Biology’s Thomas Mozdzer asks, Are our front lawns killing the wetlands?

Posted October 1st, 2014 at 12:36 pm.

Fertilizers keep farmland productive and suburban lawns lush, but what happens when the fertilizers we rely on find their way into the watershed?

Scientists working out of the Marine Biological Laboratory (MBL) at Woods Hole are looking to answer that question—with the help of Bryn Mawr’s Thomas Mozdzer and his research group including undergraduate students and Bucher-Jackson postdoctoral scientist Dr. Joshua Caplan.

An assistant professor of biology, Mozdzer is a plant ecologist whose work focuses on wetland habitats. With this study, Mozdzer is looking at the effects of chronic nutrient pollution on salt marsh plant communities. Called the TIDE Project, the study is looking at how a salt marsh ecosystem responds to chronic nutrient pollution—those fertilizers from upland agriculture and suburban lawns, discharge from sewer treatment plants, stormwater runoff, car and power plant emissions, and failing septic tanks, among others.

The TIDE Project is being conducted at an unusually large scale and over an unusually long time span. What are the advantages of that experimental design?

Many ecological studies work at a relatively small scale. For example, my experimental plots are 3 square meters in my other project—an investigation of the effects of global change on common reed, Phragmites australis, invasion at the Smithsonian Global Change Research Wetland—. But in the TIDE study, we’re working at the ecosystem scale, fertilizing 60,000 square meters with about four tons of fertilizer every week.  This is the only experiment in the world that manipulates nutrient pollution in salt marshes at this scale—and does it in a manner that mimics the way nutrients actually enter this ecosystem.  Because of this size, we can ask large-scale questions.  For example, how does nutrient pollution affect nutrient cycling, food webs, and ecosystem services?

Plus, the project is long-term—it’s been underway for 11 years. Long-term data is essential to understand how communities and ecosystems respond to large scale perturbations. For example, if the experiment had stopped after the first three years, which is a typical time frame for ecological studies, we would have had completely different results—and would not have been able to predict or observe the collapse of the salt marsh creek banks with nutrient pollution. This has actually been the most unanticipated result, as these ecosystems were supposed to benefit from increased nutrient pollution.

What has your involvement in this project been?

I began working with colleagues on TIDE Project about two years ago.  In my role as the lead plant ecologist, I collaborate with biogeochemists, community ecologists, microbial ecologists, food web scientists, and coastal geomorphologists from four institutions to understand how these ecosystems will respond to global change factors, such as nutrient pollution.

Our preliminary data show that, after 10 years of nutrient pollution, the plants in the fertilized creeks are genetically different and are less genetically diverse than those in our non-manipulated reference salt marshes. Understanding how genetic filtering of the foundation species, Spartina alterniflora, feeds back on ecosystem processes is my current focus.

Other studies have found that genetic diversity—even within a species—correlates with ecosystem resilience. For example, the greater the genetic diversity, the more robust the system, and the less vulnerable to disturbance the ecosystem is. One current hypothesis is that the loss of genetic diversity with nutrient pollution may also contribute to the collapse of the salt marsh due to a selection for particular plant traits.

So how did your findings contradict the original hypothesis?

We had expected the plants would grow more with added nutrients, allowing the marshes to keep pace with sea-level rise by building soil. Instead, we found that plants actually allocated less of their energy and resources to roots. Without those roots to hold the soils together, the banks of the salt marshes are literally falling apart. As a consequence of these changes in ecosystem structure, there are changes in  the ability of the ecosystem to provide ecosystem services such as storm buffers and habitat for commercial and recreational fisheries.

What is next for this project?

I’m very interested in understanding the effects of nutrient pollution and other global change factors on ecosystem carbon cycling. Because of their ability to store disproportionally more carbon than any other terrestrial ecosystem, salt marshes are critical ecosystems for carbon sequestration globally.  However, in addition to nutrient pollution, salt marshes are also subject to other stressors—sea-level rise, warming, and droughts. The combination of these factors may disrupt the processes that allow salt marshes to sequester carbon, which may feed back to global climate.

Finally, we want to see what happens next. As I said earlier, the long time span of this project resulted in completely unanticipated findings. The collapse of the salt marsh creek banks didn’t happen until year seven. Once the current phase of the project is over, we’re planning to submit another proposal to look at recovery. Will the marshes be able to recover if the nutrients are no longer supplied? What effects will continue? What are the implications for policy? There are many directions to pursue!

The TIDE in TIDE Project stands for Trophic cascades and Integrating control processes in a Detritus-based aquatic Ecosystem. To read more about it, visit www.mbl.edu/tide/.

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