New Technique Quickly
Predicts Salt Marsh Vulnerability
US
Geological Survey
If
coastal salt marshes are like savings accounts, with sediment
as the principal, all eight Atlantic and Pacific coast salt
marshes studied are "in the red," researchers found.
Scientists
working on a rapid assessment technique for determining which US coastal salt
marshes are most imperiled by erosion were surprised to find that all eight of
the Atlantic and Pacific Coast marshes where they field-tested their method are
losing ground, and half of them will be gone in 350 years’ time if they don’t
recapture some lost terrain.
The US Geological Survey-led
research team developed a simple method that land managers can use to assess a
coastal salt marsh’s potential to survive environmental challenges.
The method,
already in use at two national wildlife refuges, uses any one of several remote
sensing techniques, such as aerial photography, to gauge how much of an
individual marsh is open water and how much of it is covered by marsh plants.
By comparing the ratio of ponds, channels and tidal flats to marsh vegetation,
land managers can determine which marshes stand the best chance of persisting
in the face of changing conditions.
This ratio, called the Unvegetated-Vegetated Marsh Ratio or UVVR, is a good surrogate for much more labor-intensive field studies, said oceanographer Neil Ganju of the USGS Woods Hole Coastal and Marine Science Center. Ganju is the lead author of the study, which was published Jan. 23 in Nature Communications.
“Our
method does a good job of tracking the main destructive processes in marshes
- the conversion of vegetated areas to open water, and the loss of
sediment,” Ganju said. “Together these changes control the long-term fate of
the marsh.”
Salt
marshes worldwide are being lost to sea-level rise, erosion, and land use
changes. These marshes protect the coast against storms and erosion, filter
pollution, and provide habitat for fish and shellfish.
Land managers nationwide
want to know which marshes stand the best chance of enduring, so they can
concentrate marsh conservation and restoration work where it will be most
effective. But making that assessment is normally difficult and costly, the
researchers said.
To
find out whether the UVVR is a good predictor of coastal salt marshes’ fates,
the researchers applied it to eight marshes that had already been studied. The
sites were portions of Seal Beach National Wildlife Refuge and Point Mugu Naval
Air Station in California; Rachel Carson National Wildlife Refuge in Maine;
Fishing Bay Wildlife Management Area and Blackwater National Wildlife Refuge in
Maryland; Reedy Creek and Dinner Creek at New Jersey’s Edwin B. Forsythe
National Wildlife Refuge, and Schooner Creek in New Jersey.
The
researchers wanted to include marshes that seemed fairly stable and ones that
seemed unstable. And to test their method, they needed sites where scientists
had already done the field studies necessary to develop a detailed local
“sediment budget.”
“Think
of a marsh as similar to a savings account, with sediment as the principal,”
Ganju said.
“Every new deposit of sediment is like interest added to the principal, and every sediment loss is like money spent. If a marsh is gaining sediment, it can tolerate some withdrawals, and the budget will still be in the black. If it’s losing sediment, that marsh is in the red. Its sediment either gets replenished by natural processes or human intervention, or eventually the principal will all be spent.”
“Every new deposit of sediment is like interest added to the principal, and every sediment loss is like money spent. If a marsh is gaining sediment, it can tolerate some withdrawals, and the budget will still be in the black. If it’s losing sediment, that marsh is in the red. Its sediment either gets replenished by natural processes or human intervention, or eventually the principal will all be spent.”
All
eight marshes’ sediment budgets were “in the red.” And in each case that UVVR
result correlated with the sediment budget, confirming that each of those
marshes is losing ground. The researchers calculated the likely life spans of
all eight marshes. The shortest, at Maryland’s Blackwater National Wildlife
Refuge, was approximately 83 years, and the New Jersey marshes had life spans
ranging from 170 to 350 years.
The
researchers stressed that these figures are estimates with large margins of
error, and don’t necessarily mean that these marshes are doomed – only that
they will need infusions of sediment to last longer.
“Our
results can be used to distinguish marshes that are struggling to survive
from more resilient ones,” said co-author Matthew Kirwan, an assistant
professor at the Virginia Institute of Marine Science. “That’s important
because it will help prioritize restoration work.”
Staffers
at Massachusetts’ Parker River National Wildlife Refuge are working with
members of the research team to identify which marsh units are losing sediment,
and at what rates.
"This
will help us figure out where we can make a difference with restoration
techniques. It will also help us determine which areas are beyond
restoration," said Refuge Manager Bill Peterson. "This ensures that
we're using our limited resources effectively to strengthen and enhance these
valuable natural areas."
The
research paper, “Spatially integrative metrics reveal hidden vulnerability of
microtidal salt marshes,” is online at http://dx.doi.org/10.1038/NCOMMS14156. An
example of the UVVR mapped over a coastal wetland is at https://www.sciencebase.gov/catalog/item/57fe81fbe4b0824b2d148389
