A Climate-Changed Bay

By Barry A. Costa-Pierce and Alan Desbonnet

There is consensus among scientists who study the Bay that the climate change signal is distinct and strong in Narragansett Bay, with broad implications for altering the ecology and circulation of the ecosystem.

Bay water temperatures have risen about 2°C in winter and 1°C in summer over the past few decades, and it is predicted that Rhode Island air temperatures could increase 5°C in the coming years. If so, it is projected that the Narragansett Bay ecosystem will approximate coastal ecosystems currently found in South Carolina.

Over the last 50 years, the ecosystem of Narragansett Bay has been changed by nutrient loading and over- ﬁshing. Nutrients “fertilize” the water, which promotes algal growth and can lead to decreased dissolved oxygen for other aquatic plants and animals. Increases in average annual sea surface temperatures amplify these effects to exert major new forces of change on the ecosystem.

Nutrient loading to the upper Bay is beyond the threshold considered acceptable for eelgrass. Increased algal growth can block sunlight from reaching the eelgrass and diminish water quality. When warming waters due to climate change are added, eelgrass restoration efforts in the upper Bay are less than successful. Nuisance algae, common in upper Bay areas historically, now predominate in the urban reaches, but hard data are not available to track trends over time. The R.I. Department of Environmental Management ﬁsheries trawl surveys provide a convincing story of macroalgal increase in the upper Bay in recent decades: In the mid-1980s, trawls in waters less than 6 meters deep were halted after the trawl nets were rapidly ﬁlled and choked with algae upon being set. In 2004, all trawl survey stations in the upper Bay were abandoned for this reason. Warming temperatures due to climate change, combined with high nutrient loadings in the upper Bay, are at least partially responsible for this increase in algae.

Shifts in precipitation are also a major element of climate change, and the indication is that precipitation volume is increasing over time in the Bay region. The Narragansett Bay watershed is witnessing as much as a 30 percent increase in precipitation since 1900. Precipitation is critically important to the behavior and ﬂow of nutrients into Narragansett Bay.

Nutrients arrive in Narragansett Bay in pulses, driven by rainstorms that move large quantities of nitrogen from river and sewage treatment facilities with storm-water overﬂows to Bay waters. Impervious surfaces in the watershed, such as paved roads or parking lots, continue to increase as urbanization and landscape development proceed; and with increased precipitation, nutrients from runoff of these surfaces will increase.

Climate change may be a key to predicting the effects of nutrients on shallow, well-mixed ecosystems such as Narragansett Bay. A few degrees’ increase in annual surface water temperature is signiﬁcant in shallow water systems such as Narragansett Bay, exacerbating nutrient impacts to plankton, shellﬁsh, and other bottom-dwelling organisms, macroalgae, eelgrass, and ﬁsh. How climate change will fully impact the Bay ecosystem is nearly impossible to accurately predict, but certainly change will occur at various, if not all, ecosystem levels, and these changes can be unexpected and sometimes startling.

For instance, the 2°C to 3°C warming currently affecting the Bay has drastically reduced the magnitude of its fundamental food production process—the annual winter-spring phytoplankton (notably diatoms) bloom—the ecological bedrock of the Narragansett Bay ecosystem. Copepods, the minute crustaceans that eat diatoms and are an important food source for other animals, are declining. Researchers are now ﬁnding an almost complete lack of copepods during summer months in Narragansett Bay. With this link in the food web removed, impacts on other components of the ecosystem, including species declines, are not yet known.

There is speculation that if nutrient levels decline because of reduced discharges from wastewater treatment facilities, the magnitude of winter-spring blooms may be further suppressed, resulting in an even more reduced supply of nitrogen available during the summer period when there is already evidence of food limitation in the ecosystem of the mid- and lower Bay.

Fulweiler et al. (2007) found shifts in grazing and/or increased cloudiness that have caused a 40 percent decrease in primary production in Narragansett Bay over the past three decades. For the ﬁrst time anywhere in the world, these authors found that the sediments of a rich, shallow-water estuary during summer months switched from their traditional role as nitrogen sinks to being a source of nitrogen. They calculated that the Bay’s sediments now release a huge amount of nitrogen to the Bay’s water column, equal to as much as 60 percent of all of the nitrogen being added by sewage to the Bay! This massive output from the sediments has many potential ramiﬁcations for the ecology of the Bay and of nearby Rhode Island Sound.

The warming of the Bay has also increased the abundance of a ctenophore (comb jelly) that has extended its range as well as its seasonal cycle of abundance. Because of warming, the comb jellies become abundant earlier in the season. Ctenophores are voracious predators that lower the population densities of Bay zooplankton during summer months, allowing a summer phytoplankton bloom to occur in the upper Bay. Late summer blooms might heighten the problem of low oxygen (hypoxia) by providing more organic matter to bottom waters for decomposition—a process that consumes oxygen—during that period of the summer season when the upper Bay is most susceptible to hypoxia.

An additional concern is that the warming of Narragansett Bay may allow southern species, including invasive species, to expand their range northward and inﬁltrate the Bay ecosystem, further impacting its rapidly changing ecology. In recent memory, the lowly mussel ﬁshery. And the lionﬁsh, a voracious, invasive predator from more tropical waters, has been noted in waters near Jamestown.

The questions that arise and the problems posed to coastal ecosystem managers due to climate change are numerous, signiﬁcant, and surprising. There is every indication that the warming trend will continue, and it appears New England will continue to get wetter as well, at least over the near term. Any ecosystem-based management schemes must incorporate and consider climate change, which is impacting our ecosystems even as mitigation measures are being drafted and studies conducted. Management strategies will need to be ﬂexible, able to respond to changing climate and the shifts it brings to the Narragansett Bay ecosystem. To do otherwise will be neither productive nor prudent.