NARRAGANSETT, R.I. — Narragansett Bay has experienced dramatic changes during the past century, from being a dumping place for sewage and industrial pollutants to a near paradise for recreational swimming and boating. But changes continue to occur, whether from the warming climate, invasive species, fluctuating wastewater effluent, or other factors.

As University of Rhode Island oceanography professor Candace Oviatt recently told an audience of fishermen, scientists and students, “I don’t think I’ve ever seen an average day on Narragansett Bay. The bay is always changing. Every year is different. Whether we like it or not, the bay is going to keep changing.”

Oviatt’s comments on Dec. 6 were part of a daylong symposium sponsored by Rhode Island Sea Grant and aimed at creating a dialogue between fishermen — many of whom are worried that the bay has gotten so clean that there is little food left for fish to eat — and scientists whose research tells a sometimes confusing story of how the bay’s changing ecology might give that erroneous impression.

While most of the scientists claim their research suggests that the biomass of fish and other creatures living in Narragansett Bay has changed little through the years, almost all said the composition of species that call the bay home has changed dramatically.

A weekly fish trawl survey in two locations in the bay conducted since 1959 illustrates those changes. According to Jeremy Collie, the URI oceanography professor who directs the trawl, in the early years of the survey most of the species collected in the nets were fish and invertebrates that live on or near the bottom, such as lobster, winter flounder, tautog, cunner and hake. Those species also happen to prefer cooler water.

In recent years, the species that prefer warmer waters and that live higher in the water column have dominated the trawl surveys, including butterfish, scup and squid.

“Very few species are standing still or swimming in place,” said Collie, noting that similar patterns have taken place in estuaries throughout the Northeast. “All are either increasing or decreasing. And some have had really big changes.”

One of the major drivers of change in the bay is nutrient levels, primarily nitrogen and phosphorous, which largely come from discharges from wastewater treatment plants. Those nutrients stimulate the growth of plankton, which are fed upon by fish and a variety of marine invertebrates. But when nutrient levels are too high, it can cause harmful algae blooms and oxygen depletion in the water, which can lead to massive fish kills, like the one that occurred in Greenwich Bay in 2003.

Stricter discharge regulations for wastewater treatment plants since the mid-2000s has reduced nutrient levels significantly, leading to much clearer and cleaner water, especially in the upper bay. But it has prompted some fishermen, including lobsterman Al Eagles, to worry that Narragansett Bay has become “a dead bay.” Their concerns are in part because the decline of popular species such as winter flounder and lobster seemed to occur at the same time that discharge regulations were tightened.

Oviatt acknowledged that nutrient levels in the upper bay have declined by 50 percent since 2004, which resulted in a 33 percent decrease in “primary production” — the growth of plankton, especially algae. This has significantly decreased the size of the annual winter/spring plankton bloom that occurs throughout the bay and serves as the base of the food web, when it sinks to the bottom and is fed upon by crabs, fish and other species.

“When we have a short winter/spring bloom, or no bloom, we have low input of organic matter to the seafloor,” Oviatt said. “The consequence is lower biomass to the benthos [seafloor] and pelagic fish dominating the bay.”

She said this partly explains Collie’s findings that the species composition in the bay has shifted from bottom species to species living higher in the water column.

“Are there fewer fish in the bay? I don’t think so,” she said. “We’ve had a dramatic decrease in the bottom community, which started in the 1990s. Our decapods [crustaceans like lobsters and crabs] have basically left the bay.”

Oviatt said nutrients in the bay are still three times higher than they were before Europeans colonized New England. But just a few decades ago, when more sewage was being discharged into local waters, nutrient levels were up to five times higher than in pre-Colonial times.

“Wastewater treatment facilities are still the major contributor of nutrients to the bay,” she said.

Yet, Oviatt and others noted that decreases in nutrients from wastewater plants aren’t the only reason for the changes to marine life in Narragansett Bay.

Robinson Fulweiler, a professor at Boston University who studies nutrients in the bay, said the warming climate is causing some species to move northward to cooler waters and other species to arrive here from the south. And other factors complicate the situation.

Responding to a question about the “best” level of nutrients in the bay, she said it depends on what you’re looking for.

“The estuary a purist wants to go back to is not likely the one that fishermen might want,” Fulweiler said.

Rhode Island resident and author Todd McLeish runs a wildlife blog.