Current meters contribute to sustainable seaweed farming in Puget Sound

“I started using a Nortek current meter back in… oh, gosh, it’s probably 15 years ago!” says Joth Davis, co-founder and principal at Blue Dot Sea Farms, a Puget Sound-based seaweed and shellfish farming company. Davis, who received a PhD from the University of Washington in 1994, started a family-run shellfish company called Baywater Shellfish Co. back in 1990 alongside his wife Karen, raising oysters and other species including geoduck clams. His son Caleb now runs the Baywater day-to-day, allowing Davis more time for other ventures, like founding Blue Dot Sea Farms eight years ago.

Davis has used a Nortek Aquadopp for much of his shellfish and seaweed farming career, both at Baywater Shellfish Co. and at Blue Dot Sea Farms. Understanding current speed and direction at farming sites is crucial to setting them up for success on several levels.

Seaweed farming: the why and the how

At Blue Dot Sea Farms, Davis is growing sugar kelp (Saccharina latissima), a common species of kelp grown in various parts of North America. According to Davis, the best use for sugar kelp is as food. Usually, they sell the kelp to restaurants or other outlets as raw kelp, but they can also freeze it, dry it, or turn it into different types of snacks.

One of the benefits of seaweed farming is seaweed’s ability to “pull” carbon or other nutrients from seawater while growing. Dissolved carbon dioxide in seawater is a major contributor to ocean acidification, so its removal is beneficial to the marine environment.

Farmed seaweed starts out in a laboratory as a microscopic plant called a sporophyte. The sporophytes get seeded onto a “grow line,” which acts as the substrate on which the kelp will grow.

“They love it,” says Davis.

The seeding process happens around Thanksgiving (late November). The kelp then spends the winter growing before being harvested around the beginning of April.

The seaweed farm itself, according to Davis, is typically anywhere from five to 10 acres. Blue Dot Sea Farm is a 1000-foot by 250-foot rectangle. The grow lines are about 500 feet long or more, suspended about 8 feet below the sea surface and anchored to the seabed. These mooring structures enable the organized growth and harvest of seaweed.

Aquadopp current meters offer flexibility across projects

About a year ago, Davis finally replaced his original Aquadopp, after more than a decade of use around the different shellfish and seaweed farms he works on.

Davis originally started using the Aquadopp on his shellfish farm to try to figure out the flux of seston (essentially, shellfish food) over the shellfish beds. To do so, gathering information about current speed and direction was crucial, and the Aquadopp proved the right tool for the job.

Since that original Aquadopp deployment, Davis has used the Aquadopp for gathering information on his seaweed and shellfish farms in a variety of projects.

“The Aquadopp is a staple in my quiver of instrumentation that we use for all kinds of things,” says Davis. “It fits so many studies in so many different ways.”

According to Davis, a typical deployment involves suspending the Aquadopp at two to three meters depth from a floating structure or a buoy. He usually leaves the current meter out for around three weeks at a time to capture multiple tidal cycles, measuring every 10 minutes, before retrieving it to look at the data. He says future projects may follow a different pattern aimed at capturing a single tidal cycle but measuring more frequently. The varying nature of Davis’ deployments makes the flexibility of the Aquadopp is a benefit.

“That’s what’s nice about these instruments,” he says. “You know they’re easily programmed for different deployments.”

Using current meters to optimize seaweed farm production

One of the major uses for the Aquadopp on the seaweed farm is to determine optimal setup of the farm itself.

Davis has deployed the Aquadopp at multiple sites around their farm and found similar results across the area: the currents run north and south. The rectangular setup of the mooring systems on which the kelp is grown was based on this information.

“How that footprint is oriented was partly due to the Aquadopp deployment,” says Davis. “We want to be more or less perpendicular to the main current that goes through the particular area.”

The long axis of the farm’s footprint is therefore oriented in line with the average current direction through the farm.

“That way, there’s less stress on the mooring systems. If the currents are running in line with the moorings, they operate more efficiently,” explains Davis.

Davis often acts as a consultant during the setup of new seaweed farms, and the Aquadopp comes in handy nearly every time.

“The current meter is pretty much the first thing that comes out,” he says. “First thing we need to know for your site is what are your average current velocities and the direction. You can have that assist in how you place your moorings for optimal operations.”

Climate adaptation projects on seaweed farms

More recently, Davis collaborated with scientists from the University of Washington and NOAA to determine if the carbon coming through the farm in the form of dissolved carbon dioxide could help the seaweed grow. This would in theory also reduce the amount of dissolved carbon dioxide in the seawater, contributing to ocean acidification mitigation.

“The Nortek current meter was again useful in this climate adaptation project,” says Davis. “Among other things, we needed to know the velocity of the currents coming through the farm.”

In this case, their findings showed that the velocities at the seaweed farm were too high to measure a carbon uptake signal; currents going through the farm averaged about 30 cm/s, whereas the ideal for carbon uptake would be less than 10 cm/s.

“The problem was that there was too much current flow through the farm,” says Davis. “And we learned that. And you know, that’s why you do it!”

While the site they took measurements at may not be right for estimating carbon uptake, the team was able to develop a model which correlates flow, carbon and nitrogen content in seawater, and seaweed growth, which can be used to choose sites where seaweed may successfully be farmed for carbon uptake in the future.

Food and the future

Beyond consulting and running his farms, one of the newer projects Davis is working on is Blue Dot Kitchen, a seaweed food company using seaweed from Blue Dot Sea Farm. The specialty product? A seaweed puff called “Seacharrones” (at risk of ruining the joke, a play on “chicharrónes,” fried pork rinds). The sustainable snack is sold in many stores around the Pacific Northwest with hopes to expand.

All in all, Davis loves what he does, and over 40 years of getting out on the water during Washington winters have not changed that. His passion for his work shines through clearly in the way he talks about each facet of it, from shellfish and seaweed farming to climate research.

“We’re having fun. I think that’s the bottom line,” he says. “Have fun at what you do. It keeps me busy and hauls me out of bed in the morning.”