Coast to Coast
Using eddy covariance techniques, research into diverse coastal systems reveals ecosystems “on the edge”
At a coastal North Carolina research location, fall storms regularly inundate entire original hardwood forests with 1-3 feet of water that drains by the time trees begin leafing out the following spring. Researchers have evaluated the Alligator River AmeriFlux network site for over three decades to show that these long periods of inundation stress and ultimately kill the trees, interfering with the forest’s ability to sequester carbon dioxide from the atmosphere. Photo by Guofang Miao
In North Carolina, flooding caused by sea-level rise is killing native hardwood forests that help sequester carbon dioxide, while more than 700 miles away in a northern Ohio wetland on Lake Erie an unusual methane-producing soil microbe was identified, calling into question whether emissions of methane, another important greenhouse gas, have long been underestimated there.
Scientists are obtaining data from eddy covariance towers stationed within these very different coastal landscapes to measure carbon dioxide and methane fluxes circulating between soils, plants, and the atmosphere. These research sites are just two of more than 200 AmeriFlux Network sites set up along or near coastline locations across the Americas to evaluate coastal ecosystems, which are both an important contributor to the global carbon sink, and a significant source of natural methane.
Coastal research can help explain how changes in climate and land use impact ecosystems over time, and increase understanding of extreme weather events and changing sea levels, including the environmental changes that affect our drinking water supply. Scientists like Gil Bohrer, the Ohio State University professor leading research at the Old Woman Creek AmeriFlux site on Lake Erie near Huron, Ohio, see eddy covariance—the only technique capable of providing continuous monitoring of fluxes at fairly large scale in remote locations—as critical to their coastal studies.
“The advantage of eddy covariance techniques to coastal research especially is that they allow you to evaluate a much larger area than can be assessed otherwise,” said Bohrer of the towers’ ability to noninvasively measure fluxes across an area extending hundreds of meters beyond the point of measurement. “In an ecosystem as heterogeneous as a coastal wetland, flux towers are able to capture information about what is taking place at the ecosystem level in a way that other types of observations—which focus on just a few designated points—cannot.”
Coastal Systems and Eddy Covariance
This November when the Atlantic Ocean broke its own record for the all-time busiest hurricane season, it came as a reminder of the vulnerability of coastal regions, which in the U.S. are home to half the population and much of the nation’s energy infrastructure.
With an estimated 2,000-plus eddy flux towers operating throughout the world–500 of them affiliated with AmeriFlux—eddy covariance techniques afford scientists a nondestructive method for gaining insights into the intricate processes underway and evolving within and across multiple types of ecosystems from arctic tundra to arid desert over time. For coastal systems which are oftentimes difficult to access, the wide-angle continuous view that flux towers afford over years and decades helps ecologists pursue foundational knowledge that can otherwise be impossible to acquire.
While coastlines themselves play a key role in defining the Americas’ natural identity, their relevance—and function—extends far beyond their sandy shores. The coasts are connected to rivers, headwater streams, floodplains, wetlands, and estuaries by the water, carbon, other solutes, and particles cycling within and across them to form an integrated continuum from land to ocean: the land-aquatic interface.
As such, wetlands in coastal watersheds offer numerous environmental benefits: absorbing floodwaters by acting as a natural sponge, for example, and filtering water moving downstream from its starting point in the mountains, removing contaminants before it can reach the ocean. One 2016 study found that in the contiguous U.S. wetlands store 11.5 petagrams of carbon—roughly equivalent to four years of the nation’s annual carbon emissions. Despite their significance, these ecosystems—and the exchange of energy within them—remain poorly understood.
Coastal systems research has benefited from recent initiatives by the AmeriFlux Management Project (AMP) team, such as its 2019 Year of Methane campaign which produced collaborations to address the many research challenges and uncertainties that exist in understanding ecosystem methane fluxes. Operating from Berkeley Lab with funding from the Department of Energy, AMP supports the AmeriFlux Network with technical support, site visits, and an extensive data infrastructure to ensure the quality and availability of continuous, long-term ecosystem measurements.
AmeriFlux and Berkeley Lab
More than 500 research sites across the Americas from Barrow, Alaska, to Tierra del Fuego, Argentina, make up the AmeriFlux Network. Several-meter-tall micrometeorological towers are stationed at the sites to continuously monitor how fluxes of carbon dioxide (CO2), water vapor, and energy (heat) are exchanged between the biosphere and the atmosphere.
