The nitrogen cycle seems simple. As a gas, it’s the most abundant element in the atmosphere, but it can quickly be turned into organic nitrogen, ammonium or nitrate through fixation done naturally in ecosystems by microbes. It can then be used to fuel plant growth.
But there’s a catch.
“It’s a lie. A total lie,” Robinson “Wally” Fulweiler says of the readily accepted nitrogen cycle theory.
“I think it’s much less of a cycle and much more of a dynamic interacting system where things don’t always flow in a nice circular motion but instead move in complex directions to and from different areas,” she says.
Fulweiler, an ecosystem ecologist and biogeochemist at Boston University and Bethany Jenkins, a marine microbial ecologist at the University of Rhode Island’s Graduate School of Oceanography and College of Environment and Life Sciences, presented their findings and challenged what’s known about the nitrogen cycle as part of Rhode Island Sea Grant’s Coastal State Discussion Series on Thursday, February 12 at the University of Rhode Island.
Looking at the total ocean nitrogen budget, Fulweiler notes an interesting trend — there are more “sinks,” or processes for removing nitrogen, than there is nitrogen to be removed from the system.
“It’s like your credit card balance where you’re spending way more money than you have, and for the ocean nitrogen budget that doesn’t quite make sense,” she says. “But maybe we can close these gaps in the nitrogen budget by better understanding some of the environmental controls of denitrification, a removal of nitrogen, and nitrogen fixation, which is a way to add nitrogen into the system.”
Studying long-term data sets available for the Narragansett Bay and utilizing research funding from Rhode Island Sea Grant, Fulweiler found that the cycle does not adequately incorporate the environmental and biological variability that can be seen in how nitrogen flows over seasons.
The Dimming Effect
Changes in the wind could be responsible. With less wind, there’s less mixing of the water column, which limits nitrogen dispersal and therefore its availability. There’s also been a lot of variability in temperature, though an overall increasing trend is seen. This means average temperatures, as well as the range of temperatures, are in flux.
“One of the most profound changes has been the change in the water column chlorophyll.”
Fulweiler tracked more summer days above 23°C (73.4 F) and fewer winter days below 1°C (33.8°F). Warmer summers and less-cold winters generate more cloud coverage and fewer clear days. This means less light reaching Narragansett Bay. Using records dating back to the 1970’s, she and her colleagues looked at light in the bay and its changes over the years. “There’s been this strong dimming in New England over this time period,” she says.
Because phytoplankton are susceptible to light, and light has been decreasing over this period, there has been less phytoplankton growth over time.
“One of the most profound changes has been the change in the water column chlorophyll,” Fulweiler says.
Since chlorophyll is known to be a good proxy for phytoplankton, Fulweiler says she can use this information to predict ecosystem shifts, because it appears that when more chlorophyll are present, rates of denitrification are higher.
Currently, research shows that annual productivity in Narragansett Bay is declining.
“This is really interesting. It’s not what you would expect for an estuary that is surrounded by people, and has Providence River dumping nitrogen into it at a steady rate,” Fulweiler says, explaining that various tests showed that the nitrogen cycle wasn’t being driven by nitrates or ammonium, but was really being driven by organic matter, such as plant and animal plankton, in the water column.
To understand how a change in organic matter changes the nitrogen cycle, Fulweiler and her team recreated the bottom of Narragansett Bay using MESOCOSMs, which are very large-scale lab experiments that mimic an environment to test a suite of variables. Rotators mixed bay water with certain amounts of organic matter in order to measure the resulting nitrogen fluxes from bacteria in the sediment.
“Depending on what kind of organic matter you give [bacteria in these sediments] they change what they’re doing. Those that were getting lots of organic matter were denitrifying or removing nitrogen. Those that weren’t were nitrogen fixing or adding nitrogen to the system.”
“I think that’s really important to start to understand how all of these systems are interacting. Without understanding what the organic matter is doing in Narragansett Bay, we couldn’t possibly understand what’s happening with the nitrogen cycle,” she says.
