ATHENS, Ga. -- Concern over the health of our oceans has grown, in the past two decades, from worry to alarm. Coastal waters are crucial links in the food chain of the seas, and numerous disasters, many of them man-made, have threatened these waters.
A new mathematical model, coupling both physical and biological effects, could be a crucial step in predicting the health of near-shore ocean environments where rivers enter the sea. The model, which shows the dramatic importance of winds to the health of these ecosystems, could be useful in preventing the destruction of such areas as rich fishing banks.
"The key is learning how to identify, quantify and qualify which processes are at work in an ecosystem," said Dr. Changsheng Chen, a marine scientist from the University of Georgia. "In the past, we had scientists studying the physical effects and others looking at the biological effects, but they weren't talking to each other. We must understand how the two systems work together."
A new study using a coupled biological and physical model was recently published in the Journal of Marine Research. Along with Chen, others in the study were his collaborator, Denis Wiesenburg of the University of Southern Mississippi, and Chen's research associate from UGA, Liusen Xie. Their work was supported by a grant from the Office of Naval Research.
Chen's study focused on the Louisiana-Texas continental shelf in the Gulf of Mexico, an area of well-known biological activity. This shelf begins at a 20-meter depth off the coast of Louisiana and Texas and gradually increases to 200 meters at the shelf break and then rapidly drops to 500 meters at the outer edge of a slope. Researchers know from direct observation that most of the biological activity is closely tied to the seasonal discharge of nutrient-rich freshwater from the Mississippi and Atchafalaya River systems at the coast, the upwelling of water at the shelf break and local wind and tidal mixing.
The problem is there has been no way to use information about the physical world of the reef to predict biological production -- until now.
The inherent problems in understanding all the variables are daunting, to say the least. In order for a coupled model to predict biological activity it must include, among other things, water salinity, tides, turbulence and winds, not to mention the amount of dissolved nutrients taken up by phytoplankton (which are then grazed by zooplankton).
"We have for many years known a lot about these systems," said Chen. "What we haven't known is how these systems interact."
For many years, scientists have observed the various chemical, biological and physical parameters that are critically important to ecosystem functions. What they have not understood well is how these factors interact. Until now, they have lacked a technique for coupling them into a dynamic system that can also analyze the relative importance each of the components.
The importance of Chen's work is that it "fully couples" these factors in a way that allows researchers to isolate the level of influence from each of them.
"Until we understand how they interact, and which factors are most important under given circumstances, then we cannot predict ecosystem change," said Chen.
Chen's model is a tool for analyzing existing physical, chemical and biological observations. It can identify which processes are involved in a given ecosystem condition; qualify which of these processes is most important in a given ecosystem condition; and quantify the degree of importance of each.
The most crucial finding of the research appears to be confirmation of the importance of the boundary zone between freshwater and salt water -- an area called a "front." In the Texas-Louisiana Shelf, biological production is related closely to the location of this front, which is moved both by wind and the amount of freshwater flowing into the sea.
The problem for researchers is that this front is often and easily broken by physical forces, and when this break occurs, the front can stretch at greatly irregular distances from the shore. The new coupled model is the first to allow predictions about the location of this front and its influecne on biological production when certain conditions are included in the mathematical formulas.
Complications abound. For instance, the influence of light on phytoplankton production is a crucial element in understanding the biological structure of the front. Normally, light penetrates down only about 20 meters, and it is in this zone that most of the phytoplankton "bloom" takes place. However, in studies of the Georges Bank fishing area off the northeast coast of the U.S., Chen and his collaborator Peter Franks of the Scripps Institute of Oceanography found that tidal currents can take this bloom down as deep as 80 meters, a physical change that has a profound effect on production of food for larger animals.
"There are front zones in lakes, too, and I am part of a team studying them in Lake Superior and Lake Michigan," said Chen. "There, plume structures caused by sediments cause a front similar in some ways to what we see on the Texas-Louisiana Shelf."
The model predicted for the Lousiana-Texas Shelf a well-defined frontal zone and a high concentration dome of nutrients near the bottom within the frontal zone. New production of nutrients was high throughout the water column near the coast and in the upper 10 meters at the ourter edge of the front, though lower in the frontal zone where freshwater meets the salt water.
The current model study was "in reasonable agreement" with data from direct observation on the Texas-Louisiana Shelf done in the spring of 1993.
Journal
Journal of Marine Research