The AmeriFlux Management Project team operating from Berkeley Lab with funding from the Department of Energy supports these member sites with technical support, site visits, and an extensive data infrastructure to ensure the quality and availability of continuous, long-term ecosystem measurements.
The Future of Eddy Covariance and Coastal Study
Although traditionally land and aquatic systems have been the subject of separate research, that’s changing amid concerns over the impact climate extremes are having on factors such as ecosystem water use, a decisive factor affecting streamflow and the subsequent availability of sufficient water supply for forests, estuaries, and people.
Berkeley Lab staff scientist Trevor Keenan, for example, leads a new DOE-funded project to understand how drought, extreme heat, and wildfire are impacting ecosystem water use across diverse coastal regions in the western U.S. where tree mortality rates are particularly high.
“We have developed techniques that leverage machine learning, Earth system models, and information obtained from distributed sensor networks like AmeriFlux that allow us to identify the relationship between water resources and vegetation state and function,” Keenan said. “Our goal is to identify the spatial distribution of ecosystems in which water-mediated ecosystem functions are particularly susceptible to extreme events, and to track seasonal changes in water limitations over time. Data obtained at sites in the AmeriFlux Network are especially valuable to our efforts.”
Particularly in the U.S. where it is infeasible to monitor all 95,000 miles of shoreline, climate models informed by eddy flux data help provide a fuller picture of the influence of climate and environmental changes on coastal systems at national and continental scale. Keenan, a member of the AmeriFlux Management Project leadership team, sees the data obtained by flux towers at 202 independent AmeriFlux sites located in coastal regions as essential to improving understanding of these ecosystems and building effective computer models that simulate past and future ecosystem response to environmental changes.
“The quality and type of data obtained at AmeriFlux sites on the water and carbon cycles in these distinct and important ecosystems, when strategically combined in a model framework, represent a promising step in how we understand and assess coastal ecosystem function,” Keenan said.
“We have developed techniques that leverage machine learning, Earth system models, and information obtained from distributed sensor networks like AmeriFlux that allow us to identify the relationship between water resources and vegetation state and function.”
Sea-Level Rise Changing Coastal Forests
As with any research location, each AmeriFlux Network coastal site is a product of its past. The Pamlico-Albemarle Peninsula in North Carolina, nested between the estuaries of two rivers and home to the Alligator River AmeriFlux site, has escaped three historical waves of land draining for agricultural conversion because the low-lying terrain makes for slow drainage and a ditch network would allow saltwater to intrude deep inland with storm surge. While much of the surrounding landscape was converted to agriculture and later commercial forestry, the peninsula remains one of a few large contiguous protected bottomland forests in the Atlantic Coastal Plain.
Over the years about half of North Carolina’s original wetlands have been drained and converted to commercial forests, helping earn the state its rightful place in the “Timber Basket of the World” which is the Southeastern U.S.
Asko Noormets, lead co-investigator at the Alligator River AmeriFlux site, attributes the high productivity of the pine plantations to historic land management decisions, explaining, “The productive potential of this land is unleashed through site preparation and fertilization practices common in the agricultural and commercial forestry setting. This land has gone through three large waves of draining to allow agricultural use–the latest completed just prior to the passage of the Clean Water Act of 1977 that banned draining of wetlands,” Noormets said. “The benefits of lowered water tables for improved plant productivity have been plentiful, but the soil carbon costs remain largely unquantified.”
Measuring Tree Mortality Over Decades
Overstory trees were already dying at the Alligator River AmeriFlux site in 2009 when a team led by John King of North Carolina State University and Noormets (also formerly at NCSU) began measuring carbon fluxes there, and the mortality has recently picked up. A series of studies culminated in a recent summary of 34 site-years of eddy flux measurements from the unmanaged bottomland hardwood forest and three commercial loblolly pine plantations which found that despite high photosynthesis rates, the bottomland forest sequesters very little carbon compared to the pine plantations on drained soils.
As such, Noormets and King are hesitant to read too much into the differences in modern-day sequestration rates without having an estimate for the historic soil-carbon dynamics. On the other hand, the reduced net productivity of the bottomland forest is ultimately attributed to sea-level rise, which at 5.08 mm/yr (average since the start of the instrument record in 1977) is among the highest in the world due to land subsidence. Specifically, the researchers assert that slow drainage at the unmanaged site exposes the plants to a longer period of inundation, which stresses and ultimately kills the trees.
