Changing Climate, Changing Bay
Published on May 30, 2008
On Thanksgiving Day 1871, the people of Providence were celebrating. Twelve days earlier, the connection of the city‚??s pubic water supply had brought residents indoor running water.
The elaborate celebration included church bells ringing for 10 minutes, a 13-gun salute, and the ‚??sending up (of) great jets of water from service-pipes conducted to the bridges which cross the river,‚?Ě according to King‚??s Pocket-book of Rhode Island.
The Pawtuxet River supplied water to Providence homes, while the waste emptied into Narragansett Bay. Little did the people of Providence know that 135 years later a scientist would be able to pinpoint that day as the beginning of a long, and, if she‚??s right, troubling new facet of environmental change.
The story of that change begins with that most basic of scientific impulses: the study of how introducing a new factor changes a system.
The wastewater pumped into Narragansett Bay is subject to treatment designed to remove harmful chemicals and elements, including nitrogen, which at high levels can damage aquatic life and cause public health issues. In 2006, a more robust treatment process was about to be implemented.
Robinson ‚??Wally‚?Ě Fulweiler, then a Ph.D. candidate at the University of Rhode Island, knew that there had been study of nitrogen in the Bay in the 1970s, ‚??80s, and ‚??90s. Fulweiler and her colleagues collected sediment samples to learn how the Bay had changed since those measurements and to gather baseline data before the new treatment process was implemented.
They studied the sediment samples and found, according to Fulweiler, ‚??something no one would have predicted.‚?Ě
The Nitrogen Cycle
The decision to route wastewater into the Narragansett Bay was grounded in solid science, since the Bay naturally removes some nitrogen and can return it safely to the atmosphere.
The sediments in estuaries and bays perform a process called denitrifcation, in which bacteria in the sediment convert nitrogen into dinitrogen gas. Denitrification generally removes 30%-50% of the nitrogen coming into estuaries, making it a powerful tool in reducing nitrogen loading.
But when Fulweiler and her colleagues studied the sediment cores, they found that instead of removing the nitrogen, the bacteria in the Bay were taking up nitrogen, a process known as nitrogen fixing.
‚??To find that sediments were doing this kind of nitrogen fixation is really unusual,‚?Ě she says. ‚??It‚??s taking the nitrogen cycle as we know it and flipping it on its head.‚?Ě
The Role of Climate Change
Fulweiler points to climate change as a potential cause.
The Bay was characterized by an annual winter-spring bloom of phytoplankton, which ended with dead phytoplankton acting as a food source for organisms dwelling on bottom sediments, including bacteria. The bloom steadily declined through the 1990s and by 2006 ‚?? the summer that Fulweiler and her colleagues made their discovery ‚?? phytoplankton production was the lowest on record.
Fulweiler and her colleagues conducted experiments and found that when the bacteria got a sufficient supply of phytoplankton, they performed denitrification. But without enough phytoplankton, nitrogen fixation occurred.
Even though they understand what is happening, Fulweiler and her colleagues are less sure as to why. They are considering two hypotheses, both tied to global warming.
The first holds that since water temperature is increasing on the East Coast, zooplankton may be active longer and eating more phytoplankton during the traditional winter-spring bloom, leaving less phytoplankton to settle to the bottom.
The second is based on work done by David Borkman and Ted Smayda at URI, who found that warmer winters have more cloudy days. An increase in cloudy days means less nourishing sunlight for the phytoplankton.
It‚??s possible that both processes are occurring simultaneously.
She also allows for a third possibility: that the increase in nitrogen fixation in Narragansett Bay is a natural cycle that hasn‚??t been observed before.
Further Study Planned
Fulweiler cautions that her conclusions represent ‚??what we think is happening. We don‚??t know this for sure.‚?Ě
That uncertainty is exacerbated by the appearance of a large traditional winter-spring phytoplankton bloom in 2007.
Fulweiler, now a post-doc at Louisiana State University, and colleagues at the University of Rhode Island and the Woods Hole Oceanographic Institute plan to study the process in the Massachusetts Bay, Rhode Island Sound, and the continental shelf, as well as Narragansett Bay. If further research supports their conclusions ‚?? that climate change is influencing one of the ocean‚??s most important cycles of an element essential for life ‚?? the implications are potentially global.
‚??Nutrient cycling is an important and fundamental process. When we start messing with that, there‚??s unintended consequences that we haven‚??t even imagined,‚?Ě says Fulweiler.
Though nitrogen fixation has only been measured during one summer and needs to be reconfirmed, she says the discovery should influence how we consider the implications of climate change.
‚??There are a lot of things that we have yet to even think about how climate change may affect us,‚?Ě says Fulweiler.
Interview and Story by Sam Costello