Marine

URI Researcher Proposes Limited Return of Nutrients to Narragansett Bay

The level of nutrient reduction could be managed, which might in turn boost quahog populations

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State leaders announced earlier this month that 180 new acres in Greenwich Bay would be opened to conditional shellfishing, the first time since 2002. (Rob Smith/ecoRI News)

NARRAGANSETT, R.I. – University of Rhode Island researcher Candace Oviatt made a bold proposal at a Sept. 15 webinar on “A New Narragansett Bay”: a seasonal replenishment of nutrients that would nourish organisms in the bay.

Once vulnerable to oxygen depletion, the bay is now much cleaner since federally-funded, advanced wastewater treatment systems began to remove most of the nitrogen from discharged effluent. But the quahog harvest continues to decline, leading some in the commercial clam fishery to attribute the lower clam population to a lack of nutrients in the water.

In the first of three talks in the Coastal State Discussion Series webinars, sponsored by the University of Rhode Island’s Graduate School of Oceanography, Rhode Island Sea Grant and Boston University, Oviatt explained that in just a few years, advanced wastewater treatment, which began in 2005, had reduced nutrients entering the bay by almost 60%.

“It has been a huge success in decreasing the upper bay hypoxia in the summertime,” she said. “It’s not a total success yet, in terms of completely eliminating hypoxia, but it’s been a huge, huge improvement.”

Hypoxia occurs when nutrients produce an overabundance of algae. When the algae decompose, they consume the oxygen in the water that fish and shellfish need to survive.

Plankton growth in Narragansett Bay peaks when the water is cold. Oviatt said wastewater treatment plants are required to reduce nitrogen and phosphorus year-round, even though hypoxia occurs not in winter, but in the summer, when the water is warm.

The level of nutrient reduction could be managed, Oviatt said, which might in turn boost quahog populations.

“I’m suggesting that we could release nutrients from the wastewater treatment facilities in winter,” she said. “As I’ve said, the process goes on year-round, and we actually only need it in the summertime.”

Oviatt noted that commercial quahog landings have declined since nutrients were reduced.

“In recent years, around 20 million quahogs with a value of $5 million have been harvested annually from the bay and, since the nutrient reduction, the quahog production has declined by 35 to 40%,” she said.

Studies of squid in the bay show that their populations have also declined.

“The reduction in nutrients from the Providence River wastewater treatment facilities after 2012 reduced the nutrient concentrations, perhaps reducing clam condition and squid condition, and perhaps causing recruitment failure,” she said. “These results suggest that the managed nitrogen may not be as necessary in colder months as it’s been in warmer months, and I further suggest that we could release nutrients in the wintertime and it could have a beneficial effect on wild fisheries and aquaculture in the bay.”

In her presentation, Boston University researcher Robinson Fulweiler, who studies long-term changes in the bay, cited regional and global atmospheric processes beyond the treatment plants.

One important change, first observed in the 1980s, is a decline in chlorophyll, an indicator of phytoplankton abundance.

“There was a decline in this chlorophyll record long before we started to take out the nitrogen,” Fulweiler said. “And so, by taking out the nitrogen, we were likely adding even more of a decline to this. But I have hard time getting behind the idea that the nitrogen reduction is causing this decrease, because we hadn’t done the nitrogen reduction yet.”

There are multiple, complex factors that impact the Narragansett Bay ecosystem, including an increase in the number of overcast days, fresh water flowing into the bay, and nitrogen entering the estuary from the deeper ocean.

Fulweiler is trying to determine how much nitrogen is transported to the bay in deep ocean water.

“What we’re looking at here is trying to understand how much nitrogen really is coming from these oceanic bottom sources, basically, off the Continental Shelf, and then up and into Narragansett Bay,” she said. “…There are pathways of water into Narragansett Bay and one of the big pathways is really off the East Passage, and at different times of the year, there are these intrusions, literally, there’s the deep bottom water coming off the shelf and into Narragansett Bay, with fairly high amounts of nitrogen.”

Contacted after the webinar, Bob Rheault, executive director of the East Coast Shellfish Growers Association, said it would be premature to discuss replenishing nutrients in the bay without a better understanding of the full extent of deep-water nitrogen inputs.

Rheault cited URI researcher Christopher Kincade, who has determined that nitrogen transported to the bay in deep ocean water is more significant than previously thought.

“While I don’t know that Chris Kincade has really been able to fully put bounds on the amount of that input, obviously, he’s presenting a range, and the range is vast,” he said. “It was certainly an eye-opener to everyone when he presented that last year.”

More research is needed, Rheault said, to understand the reasons for the decline in the bay’s quahog population, which might be attributable, at least in part, to the predation of quahog larvae by shrimp and fish.

“I look at what drives recruitment,” he said. “Typically, it’s not a lack of larvae. There’s pretty much no evidence that these systems are larvae-limited. There’s plenty of larvae. What drives recruitment to the fishery is whether they survive the first couple months of their life.”

Rheault agreed that the nutrient replenishment idea should be studied, but he also noted that with hundreds of millions of dollars invested in upgrading wastewater treatment plants, it would be unlikely that the federal government would support a proposal to put nutrients back in the water.

“I would be surprised if they would let us,” he said. “We haven’t really achieved the nitrogen reduction targets that we were supposed to …We reduced the easy part, the 95% and if you want to reduce, you know, the last 3 or 5% that’s going to be another tenfold investment and incredibly expensive.”

The quahog decline, Rheault said, is more complex than a lack of nutrients.

“I know the diggers are making all sorts of conclusions that this is why the clam population is taking a dive,” he said. “But this is a long- term trend that has been going on since well before even the nitrogen reduction started and is much wider scale than just Narragansett Bay.”

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  1. I wonder if the salt ponds are being depleted of calcium and magnesium from the oyster industry as they are major components of the shells. I would like to see a circular system in which those shells are returned to the ponds.
    Another advantage to that would be raising the pH of the water from the acid levels increased by algae, etc.

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