March 12, 2013
Healthy salt marsh creeks at Plum Island Estuary, Massachusetts, are lined
with lush, smooth cordgrass. The plant’s below-ground roots and above-ground
leaves build and maintain salt marshes. Credit: David S. Johnson
Mosquitos by the droves. Polluted coastal waters.
Increased storm surge vulnerability. Loss of habitat for crabs, shellfish and
vast numbers of beautiful bird species including sparrows and rails [1]. These
are just some of the potential consequences of loss of salt marshes around the
country, many of which are now listed as “habitats of concern.”[2]
Salt marshes are among the most ecologically
productive and diverse ecosystems in the United States. They provide important services such as floodwater
storage and storm protection for coastal cities such as New Orleans. Healthy marshes also serve essential roles in
carbon sequestration, a service of primary concern at current emission rates of
the greenhouse gas carbon dioxide, nutrient removal and water purification.
However, global climate change and sea level rise,
agricultural and industrial development and loss of sediment supply are
contributing to dramatic rates of wetland loss worldwide. In the Gulf Coast region, these and other factors – many still largely
under-studied – are driving salt marsh loss at unprecedented rates. While salt
marches are famously valued for their function in nutrient removal, improving
water quality by filtering runoff and removing sediment, nutrients, pesticides,
metals, and other pollutants [3], new research suggests that these marshes are
not impervious to the damaging effects of natural and artificial nutrient
accumulation.
In October 2012, seven researchers from various
universities and laboratories across the country published a study in
prestigious Nature magazine investigating the effects of coastal
eutrophication, or the response of aquatic systems to the addition of natural
and artificial nutrients such as nitrates and phosphates present in fertilizers
and sewage, on salt marsh loss. Linda Deegan, Louisiana State University alumna
and senior scientist in the Ecosystems Center at the Marine Biological
Laboratory at Woods Hole, headed a rigorous nine-year whole-ecosystem
nutrient-enrichment experiment to investigate the effects of coastal
eutrophication, assisted by David Samuel Johnson, R. Scott Warren, Bruce J.
Peterson, Sergio Fagherazzi, Wilfred M. Wollheim and John W. Fleeger, professor
emeritus in the Department of Biological Sciences at Louisiana State
University.
“We wanted to understand the impacts of increased
nutrients including nitrogen and phosphorus – also known as coastal
eutrophication – on all aspects of saltmarshes, from plant production, to
decomposition, to food webs that lead to fish and birds, to the long-term
ability of marshes to keep up with sea-level rise,” Deegan said.
After eight years of nutrient enrichment, the edge of
this salt marsh at Plum Island Estuary has
fragmented and turf has slumped into the channel. Credit: Christopher Neill
In a nine-year whole-ecosystem experiment, Deegan and
colleagues used a microcomputer to add controlled amounts of a solution of
concentrated nitrogen and phosphorous to incoming tidal water in tidal creeks
in Plum Island Estuary [4], allowing the water to flood the marsh the way
enriched coastal waters would in real-world processes. The site of the study, a
large marsh in northeastern Massachusetts, is otherwise generally untouched by nutrient
pollution. The experiment involved adding nutrients to the twice-daily flooding
tides for a total of nine years, from 2004 to 2012, during growing seasons,
enriching about 30,000 square meters of marsh in several experimental creek
systems. In this way, the researchers could definitively study the impacts of
nutrient addition on salt marsh health.
“This experiment is unique in the world – and given some
of the difficulties we encountered we have a better appreciation for why!”
Deegan said. “For example, it can be challenging to keep electrical components
and computers working 24 hours a day, 7 days a week during the growing season
for 9 years in a salt water environment.”
Despite physical challenges, the whole-ecosystem
experiment paid off in a big way, providing results not predicted by previous
marsh models based on small plot experiments.
“Our biggest success is that we have found responses
that simply would not be observed if we had added dried fertilizer to small
sections of the marsh as is typically done. Our experiment allowed the
interaction of many parts of the marsh resulting in an unexpected response –
the creek banks fell apart.”
“In only five to seven years, the edge of the marsh
is literally falling apart,” Fleeger said in an official university press
release [5].
As Deegan explains, the breakdown in the creekbanks
of the nutrient-enriched marsh happened in several stages. In the first few
years of the experiment, the nutrients caused the marsh grass – primarily
Spartina cordgrass (Spartina
spp) [6] – along the creek edges to grow greener and taller, in a process
similar to what happens when you add fertilizer to your garden. These taller Spartina
cordgrasses, however, produce fewer of the roots and rhizomes that normally
help stabilize the edge of the marsh creek. Added nutrients also boosted
microbial decomposition of leaves, stems, and other biomass in the marsh peat,
further destabilizing the creek banks.
“Eventually, the poorly rooted grass grew too tall
and fell over, where the twice-daily tides tugged and pulled it,” Deegan said,
“The weakened, decomposed peat in the creek bank then cracked and chunks of the
creek bank fell into the creek.”
“When we first started this work, it was thought that
salt marshes would be able to sequester excess nutrients and neutralize them
with little impact on the marsh itself, but that hasn’t proven to be the case,”
Fleeger said in the LSU press release [7]. “While they are in effect ‘grabbing’
the nutrients from the water, it is most definitely having an impact on the
stability and function of the ecosystem.”