“We see the Alligator River site as a canary in the coal mine,” said King, an ecosystem scientist who leads forest productivity and soil-carbon cycling studies.
“The patterns that we are observing at Alligator River can serve as an indicator of changes to come in broader areas of the East Coast,” he said. “The evidence is clear: Sea level will continue to rise, perhaps at increasing rates in the not-too-distant future, stressing coastal vegetation.
“The question is what happens to the forest: Does it become so inundated that it converts to marsh? Will that marsh be resilient enough to keep the land from becoming part of the sea? Or will it not?”
The conversion from forest to marsh requires new soil formation to keep pace with sea-level rise, which at these accelerating rates it may not. The loss of land at the periphery of the Pamlico-Albemarle Peninsula has been occurring at several meters per year since 1970, the latest two decades being easily observable on Google Earth.
“The evidence is clear: Sea level will continue to rise, perhaps at increasing rates in the not-too-distant future, stressing coastal vegetation. The question is what happens to the forest: Does it become so inundated that it converts to marsh? Will that marsh be resilient enough to keep the land from becoming part of the sea? Or will it not?”
A Fresh Look at Inland Systems
Gil Bohrer is using the eddy covariance technique to ask similar questions–this time at an AmeriFlux site located within a 573-acre freshwater wetland reserve on the southern point of Lake Erie near Huron, Ohio. Since 2013 when Bohrer built an eddy flux tower at the Old Woman Creek AmeriFlux site, this mineral-soil estuarine marsh has been the subject of multiple studies investigating carbon dioxide and methane fluxes in soils and the atmosphere there.
While it’s undisputed that wetlands do a lot of good for the environment–from filtering contaminants out of water to storing a substantial amount of carbon dioxide in their soils, wetlands emit more methane than any other natural source.
The Ohio State University researchers have accomplished some significant discoveries about this potent greenhouse gas at the Old Woman Creek AmeriFlux site. In a 2017 paper, they described their discovery that methane-producing microbes, typically active only in oxygen-poor soil, produced significant amounts of methane in the oxygen-rich, shallow soils there. For this first quantitative assessment of this phenomenon, called “the methane paradox,” the researchers combined information about site-wide methane flux obtained using eddy covariance with chamber measurements showing methane fluxes on the ground and over open water, and pore-water samplers which measure the concentration of methane in pore water in the soil and allow determination of the depths at which methane is produced. In addition to the flux analyses, the team collected soil samples from across the site in order to map the metabolism of microbes there.
How Methane Concentration Varies
Although they set out to better understand how methane was produced in general at this wetland, the researchers were surprised to find that samples of soils from shallow depths that were rich in oxygen–believed to be toxic to such microbes–contained more methane than deeper samples of soils lacking in oxygen. The researchers sequenced the genome of the microbial community in the samples, and determined that the newly named Candidatus Methanothrix paradoxum thrives in this wetland. During some time periods, this microbe’s activity at the oxygenated soil depths was responsible for up to 80% of the total methane fluxes from the wetland. Because this organism has now been found at more than 100 diverse methane-emitting ecosystems, the researchers believe their discovery could have global significance.
The researchers have also tried to better understand the complex role of wetland vegetation in controlling methane production. A recent study compared how four different types of plant cover assist in the transport of methane from these wetland soils to the air, again using a combination of eddy flux and chamber measurements to provide estimates of the continuous contributions of each land cover to the total methane emissions of the wetland. Their recent paper describes how they found a big difference between cattail and water lily in the relationship between methane flux and carbon dioxide uptake from their leaves, and at different times of the day.
Above: Interactive map of AmeriFlux coastal sites.
A Future in Flux?
Like King who sees changes occurring at Alligator River as indicators of changes to come in other areas of the East Coast, Bohrer hopes to find lessons that can be taken from this Lake Erie estuarine marsh and applied to other coastal systems, even those of different types.
“Algal bloom problems and hyporheic dead zones and nutrient overload are typical of both the Great Lakes and the Atlantic and Pacific Oceans,” Bohrer said. “The Great Lakes are huge freshwater bodies and have a lot in common with the oceans on the East and West Coasts.”
With more than 200 sites in the coastal zone, the AmeriFlux Network is a remarkable resource for understanding this vital part of our landscape.