Given that microbes use and transform nitrogen, how do they impact the nitrogen cycle?
Jenkins spoke about how marine microbes, such as becteria, drive and impact the nitrogen cycle. Marine microbes respond to the presence of organic matter and have been found to play a pivotal role in nitrogen fixation — a reversal of previous thought about the nitrogen cycle.
Jenkins says that by better understanding these organisms, specifically “nitrogen fixers,” scientists can begin to better understand the ecology of Narragansett Bay and similar urban-impacted estuaries.
“The big question my lab tries to answer is the interplay between genetic capacity and the functioning of micro-organisms in the environment,” says Jenkins, who has worked with Fulweiler to better understand microbial and nitrogen dynamics, and has utilized a method to identify certain genetic traits that are responsible for nitrogen fixation and denitrification processes.
Taking samples from the Providence River, the upper bay, and mid bay sites, Jenkins and her team discovered two major microbes that are responsible for adding nitrogen to the system — both of which are anaerobic (breathe without oxygen) and are more closely related to sulfur-using microbes found at hydrothermal vents and other oxygen-deprived areas.
These organisms have been found to be highly active during periods of seasonal hypoxia, like the summer of 2006.
“What we saw in 2013 is this significant relationship between low D.O. (dissolved oxygen) and elevated rates of nitrogen fixation,” Jenkins says. “We think D.O. is really playing a strong role in stimulating the activity of these organisms.”
Taken then to the lab, Jenkins and her team studied the genes essential for nitrogen fixation. “Not only is that gene present in the environment, but the organisms are actually turning that gene on and off. So there’s active engagement of that genetic machinery in Narragansett Bay sediments,” she says, noting results found using the same MESOCOSM experiment as Fulweiler.
Jenkins also found a greater range of diversity, as well as abundance and activity among nitrogen-fixing organisms near the Providence River estuary compared to Rhode Island Sound. This difference may be linked to lower oxygen levels found in the upper bay.
“We know there’s something driving diversity in a down-bay way, and we’re continuing to investigate,” Jenkins says, noting the importance of understanding different pathways for nitrogen fixation.
“These sequences from Narragansett Bay are not unique. They’re present in databases from other estuaries,” Jenkins says. “So given the right conditions in other estuarine systems such as the Chesapeake Bay, we’re likely to see the same types of activity.”
These organisms and associated processes may be broadly applied to other estuary systems like Narragansett Bay. And perhaps these organisms could be utilized to move nitrogen in different directions to help balance the nitrogen budget, though this concept needs more work says Jenkins.
“I think if we begin to adapt our brains to thinking about it like this we will actually make further advances in our understanding of what’s happening to it [nitrogen cycle] and how it changes when the environmental conditions change,” Fulweiler says.
Both Jenkins’ and Fulweiler’s work could have implications for resource managers who are already employing nitrogen reduction efforts via wastewater treatment facilities as a way to improve water quality. This fall, the federal Environmental Protection Agency awarded Rhode Island with $240,000 to help wastewater treatment systems reduce nitrogen discharges into Narragansett Bay.
“I think more information is needed, especially now as tertiary treatment will be decreasing nitrogen loading to the bay, and we know that the bay is warming. The only way to figure this out is through long-term data collection,” Fulweiler says. “In my opinion, such data are the key to the best management of Narragansett Bay.”
By Kelsey Quinn | Rhode Island Sea Grant Communications Intern and URI Journalism student[divider style=”solid” color=”#eeeeee” width=”1px”]
The Coastal State Discussion Series is a forum dedicated to highlighting current scientific research, finding solutions, and building partnerships centered around coastal issues impacting Rhode Island’s coastal communities and environment.
This series is sponsored by Rhode Island Sea Grant with the support of the University of Rhode Island’s Coastal Institute, College of Environment and Life Sciences, and the Graduate School of Oceanography.