Two people look at a drone in the sky near the ocean.
PiNC team members Alex Simpson and Rob Grenzeback used a drone to capture detailed aerial footage of the study site during the experiment. Photo: Erik Jepsen/UC San Diego

All the dye releases are timed to occur during an ebb tide—the tidal phase when the water level is falling—to ensure that the dye is carried out of the estuary into the coastal ocean. Once released, the pink dye is visible to the naked eye for several hours after the deployment, with small traces detectable by instruments for approximately 24 hours. The dye is non-toxic, so it poses no threat to humans, wildlife, or the environment. However, beachgoers are advised to recreate further south or north of the estuary on the release dates due to the active research underway.

Scripps Oceanography scientists have successfully used pink dye to conduct other nearshore experiments, including an international study tracking beach pollution dynamics near the U.S-Mexico border in 2015, and earlier work based at Imperial Beach and Huntington State Beach.

Pink waves at a beach.
Pink waves at Torrey Pines State Beach on Jan. 20, 2023. Photo: Erik Jepsen/UC San Diego

The PiNC experiment is sponsored by NSF award #1924005. Complementary data at the same site examining estuarine dynamics and estuary mouth sedimentation is funded by the California Department of Parks and Recreation Natural Resources Division, Oceanography Program. The researchers have secured permits and approval from all necessary agencies to conduct this study. 

In addition to Scripps Oceanography and the University of Washington, collaborators on this project include Universidad Técnica Federico Santa María in Chile, UC Irvine, UC San Diego Mechanical Engineering, and the Naval Postgraduate School.

Visit the PiNC website and read the FAQ to learn more about this experiment.

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