By Meredith Haas
Narragansett Bay today is not what it once was 100, or even 10, years ago. Coastal development and growing populations over time have meant changes to the bay’s geography and a greater influx of nutrients, such as nitrogen, as well as metals and other toxins. These human influences interplay with a host of complex natural systems, including climate, all of which are continuously changing the bay’s ecology in unknown and unpredictable ways.
Over 50 percent of Rhode Island’s salt marshes have disappeared and the once vast green carpets of eelgrass are all but gone from Narragansett Bay due to the impacts of landfilling and dredging, stormwater runoff, and wastewater discharge since the Industrial Revolution in the late 1800s. Nutrient reduction efforts and climate change are triggering further changes in the bay and will continue to do so.
“As the Industrial Revolution progressed, biodiversity declined until the 1980s, then appeared to recover in the 1990s and following decades, but not up to the same level,” said Stephen Hale, an ecologist at the Environmental Protection Agency (EPA), discussing changes in the benthic, or bottom-dwelling, communities of Narragansett Bay during the annual Ronald C. Baird Sea Grant Science Symposium in December, which focused on the ecological changes of the bay.
The reason for the slight rebound, he said, is still not fully known but is believed to be, in part, a result of reduced stressors entering the bay, such as nitrogen and metal inputs, thanks to legislation in the 1970s, such as the Clean Water Act.
A Shift in the Benthos
Although the overall diversity of the bay has declined, it is still relatively high when considering benthic organisms, said Hale.
“The bay has a very high benthic biodiversity because we get warm/temperate species from the south as well as [cooler water species from the north]. And since we have a deep east passage, we have species more common to the shelf, as well as rocky shores [that are not as common in other estuaries],” he said, noting that about 21 phyla of invertebrates – mostly worms, crustaceans, and mollusks – have been found in Narragansett Bay. “That’s 60 percent of the whole animal phyla on the planet.”
About 1,056 species have been recorded with hundreds expected to still be unknown. The majority of these species live in the top 10 centimeters of the bay’s sediment and support recreational and commercial fisheries, filter excess pollutants and nutrients from the water, store carbon, and help stabilize sediment against erosion.
While one of the EPA’s sample sites at the north end of Jamestown showed no real change in biodiversity since the 1990s, there was a dramatic decline near Spar Island in Mount Hope Bay – an area more vulnerable to human activity and impacted by wastewater discharge from the Taunton River and Brayton Point power plant. As a result, “the biodiversity crashed” there said Hale, leaving behind those species more tolerant of pollution.
Hypoxia (low oxygen) resulting from excess organic material from algal blooms induced by a surplus of nitrogen from wastewater discharge is more common at these locations and is a primary reason for such a response. As oxygen was depleted from the sediments, there was a loss of deep, long-lived species (such as quahogs).
“The prediction is that species will come back when you remove the [low oxygen and high nutrient] conditions,” said Hale, adding that while some stressors have improved in recent years (such as reductions in nitrogen from wastewater treatment facilities) there are still some lingering metal and toxin contamination of sediments from past industrial discharges and new challenges in the form of invasive species that thrive in increasing water temperatures, which have risen nearly 2˚C since 1959.”
“Temperature has a direct effect on the benthos. We’re losing cold water species to the north and gaining warm water species from the south,” he added.
One glaring knowledge gap, however, is the biomass of benthic communities. To better understand how these communities will respond to future changes in climate and nutrient reductions, and what that implies for the rest of the ecosystem – we need to measure their biomass, said Hale.
“We don’t have the biomass data to calculate secondary production from benthic communities,” he said, explaining that such production provides necessary food for shellfish and bottom fish. “We know almost nothing about this. It’s hardly been measured.”
Another knowledge gap concerns the loss of cancer crabs and lobsters in Narragansett Bay.
“We had a very dramatic decrease in the decapods; the cancer crabs, the lobsters; that started in the nineties,” said Candace Oviatt, a professor at the University of Rhode Island’s Graduate School of Oceanography (GSO) specializing in biological oceanography. “Decapods have basically left the bay and why that has happened, I think, is a huge research question. Some think it’s temperature, others think it could also be predation.”
A Shift in Fish
After noticing the decline in decapods, Oviatt and her team looked at fish biomass data in Narragansett Bay from DEM’s trawl surveys after 2009 and found various responses to fish abundance.
“In many areas, the upper bay for example, at Ohio Ledge, it looked like fish have increased,” she said. “In other areas like Hope Island, it doesn’t look like they’ve changed at all, and in the lower bay, there appears to be a slight decrease, about 10 percent. And it’s in the lower bay where the nutrients haven’t changed that much.”
The more noticeable changes appear to be reflected in the species type as iconic cold-water species are vacating the bay and warm-water species are moving in.
“When you look at how the fish community has changed, it’s basically shifted from resident species to summer visitors,” said Jeremy Collie, a researcher at GSO specializing in fish population and dynamics. “From a conservation perspective, we should be concerned about the resident species because they’ve evolved here. From a productivity and a fisheries perspective, we also need to think about what the summer migrants are doing.
These migrants include black sea bass, scup, squid, summer flounder, and butterfish. Those on the losing end of warming waters include silver hake, tautog, and winter flounder. Even though there is a lot of interannual variability with stocks rising and falling in any given year, there are very few species, said Collie, that aren’t being impacted. The trend lines between increasing temperatures and increasing warm water species show that the fish are responding to temperature.
“The preferred temperature of the fish community has increased to keep pace with water temperature,” said Collie, adding that the total consumer biomass is increasingly concentrated in the summer months and declining in the winter.
The timing of the migrations, he added, is shifting even more than temperature change would suggest. Temperature can also have indirect effects on the bay’s ecology. Warmer winters mean more cloudy days and less light to activate the winter-spring algal bloom, which ideally lasts for several weeks, raining down organic matter and feeding the benthos that supports demersal (bottom-feeding) fish like winter flounder. In addition to changes in algal productivity levels, warmer temperatures mean zooplankton and other species that feed on phytoplankton are active and eating away the bloom.
“Without the strong winter-spring bloom, we have less production sinking to the bottom and feeding [demersal fish species],” said Collie. “We’re seeing a shift from a benthic-dominated community to a pelagic-dominated community. It’s a working hypothesis about how the changes in temperature are amplified in the food web.”
But there isn’t just one explanation. It’s not all temperature, excess nutrients, or predators. It’s a combination of all these factors.
“I think a lot of us are looking for a single explanation when there are multiple explanations,” noted Collie, who is currently working on a Sea Grant-funded project to look at where winter flounder are vulnerable in their life cycle to better understand why this species has left Narragansett Bay. “Preliminary results suggest that the first and second winter of their life are a survival bottleneck. That should help identify the agents, causes, of mortality. It’s likely to be more than one.”
More information about the Baird Symposium, including links to videos of the morning and afternoon sessions and the PowerPoint presentations, is available online. Results will be used to inform the upcoming Rhode Island Sea Grant research request for proposals and the Rhode Island Coastal Resources Management Council’s Narragansett Bay Special Area Management Plan.
The symposium was sponsored by GSO, Rhode Island Sea Grant, the URI Coastal Resources Center, and the van Beuren Charitable Foundation.