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 . 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.”
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 , 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.
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 , 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 .
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)  – 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 . “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.”
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 . “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 https://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 https://www.nature.com/nature/journal/v490/n7420/abs/nature11533.html.