At the Heart of Plentitude:
The Bay's Complex Circulatory System

By Merrill Leffler

Traversing the Chesapeake from the Northern Bay to the Virginia capes, researchers seek clues to the Bay's productivity

While there is good reason to be concerned about the healthy recovery of the Chesapeake and its diminished resources, this shallow bay still ranks high among the nation's most productive estuaries. What accounts for this plenitude? What makes it such a rich source of fish, shellfish and other organisms? At least one team of scientists believes that the answer lies in the complex physical forces that link the Bay's chemistry and its biology - a watery circulatory system that nourishes a massive productivity.

Those physical forces are many - the estuary's sharp layering between outflowing fresh river water and incoming ocean water (the pycnocline) or between cold and warm water (the thermocline), its currents, its waves, its seiches (wind-driven water sloshing back and forth across the Bay), its many types of fronts. Like arteries, these physical forces deliver nutrients and other compounds, and move and attract living organisms - from microscopic plankton to large fish.

Determining just how this circulatory system interacts with organisms is a central challenge of TIES (for Trophic Interactions in Estuarine Systems), a unique and ambitious research effort by scientists at the University System of Maryland Center for Environmental Science (UMCES).

They have set a daunting task for themselves - Walter Boynton and William Boicourt admit that they were thrilled and a little scared when they received word from the National Science Foundation that their six-year research project had been funded. With this award they have an opportunity to pull together an unparalleled understanding of one of the world's most productive water bodies, to literally map the way it functions.

Taking the Pulse of the Bay

The Chesapeake operates in pulses, says Boynton, pulses of water that may shuttle organisms or even trap them: hurtling springtime river flows, ocean tides, and jets of scooting bottom water that result from sharp changes in topography. "Pulses play a role in setting up the physics of the estuary," says Boynton. "We would not have the same physics if river flow were constant."

Each spring, river waters cascading into the Chesapeake encounter salty tidal waters coming up Bay from the ocean. The density differences betwen fresh and salty water -- influenced as well by winds, tides, gravity, and bottom topography -- result in fronts that scientists believe are highly fertile zones for the production of fish.

Pulses help set up physical structures, such as fronts - areas where, as with weather fronts, temperature and density differences create dynamic transition zones. The UMCES researchers want to find out if secondary production - of zooplankton and fish - forms around these fronts in the way that primary production (in the form of algal blooms) appears to.

One kind of front forms sharp boundaries that result when different water masses meet, lighter layers often lying atop denser layers. Fronts come in different sizes and can last for varying lengths of time. Some may come and go in a few hours - the result of wind, for example - some may last for a few days, others for two or three weeks. The best known example of a long-term front is the so-called "maximum turbidity zone" - a recurring physical structure where freshwater flowing down a river hits brackish estuarine water, the river's single flow mixing with the estuary's more complex tidal flow.

"A decade ago we didn't know enough about nutrient cycles or primary production to reasonably attempt this."

The maximum turbidity zone serves as the scene for intense chemical and physical activity in a tributary or estuary, with some river-borne particles settling out and others becoming entrained in the frontal zone. In the Chesapeake Bay, the most notable maximum turbidity zone occurs north of the Bay Bridge where the largest influx of freshwater in the Bay flows from the Susquehanna River, entraining sediments, algae and other particles. "It is such zones - both in the rivers and mainstem Bay - that we're interested in," says Boicourt.

The researchers hypothesize that seasonal pulses of nutrients from land, air and ocean accumulate in frontal zones and set in motion the processes that produce algal blooms. Are these major zones for secondary production as well - for zooplankton, fish and other organisms that feed on the Bay's plant life? "That's what we're guessing," says Boynton. Their belief is, according to Boynton, that such pulsing systems have an inherently high capability for secondary production. Zooplankton and larvae may be attracted to the algal growth, or they may themselves be caught up in these physical structures.

In the coming year physical oceanographer Larry Sanford (on right) will join the TIES effort to link the Bay's physics with its biological abundance. Sanford is pictured here with graduate student Weiqi Lin, as they study Bay wave patterns in a current Sea Grant-funded study..

The timing and strength of these pulses shift from year to year because of climatic events - in particular variations in the amount of rain and snowfall - that fuel the flow of freshwater into the Bay. In the spring of 1995, for example, pulses of freshwater came early and were relatively small. Measurements this past spring should give researchers a good baseline for comparisons over the next five years, when there will almost certainly be higher flows, Boynton says, with larger pulses of freshwater at varying times. The researchers wonder, for example, how major forage fish (like anchovies) and their predators will school and behave as the system moves from low-pulse, low-flow years to years with a heavy spring flow.

