New Sea Grant research may hold the answer to $100 million question

A series of mesocosm tanks hold samples from Narragansett Bay.

Mesocosm tanks at the URI Graduate School of Oceanography allow researchers to bring a bit of Narragansett Bay into the lab. Photo by Malia Schwartz.

Researchers take a sediment core from a mesocosm tank.

Wally Fulweiler, recent URI oceanography Ph.D. graduate, and URI student Adam Pimenta take a sediment core from the mesocosm tanks to measure the exchange of nitrogen gas across the sediment-water interface. Photo by Malia Schwartz.

The graph shows average chlorophyll concentrations have decreased by 50% since 1970.

Long-term monitoring of the phytoplankton at a sampling station near the middle of Narragansett Bay shows that the average annual water column chlorophyll (Chla) concentrations have decreased by about 50 percent since 1970. (larger image)

Researchers in Scott Nixon’s Coastal Ecology Laboratory want to know what’s going to happen to Narragansett Bay, R.I., when nitrogen discharges from the Bay’s sewage treatment plants are markedly reduced over the next few years. This reduction is following already documented increases in temperature due to global climate change and important changes in phytoplankton composition and abundance, at least in the mid-Bay. “The Bay is poised to become the subject of a great but uncontrolled experiment in which both climate and nitrogen are changing,” says Nixon, University of Rhode Island (URI) oceanography professor. “During the past 25 years, it appears that the changing climate has reduced the amount of phytoplankton in the Bay. Now the nitrogen reductions are designed to reduce the plankton even more.”

“The Bay is poised to become the subject of a great but uncontrolled experiment in which both climate and nitrogen are changing.” —Scott Nixon, URI oceanography professor

Phytoplankton need nitrogen to live. But excess dissolved nitrogen can lead to excessive plankton growth and oxygen depletion when the plankton die—a process called eutrophication. While eelgrass losses, nuisance algal blooms, and fish kills have been blamed on eutrophication, no one is sure what a significant reduction in nitrogen will mean for water quality, habitat, or the organisms that live in the Bay.

The sewage treatment facilities at Field’s Point in Providence and Bucklin Point in East Providence have agreed to reduce nitrogen discharges from May through October by going to tertiary treatment—nitrogen removal through denitrification—at a price tag for Narragansett Bay Commission ratepayers of some $100 million.

In a Rhode Island Sea Grant-funded study, Nixon and colleagues have taken a bit of Narragansett Bay into the lab to determine what’s happening in Bay sediments and in the sediment-water interface (the water layer just above the bottom sediments). Using mesocosms—a series of holding tanks used to replicate the Bay environment—the researchers added either a full dose (estimated from historical data) of spray-dried phytoplankton; a half-dose of that historical input; or no plankton (control) on a monthly basis. The additions were meant to simulate bottom depositions from phytoplankton blooms.

The rate at which the bottom communities consumed oxygen and regenerated nutrients was measured each month, and the sediments were sampled at the end of summer to measure the exchange of nitrogen gas across the sediment-water interface.

The researchers found a marked decrease in sediment oxygen consumption, in the fluxes of ammonia and phosphate from sediments to the overlying water, and in sediment denitrification. According to Nixon and Robinson “Wally” Fulweiler, a recent Ph.D. graduate from Nixon’s lab, “We believe that these changes are the result of about a 50 percent drop in average annual chlorophyll concentration since 1970 as the Bay has shifted from a system characterized by a dominant winter-spring phytoplankton bloom to one supported by more ephemeral and less intense summer-fall blooms. The loss of the winter-spring bloom is particularly stressing for a benthic community that is already food-limited by the end of summer.”

Fulweiler’s and Nixon’s study indicates that “since climate change has already decreased primary production in a once highly productive Narragansett Bay, it’s possible that reductions of nitrogen input during the growing season will have a significant negative impact on secondary production—zooplankton, invertebrates, fish, and other animals higher up the food chain—throughout the mid- and lower Bay.”