The results of the experiment have important
consequences for marsh ecosystems worldwide. Salt marshes are a critical
interface between the land and sea. They provide habitat for fish, birds, and
shellfish, protect coastal cities from storms and absorb nutrients out of the
water coming from upland areas, protecting coastal bays from over-pollution.
“If we lose our marshes, we will lose all these
important ecosystem services,” Deegan said.
But where do these deleterious nitrogen and
phosphorous nutrients come from in real-world processes that are now known to
harm salt marches more than previously thought? The modern use of ammonium
fertilizers and combustion of fossil fuels has created an accelerated global
nitrogen, contributing to high nutrient levels in uplands that eventually
affect coastal waters.
“These actions are a combination of agricultural runoff,
waste water from cities and towns and atmospheric deposition,” Deegan said.
“The proportion of the nutrient problem that is from these different sources
can vary from location to location. Here in New England, the most of the nutrients are from sewer and septic
wastewater from cities and towns, while in other regions the biggest problem is
agricultural runoff.”
Deegan suggests that agricultural practices use a
combination of ‘tried and true’ as well as new approaches to control fertilizer
runoff. Strips of land in natural vegetation can be used as buffers
between agricultural land and coastal areas to remove fertilizers and minimize
runoff. We could also do a better job of targeting applied nutrients on an
as-needed basis.
“We can also use new technology to be smarter about
how much, when and how we apply fertilizer,” Deegan said. “A lot of fertilizer
is ‘over applied’ to make sure that the agricultural crop has more nitrogen
that it actually needs. That causes pollution problems and is expensive. Doing
a better job of targeting the applied nutrients is a ‘win/win’. We can do
a better job of timing the application of fertilizer to when the plants need it
by keeping close track of the plant growth. We also now have the ability to
test soils for nitrogen content in real-time, and tractors that can be
programed to apply the specific amount of fertilizer for a particular section
of ground rather than just broadcast general average for the region. Putting
all these old and new techniques in place could contribute to less fertilizer
runoff.”
Linda Deegan sampling water in Plum Island saltmarsh.
Credit: Christopher Neill
Deegan and colleagues have provided substantial
evidence that salt marshes can’t take nutrient overload without harmful
consequences for natural ecosystem services. Unfortunately, the damage
doesn’t end there. Deegan and colleagues suggest in their paper that
simultaneous increases in nutrient loading and sea-level rise as a consequence
of global warming could result in synergistic march loss, with higher wave
energy and flow velocities associated with sea level rise combining with
nutrient effects on creek-bank stability to accelerate land erosion.
“Now we understand that nutrient enrichment also
causes a very important loss of salt marsh habitat for fish and shellfish,”
Deegan said in a Woods Hole press release [8]. “This is one more reason why we
need better treatment of household waste in our towns and cities.”
“We also recognize that marsh loss has many causes,”
Fleeger said. “In some places, herbivores –consumers of algae or marsh grass –
have increased, often because top predators are reduced in numbers, and have
reduced the area of vegetated marsh as a result of overgrazing. Other
places have experienced marsh loss because the supply of sediment is reduced
and marshes subside.”
Despite the many and complex factors contributing to
marsh loss, Deegan provides some hope for marsh ecosystems if appropriate
measures are taken.
“We feel certain that if we control the levels of
nutrients in the water, given continued sediment availability, the marshes will
rebuild using the same natural processes that built them in the first place,”
Deegan said. “It may take a couple of decades, but we think they will recover.”
~~~
For a video interview with Dr. Linda Deegan about salt marsh loss, visit http://www.youtube.com/watch?v=eP3hRkX03Q8&feature=youtu.be.
For the original Woods Hole press release, visit http://www.mbl.edu/blog/why-are-our-salt-marshes-falling-apart/.
For the original Louisiana State University press release, visit http://www.lsu.edu/ur/ocur/lsunews/MediaCenter/News/2012/10/item53764.html.
For the original Nature article, visit http://www.nature.com/nature/journal/v490/n7420/abs/nature11533.html.
Notes:
1 http://saltmarshlife.com/salt-marsh/birds.html
2 http://udel.edu/~gshriver/research/saltmarsh.html
3 http://healthygulf.org/our-work/wetlands/wetland-importance
4 http://www.lternet.edu/sites/pie
5 http://www.lsu.edu/ur/ocur/lsunews/MediaCenter/News/2012/10/item53764.html
6 http://www.psparchives.com/our_work/protect_habitat/ans/spartina.htm
7 http://www.lsu.edu/ur/ocur/lsunews/MediaCenter/News/2012/10/item53764.html
8 http://www.mbl.edu/blog/why-are-our-salt-marshes-falling-apart/
Paige Brown is a 1st year PhD student in Mass
Communication at the Manship School, Louisiana State University. In her
research, she focuses on science and environmental communications and message
effectiveness. She also holds B.S. and M.S. degrees in Biological and
Agricultural Engineering from Louisiana State University. Paige is the author of the popular science blog From
The Lab Bench, hosted on SciLogs.com. Although a scientist by trade, she is
a writer at heart. You can follow her on Twitter @FromTheLabBench
and on Facebook. Follow on Twitter @FromTheLabBench.
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