Answers to such questions could tell us a lot about how the circulatory system of the Chesapeake helps explain the productivity of what H. L. Mencken called the Bay's "protein factory."

According to Walter Boynton, such an ambitious analysis of the Bay's secondary production would have been impossible before now. "A decade ago we didn't know enough about nutrient cycles or primary [algal] production to reasonably attempt this," he says. Nor did the researchers have the technology to take thousands of rapid measure-ments up and down the Bay system, from the Virginia capes to the Chesapeake and Delaware Canal.

Now, says Boynton, they have both the science and the machine - in the form of the remarkable Scanfish (see sidebar on "High Tech Fish").

Building on the Past

As a scientist, what excites Boynton is always the next set of questions. Though we don't have the whole story about nutrients, he says, the programs we've worked on have taught us a lot. For example, extensive monitoring by the Maryland Department of Natural Resources and scientific projects such as PROTEUS (for Processes of Re-cycling, Organic Transformations and Exchanges between Uplands and the Sea) have supplied researchers with a wealth of data. PROTEUS in particular (part of the National Science Foundation's Land Margin Ecosystem Research program) expanded on knowledge resulting from the multi-state dissolved oxygen project supported by the National Oceanic and Atmospheric Administration (NOAA) and the Maryland and Virginia Sea Grant programs. According to Boynton and other scientists, these and related research programs have now positioned us to ask the all-important next set of questions, why is secondary production so efficient in the Chesapeake?

PROTEUS helped researchers establish what Boynton calls the "first-order dynamics" of how nutrient inputs, varying as they do from year-to-year because of climatic effects, drive nutrient cycling and primary production. It showed, says Boynton, that annual changes in such processes as primary production and nutrient recycling are directly related to changes in inputs of nutrients from the watershed. Those shifts in nutrient levels and the timing of their delivery result in turn from annual differences in river flow and other physical factors such as the strength of the pycnocline. The pycnocline forms an invisible boundary - between buoyant, downflowing river water and dense, inflowing saltwater. The strength and extent of the pycnocline, just one of the Bay's fronts, have important effects on ecological processes - such as the delivery of oxygen - throughout the estuary.

UMCES Researcher Ed Houde tracks the distribution of fish throughout the Chesapeake. Intensive sampling over the next five years should begin to reveal if zooplankton and fish are drawn to special productivity zones.

While research on dissolved oxygen gathered five years of data at several lateral locations across the Bay between the Choptank and Patuxent rivers, the new TIES project calls for extensive shipboard measurements over the entire length of the Bay for several years. Coordinating with weekly Baywide airborne surveys of algal abundance and distribution by UMCES researcher Larry Harding, scientists will gather enough data to further confirm that ecological processes such as the extensive patchiness of algal blooms are driven by pulses of nutrient inputs and the timing of physical forces.

"It may be that 90 percent of the secondary production is occurring in 10 percent of the Bay's area."

To date, research suggests that some physical structures recur in predictable regions - for example, gravity currents in what physicist William Boicourt refers to as the "hydraulic control point," the region in the Bay between the Rappa-hannock and Potomac rivers where a steep change in bottom topography accelerates water flow. (The Bay deepens from the Potomac north to Kent Island.) Others, like the turbidity maximum zone north of the Bay Bridge, will shift depending on the strength and timing of spring flow from the dominant Susquehanna River.

But there are many other physical structures that form as the result of pulses - they may often occur in more limited regions and for shorter lengths of time. Fronts, for example, can form during tidal exchanges, or when a mass of moving water at one temperature meets another mass at a different temperature or salinity. While differences in density segregate these water masses, gravity - along with tides and winds and freshwater flow - drives them through the estuary, often mixing them as water moves in and around the Bay's many twists and turns, over its tidal flats, its narrow creek mouths, its deep river channels.

The Chesapeake operates in pulses, says Walter Boynton, pulses of rolling water that shuttle organisms or may even trap them.

The more researchers learn about how these different areas behave, the better they may be able to suggest to managers which parts of the Bay are significant in fostering production of fish and shellfish. "It may be," says Boynton, "that 90 percent of secondary production is occurring in 10 percent of the Bay's area. We don't know this, of course. But it's a central question -a hypothesis we're working with."

If this hypothesis proves to be true, its implications could lead not only to improved management of fisheries but to a more effective targetting of Bay clean-up efforts.

Over the next five years, the team of UMCES scientists led by Walter Boynton (pictured above) will concentrate their research efforts in the Bay on key physcial features such as maximum turbidity zones and other frontal regions.

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		Contents of this issue	Other Issues

At the Heart of Plentitude: The Bay's Complex Circulatory System

Taking the Pulse of the Bay

Building on the Past

At the Heart of Plentitude:
The Bay's Complex Circulatory System