Alumnae Bulletin May 2009

More connected than we thought

By Alicia Bessette

You might think a creature's everyday comings and goings have little effect on the actual chemical composition of its environment. But in a novel study of a seagrass food web, Amanda Spivak '01 has proven otherwise.

Spivak got her Ph.D. in the College of William and Mary's School of Marine Science at the Virginia Institute of Marine Science, where she was deemed one of the most accomplished interdisciplinary students ever enrolled in the program. Her dissertation was lauded for its broader relevance to issues of habitat restoration and biodiversity, as it shed light on how food webs influence the quality and cycling of organic carbon. A key finding was that food web diversity influences the microbial communities in sediment.

At VIMS she set out to define the relationship between biogeochemical cycles and animal community diversity in seagrass beds of the Chesapeake Bay. "We know a lot about the organisms that live in the Chesapeake Bay and we know broadly about the dynamics of nutrients like carbon and nitrogen," Spivak says. "But we didn't know how plants and animals in the Chesapeake Bay influenced elemental cycles.

"The problem is that seagrass beds in the Chesapeake Bay are impacted by commercial and recreational fishing and crabbing. As a result we're rapidly losing seagrass habitats and the important ecosystem services they provide. If we want to restore seagrass habitat functions we first need to understand how the ecology and biogeochemistry of the system interact. Understanding how food web diversity influences organic matter cycling is a first step."

Spivak's study, which took six years to complete, blended experimental ecological approaches with the application of state-of-the-art geochemical tools called fatty acid biomarkers. Essentially, fatty acid biomarkers are chemical fingerprints that identify the organic matter source.

"You take a sample and extract the fatty acid compounds to identify whether the organic matter came from vascular plants, algae, animals, bacteria or some combination thereof," she says. "It's like a big jigsaw puzzle, and you have to tease apart which plants and animals are important contributors to the sediment organic matter pool.

"You wouldn't think that crabs or fish could influence the sediment microbial community, but they do. Crabs, fish, and other top predators create trophic cascades whereby they affect organisms lower in the food chain. We're trained to think that trophic cascades stop at the lowest trophic level: plants and algae. However, my work shows that cascading trophic effects can extend to sediment microbial communities, and this has implications for nutrient regeneration and carbon storage. So, changing the food web actually changes the sediment chemistry."

Now Spivak is a postdoctoral fellow at Miami University in the zoology department, and is hard at work on two freshwater projects. One project analyzes what the appropriate size scale should be for simple freshwater experiments. Because many people use experiments to understand how ecosystems function, it is important that experimental systems closely mimic the natural environment; experimental container size is a factor that may strongly influence the outcome of a study.

In another project she is making a mathematical model of carbon dynamics in freshwater reservoirs with different food webs. "I wanted to expand my skill set," she says of her new work with freshwater environments. "In particular, I am curious to learn whether the food web can affect the carbon budget of an entire lake or reservoir. It's important to understand how organisms and trophic interactions can affect the internal carbon dynamics of freshwater lakes and reservoirs, since those bodies may be integral components of the global carbon budget."

Spivak's love of aquatic environments was sparked at Bryn Mawr, where she majored in biology with a concentration in environmental science, and minored in English. Two classes she took with Senior Lecturer Stephen L. Gardiner—one on evolution and another on biological oceanography—were highly influential. "The biological oceanography class was really what did it for me," she says, mainly because Gardiner introduced students to a diversity of marine life and allowed them to analyze zooplankton samples from oceans all over the world. "I got to see diversity that you can't find in Pennsylvania," she says.

During her time as a Bryn Mawr student and in the year following graduation, Spivak completed four internships—at the University of Maine's Ira C. Darling Center; the University of Miami's Rosenstiel School of Marine and Atmospheric Science in Florida; the Dauphin Island Sea Lab in Alabama; and the Smithsonian Environmental Research Center in Maryland.

During her junior year, she spent a semester at the Marine Biological Laboratory in Woods Hole, Massachusetts, studying environmental sciences and sampling aquatic and terrestrial ecosystems. "That was instrumental in pointing me in this direction," she says. "Some of my internship groups were definitely male dominated. By the time I got to grad school, my first-year class had just as many women as men. And now, six years later, it seems that the incoming graduate classes in marine science are mostly women. I hope more young women from Bryn Mawr go into the marine and environmental sciences. There are a lot of opportunities when you're in college, but you have to put some effort into finding out your options."

In the not-so-distant future, Spivak might put her English minor to good use; she's interested in improving com­munication among policymakers, scientists, and people using the land.

"Research is really important," she says, "and yet I don't think researchers always communicate very well. I think after my post-doc I may transition to working with environmental NGOs, to incorporate science with policy, and to help get the information we have out into the world in a more applied sense."

But for now Spivak is enjoying where she is. "On gorgeous sunny days I get to be outside on the water. That's the best part of my job, and enjoying my coffee outside in the fresh air is a great perk."