A salt marsh has a story to tell.
How it was born, what makes it thrive, what it can or cannot tolerate and still survive. And it takes a dedicated, attuned ecologist to listen to a marsh over many years and hear its tale.
Half a century ago, Ivan Valiela pioneered ecological research at Great Sippewissett Marsh in Falmouth, Mass., that (astonishingly) is still going strong today.
Along the way, Valiela and colleagues have coaxed from this deeply studied marsh many of the fundamental truths about salt marsh function and wise coastal management that we take for granted today (Valiela, Estuaries and Coasts, 2015).
Photo: MBL Scientist Ivan Valiela in Great Sippewissett Marsh, 2014. Credit: Daniel Cojanu
Most recently, Valiela’s research has become a testbed for a critical question: "Will coastal wetlands adapt to the rising seas that accompany global warming, or will they drown?" The ultimate fate of wetlands will vary globally. But Great Sippewissett Marsh will be underwater within this century, Valiela predicts.
"Will coastal wetlands adapt to the rising seas that accompany global warming, or will they drown?”
This lush habitat, along with other salt marshes that protect Cape Cod from storm surge and flooding, will become shallow bays by 2099, unless global warming takes a markedly different trajectory than the one it’s on now.
Groundbreaking Research on Coastal Impacts of Nitrogen
Valiela first arrived at the Marine Biological Laboratory (MBL) in 1969, a young Ph.D. accepting a joint appointment in the MBL Systematics Ecology Program and in the Boston University Marine Program at the MBL. It was a “period of gestational thinking in ecology,” Valiela has written, with many heated debates about what controls growth and activity in ecological communities, such as forests or lakes.
These controversies had prompted scientists to pioneer a novel approach to ecology: Experimental manipulation of lakes, ponds, and other ecosystems to tease out the factors that control their function and processes.
While Valiela knew very little about marine science when he arrived at the MBL, he hoped to reveal the controls on a coastal ecosystem through manipulative experiments, which had never been done before.
Through interactions with eminent colleagues at Woods Hole Oceanographic Institution including John Teal, Alfred Redfield, John Ryther, Buck Ketchum and others, Valiela and Teal developed a plan to use Great Sippewissett Marsh for long-term studies.
After gaining permission to run experiments in Great Sippewissett Marsh (“not a simple thing,” Valiela says), he and Teal started by asking, “How do nitrogen and phosphorus (often called nutrients) control activity in the salt marsh, such as production of fish and grasses?”
They established nearly identical plots of marshland and gave each one a low, medium, or high dose of hand-scattered fertilizer, or no nutrients at all. Their first major finding was that nitrogen, not phosphorus, controls essentially all activity in the marsh (Valiela and Teal, 1974)
Photo: Aerial of Great Sippewissett Marsh in 2015. Some of Ivan Valiela’s experimental plots are located in lower right of the photo. Credit: Rhys Probyn
They had to wrestle to balance the budget, though. After summing all the processes in the marsh that added, removed or transformed nitrogen, they had far more nitrogen coming into the marsh than was going out to sea. The missing input, they discovered, was nitrogen entering via groundwater from land.
“Then we asked, ‘Whoa, how did all that nitrogen get in the groundwater?” Valiela says. “We determined most of it came from wastewater, largely discharged from septic systems.”
Today, this foundational discovery is routinely applied in coastal management strategies. We now know that nitrogen loading to coastal waters triggers a variety of effects, referred to as eutrophication, that can negatively affect water quality, food webs, and shell and finfish stocks.
The need to remove nitrogen from wastewater was buttressed by Valiela’s larger landscape-level studies, beginning in the mid-1980s. Through research carried out at Waquoit Bay, Falmouth, and other sites, his teams showed that the more land is covered by urban development, the greater the nitrogen load to the coast and the lower the water quality.
When they expanded their inquiry to landscape-scale studies at tropical mangroves—which function similarly to salt marshes—in Panama, Brazil, and Trinidad, they found similar results.
Valiela and collaborators also began studying how urban development affects coastal marine species, research that continues today (Pierce et al., Can. J. Fish Aquat. Sci, 2020). Microplastics, too, accumulate in salt marshes, they discovered, creating a record of human plastic use (Lloret et al, Environ. Adv., 2021).
The Coastal Squeeze
The experimental plots at Great Sippewissett Marsh continue to surprise.
After four decades of enriching the plots with nitrogen, the plots still intercepted up to 90 percent of the nitrogen added (Brin et al., Marine Ecology Progress, 2010). Why?
As the experiment continued, the dominant native grass in the marsh (Spartina alterniflora) increased in mass. But after many years, other nitrogen-loving species began to proliferate, particularly the opportunistic Distichlis spicata. Because Distichlis doesn’t decay nearly as fast as Spartina, when it dies it forms a thick thatch on the marsh that collects particles of sediment. This builds up the sediment platform, allowing the marsh to “keep up” with rising sea level.
Why, then, does Valiela predict that Great Sippewissett Marsh will be underwater within this century? Because, as sea level rises, a marsh adapts through migration of its salt-tolerant vegetation toward land.
In a predicament called “The Coastal Squeeze,” Great Sippewissett Marsh’s landward retreat will be stopped by a railroad trestle, now converted to a bike path, and seawalls. The marsh will only remain above water if these barriers to its landward incursion are removed.
Valiela’s vast contributions to our understanding of coastal wetlands continues. Today, he is working on a synthesis of his 49 years of research at Great Sippewissett Marsh—priceless knowledge of landscape change that can only emerge from a dedicated lifetime of study.
Photos Courtesy: Kelsey Chenoweth, Daniel Cojanu, Simon Minor, and Rhys Probyn