Most of world's salt marshes likely to be underwater by 2100
Marine Biological Laboratory
Cape Cod's salt marshes are as iconic as they are important. These beautiful, low-lying wetlands are some of the most biologically productive ecosystems on Earth. They play an outsized role in nitrogen cycling, act as carbon sinks, protect coastal development from storm surge, and provide critical habitats and nurseries for many fish, shellfish, and coastal birds.
And, according to new research from the
Marine Biological Laboratory (MBL), more than 90 percent of the world's salt
marshes are likely to be underwater by the end of the century.
The findings come from a 50-year study in
Great Sippewissett Marsh in Falmouth, Massachusetts. Since 1971, scientists
from the MBL Ecosystems Center have mapped vegetative cover in experimental
plots in this marsh to examine whether increased nitrogen in the environment
would impact species of marsh grass. Due to the study's length, they also were
able to detect the effects of climate change on the ecosystem, especially those
driven by accelerating sea level rise.
The researchers found that increased nitrogen favored higher levels of vegetation and accretion of the marsh surface, but that no matter what the concentration of nitrogen they applied to the marsh, these ecosystems won't be able to outpace submergence from global sea level rise.
"Places like Great Sippewissett Marsh
will likely become shallow inlets by the turn of the century," says MBL
Distinguished Scientist Ivan Valiela, lead author of the study. "Even
under conservative sea level estimates…more than 90% of the salt marshes of the
world will likely be submerged and disappear or be diminished by the end of the
century."
"This is not a prediction from
isolated scientists worried about little details. Major changes are going to be
taking place on the surface of the Earth that will change the nature of coastal
environments," says Valiela.
An Ecosystem Engineer
Salt marshes are gently sloping ecosystems
and their plants have very narrow preferences for the elevations in which they
can grow. Different species grow in the upper elevations (high marsh) versus
the low elevation closer to the ocean (low marsh) and have different responses
to changes in nitrogen supply. When change happens slowly enough, the grasses
can migrate to their preferred elevation.
In the low marsh, cordgrass (Spartina alterniflora) prospered as scientists increased the nitrogen supply. Among high marsh species, the abundance of marsh hay (Spartina patens) in the experimental plots decreased with sea level rise.
Saltgrass (Distichlis
spicata) increased with nitrogen supply and also acted as what the
researchers called an "ecosystem engineer" -- increasing the rate at
which marsh elevation rose. Accretion of biomass left behind by the decomposing
saltgrass compensated for the increased submergence resulting from rising sea
level in these areas.
"Saltgrass disappeared after a few
decades, but it left a legacy behind," says MBL Research Scientist Javier
Lloret, adding that it was "extremely cool to see that interaction in the
dataset."
Regardless of how much nitrogen was added
to the environment, the research showed that at the current and future
forecasted sea level rise, low marsh species will completely replace high marsh
species. As sea levels continue to rise, even these species will be submerged.
"At some point, if sea level continues
to increase at the rates that we anticipate, there will even be no more room
for the low marsh plants. They're just going to be too submerged to
survive." says Valiela.
The only alternative would be for salt
marshes to migrate landward.
A Coastal Squeeze
Marshes around the world face what Lloret
calls a "coastal squeeze," where sea level rise pushes from one
direction and human development pushes from the other. A seawall that may
protect a home from flooding will prevent the migration of a marsh naturally
moving to higher ground.
"These barriers, whether they be
geographic like a hill or a cliff, or people building along the edges of the
ecosystem, constrain the potential for landward marsh migration," says MBL
Research Assistant Kelsey Chenoweth. "On top of that, sea level rise is
accelerating and marshes are having a hard time keeping up."
In a sea level rise scenario like the one
we're facing, "the only solution for the plants will be to colonize new
areas, to go uphill," says Lloret. "But that migration may just be
impossible in some places."
"Sea level rise is the most important
threat to salt marshes. We really need to figure out what's going to happen to
these ecosystems and learn how to prevent some of the losses from happening or
try to adapt to them, so marshes can continue to play these important roles for
nature as well as humans," says Lloret.
Half a Century of Science
In 1971, the scientists at the MBL
Ecosystems Center had no idea they would be using their data to study global
sea level rise.
"This was an experiment that started
looking at one ecological control (nitrogen), and then because of the longevity
of the project, we were able to add new knowledge about this major accelerating
agent of global change -- global sea level rise," says Valiela.
That's the benefit of long-term datasets
like the one at Great Sippewissett Marsh.
"You're setting a baseline to the
problems that haven't even happened yet," says Chenoweth.
When measuring ecological processes like
climate change and eutrophication, the data can ebb and flow over the course of
years as the ecosystem responds to external stimuli. The changes operate on a
much longer time scale than changes on other biological systems.
"To study a tree, you look at changes
through seasons and you should be able to see its whole cycle. For a leaf, you
look at patterns between day and night. In single cells, you look at processes
that take place at the timescale of minutes or seconds … but for an entire
ecosystem, we're talking many years or decades," says Lloret. "You
need to be thinking at the scale of decades or even centuries in order to be
able to see substantial changes."
This study includes co-authors from University of Massachusetts-Dartmouth and Woods Hole Oceanographic Institution.
