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Banking on Blue Crabs

From Ecology to Economics

Planting Oysters in the Chesapeake

High School Aquaculture

Mathias Medal Awarded to Outstanding Scientist

By Merrill Leffler

How do high nutrient loads interact with trace metals and varying concentrations of dissolved oxygen to affect the Chesapeake Bay and other coastal systems? Denise Breitburg (left, front) explains to Sea Grant's Gail Mackiernan (left, back) how experiments in an array of twenty controlled ecosystems (called mesocosms) will provide the first findings to answer this question.

Along the banks of St. Leonard's Creek on the Patuxent River, scientists at the Benedict Estuarine Research Center have begun a long-term study that should greatly improve our ability to manage complex coastal systems like the Chesapeake Bay.

Today's resource managers rely heavily on the use of computer models, such as the Chesapeake Bay Program's Water Quality model, to test various control strategies, for example, the effects of reducing nutrient inputs on dissolved oxygen in bottom water. Over the last decade, continuing research and monitoring in coastal systems - as well as improved computer power - have greatly increased the sophistication of these models. They now mimic, to a degree, basic food web and other ecological relationships, and can be used to predict ecological responses to various management actions.

Nevertheless, existing models are still limited in their predictive accuracy, says Benedict researcher Denise Breitburg, because modelers are unable to include key ecological relationships. That is because the basic research to clarify these often complex relationships has not yet been done - it requires a large coordinated effort of scientists with diverse expertise, it is time-consuming, and it is, therefore, costly.

The major issue is that organisms do not respond to a single factor at a time, but to a suite of stresses acting together. And simultaneously, they are being influenced by the activities of other species, which in turn are being affected by multiple environmental factors.

In the Patuxent River and the Chesapeake Bay, those key factors include the interactive effects of high nutrient loading, toxic contaminants and dissolved oxygen levels which can vary from total absence to super-saturation. How do such complex interactions reverberate though the ecosystem? How does the system respond, from clouds of algae to schools of large fish? And most importantly, how can we achieve a basic understanding of these responses so that we can better predict environmental change and improve management of our coastal ecosystems?

A Question of Complexity

Scientific research has clarified a good number of cause-and-effect relationships when it comes to individual environmental stresses such as high nutrient loading and low dissolved oxygen. We also know how heavy nutrient loading and changing temperature affect phytoplankton production, or how phytoplankton may respond to contaminants under a range of oxygen conditions.

But the open environment is considerably more complex: organisms from phytoplankton to sediment-dwelling worms to oysters to anchovies and striped bass are all responding to multiple stresses that continually occur in unknown combinations.

Until now, controlled experiments designed to measure the effects of such interactive stresses, under different conditions, have been limited - and predictive models are the poorer.

"Clearly," says Donald Scavia of the National Oceanic and Atmospheric Administration's Coastal Ocean Program, which is funding the project, "the interactions are what we know least about. If you base management decisions on isolated cause-and-effect relationships, you're missing the important interactions."

It is those interactions, says Denise Breitburg, that scientists at the Benedict lab, along with other researchers, are after. And in a big way.

A wide-ranging multi-disciplinary team has begun intensive experiments on the ecological effects of multiple stresses that will run over five years, experiments that make use of mesocosms and large field enclosures set in the Patuxent River.

"These experiments will not only help us compare the effects of single and multiple stressors, they will enable us to test the importance of complexity itself."

Researchers will model experimental results and then begin linking those results to a network of other models designed to predict the effects of management actions on the Patuxent River.

"If all goes well," says Jim Sanders, director of the Benedict lab, "then for the Patuxent and other coastal systems, we'll have a mechanism for being able to better predict what might happen within a system. "And," Sanders adds, "we'll also be able to pragmatically step back from those predictions and ask if a management response is worth the cost."

The goal is ambitious and chancy. "But research is all about gambling," says Donald Scavia, "and there has been no such program to deal with interactions on this scale." After all, he says, "this project is looking at multiple impacts on a full spectrum from mesocosms to field enclosures to field studies to modeling for watershed dynamics to looking at the economic impacts of multiple stressors."

It is a challenging undertaking, which begins with adept experimental design.

Designing Experiments

"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.

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