"The centerpiece of our study," says Denise Breitburg, "is a series of mesocosm and large-enclosure field experiments." Mesocosms have been designed to run controlled studies of single and multiple stressors at different levels of ecological diversity, from very simple to more complex environments. The larger field enclosures will actually be located in the Patuxent River, so that researchers can examine the impacts of multiple stressors on environments of greater complexity than the mesocosms.
"These experiments" says Breitburg, "will not only help us compare the effects of single and multiple stressors, they will enable us to test the importance of complexity itself."
For Breitburg, there are at least several issues that involve ecosystem complexity. The first involves the effects of multiple stressors such as high levels of toxic chemicals and nutrients, and low dissolved oxygen: their interactions could prove more devastating than one might predict when looking at those same stresses individually.
A second issue involves the effect different levels of ecological complexity may have in modifying multiple stresses, for example, does a more complex, or diverse, ecosystem differ in its capacity to handle multiple stresses?
A third issue has to do with how much useful information the different mesocosm environments can give, compared, for example, with the large field enclosures.
The starting place for these questions are 20 one-cubic meter mesocosms - they have been designed to run replicate experiments that span five levels of complexity, from environments containing single-celled phytoplankton to environments with fish and bottom dwelling organisms.
First level: Phytoplankton
Second level: Phytoplankton + copepods + fish
Third level: Phytoplankton + copepods + fish
Fourth level: Phytoplankton + copepods + fish + sediments
Fifth level: Phytoplankton + copepods + fish + sediments + benthos
To sort through the effects of cumulative stress, researchers need to distinguish how each of these environments responds to single stresses, for example, to different nutrient concentrations, then to different trace metal concentrations. The metal concentrations are based on the findings of Benedict scientist Gerhardt Riedel who has taken intensive measurements seasonally throughout the Patuxent River.
Researchers have already begun blending mixtures of nutrients and trace metals in the mesocosm experiments.
Each year, experiments will be run several times over five-week periods - the reason, says Breitburg, is to account for variations that occur naturally and seasonally.
What is the relation of mesocosms to the Patuxent environment? How well will the findings - and the computer models that are developed to make ecological predictions from those findings - reflect what managers can expect in the Patuxent?
One way the Benedict researchers propose to answer such questions is with experiments in larger tanks, tanks more diverse in ecological complexity and therefore more representative of the Patuxent River.
To do this, says Breitburg, they conducted trial runs this summer with two 10-cubic-meter tanks that they placed near shore in the Patuxent. These tanks, which filter water from the river, make it possible to increase species diversity: sandy areas, oyster reefs, a wider variety of fish, benthic invertebrates, clams and oysters.
By next summer, ten more tanks will be operational. Together with the mesocosm studies, says Jim Sanders, the experimental project is "immense." It involves a "careful dance of many institutions and senior investigators with different expertise who are trying to get information on time scales that are interpretable."
As a scientist with expertise in the effects of trace contaminants on phytoplankton, he has had to "step back," he says. "I had to open up my biases and think of what a fish ecologist wanted out of this study, what a modeler wanted out of it. My personal interest became more subservient to the whole."
The experiments, as he says, are only the first stage - other researchers waiting for the numbers so that they can then link the results in St. Leonard's Creek with the entire Patuxent River. Their findings will shed new light not only on the workings of Chesapeake creeks and rivers but on coastal systems throughout the world.