Rhode Island Sound’s Role in Fueling Harmful Algal Blooms in Narragansett Bay

Anchored boats and buoys slowly emerge from and vanish back into the fog that lays thick across the water’s surface in the early morning hours as we slowly motor out of Wickford Harbor. It’s the beginning of May and I’m with GSO researchers Lucie Maranda, Dave Ullman, and their student, Joe Barnes, in a 16-foot center-console heading to a mussel farm in Narragansett Bay’s east passage.

The team is going to collect mussels from American Mussel Harvesters that will be used to monitor domoic acid, a toxin produced by certain species from the phytoplankton genus, Pseudo-nitzschia, which can infect shellfish.

“Mussels filter a lot of water and are generally the first shellfish to become toxic,” says Maranda, explaining why mussels are used for monitoring research.

The fog had started to burn off by the time we rafted up to the American Mussel Harvesters’ workboat amidst the rows of black buoys marking the mussel lines. The crew passed us a crate of recently harvested mussels, which the research team divided into 16 orange mesh bags. They attached these to two 12-foot lines and deployed them on moorings at a sentinel station south of Brenton Point to begin monitoring domoic acid uptake.

“We will collect them biweekly and run an [amnesic shellfish poisoning] test,” says Maranda, adding that they will also test wild mussel stocks. “The test looks like a pregnancy test. It’s either yes or no, positive or negative, for domoic acid.”

 

GSO researchers Lucie Miranda and Dave Ullman, along with student Joe Barnes collect mussels from American Mussel Harvesters and transplant them past Brenton Point to monitor harmful algal blooms in Rhode Island Sound.

The research is funded by Rhode Island Sea Grant to better understand and predict blooms of Pseudo-nitzschia in response to an unprecedented bloom that spanned from Long Island to Maine in 2016.

Although these species can be found throughout the bay and elsewhere at any time of year, it was the first-ever shellfishing ban in Narragansett Bay in response to a toxic algal bloom. After five tons of shellfish were recalled in Maine that September and a rapid increase of Pseudo-nitzschia was detected outside of Newport Harbor in early October, a shellfish ban was issued for Narragansett Bay, the Sakonnet River, and Mount Hope Bay.

Rhode Island required dealers to hold all shellfish until tests could be made to determine whether the toxins were present at levels of concern.

“We had no idea what was going to happen or how quickly,” says Angelo Liberti, chief of Water Resources at the Rhode Island Department of Environmental Management, during a public debrief shortly after the event. “Our first priority was whether they could release the dealer stock safely.”

If humans consume highly contaminated shellfish, illnesses can range from gastrointestinal problems and lethargy to more severe cases of short-term memory loss and seizures. While levels of domoic acid remained far below standards considered unsafe for consumption and no illnesses were reported in Rhode Island, sales dipped for some local harvesters, and many state agencies were left scratching their heads as to why this bloom occurred.

Pseudo-nitzschia is comprised of a number of species, some of which produce the toxin and some that don’t, and the species that produce the toxin don’t always produce the same amount or any toxin at all. So that’s what makes it hard to predict and understand,” said Tatiana Rynearson, a plankton expert at GSO, in an interview last winter.

One theory that Maranda and Ullman, along with fellow GSO oceanographer Christopher Kincaid, are investigating is that Rhode Island Sound may be a potential source for harmful algal blooms since the 2016 bloom was concentrated in the mid- and lower bay, with its longest duration in the Sound.

To date, only two plankton samples near the Jamestown Conanicut Marine dock have shown positive results for domoic acid. Mussels from both the sentinel stations, as well as wild populations near Fort Wetherill, have produced negative results. Maranda’s and Ullman’s work will eventually help determine the abundance and distribution of Pseudo-nitzschia and two other toxin-producing species (Alexandrium and Dinophysis) in relation to physical parameters within Rhode Island Sound and lower Narragansett Bay. This includes looking at whether these species are introduced into the bay from the Sound or whether they develop into blooms from already established populations. The results will be coupled with Kincaid’s work looking at the intrusion rates from the Sound into the bay.

“One of the important things missing is the water coming in from the shelf,” says Kincaid. “There’s a deep pool of nitrogen [offshore] in the bottom water in the summer. We have current meters that show steady intrusions of water that comes in, plus these big wind-driven intrusions, but we don’t have good measurements of the two of them together.”

The knowledge gap, he says, is understanding the nitrogen budget and figuring out if, and how, this nitrogen-rich bottom water offshore is making its way to the photic zone in the bay where it can fuel harmful algal blooms. “We need a true nitrogen budget to understand circulation patterns and how that impacts the ecosystem.”

Kincaid and his team have set up several moored ADCP and CTD sensors along an east-west transect in the lower East Passage, which is the main conduit for water entering the bay from the Sound. Data on the velocity of water flow at various depths as well as temperature and salinity obtained from these sensors will be used to improve model estimates of nutrient
intrusion flux.

“This approach to nutrient monitoring is like the West Coast mentality where they have a narrow continental shelf and pay attention to the upwelling of nutrients from the deep water, whereas in the East Coast we have a broad continental shelf and very dense cities, so there’s more focus on the land sources,” says Kevin Rosa, a Ph.D. candidate in physical oceanography working with Kincaid. “But there are a lot of nutrients in Rhode Island Sound. It looks like the exchange between Narragansett Bay and Rhode Island Sound could be an important nutrient source.”

– Meredith Haas | Rhode Island Sea Grant