Study: Answering big questions on stream ecosystems
Susan Ziegler, a professor of biological sciences, has shed some light on the ecology of a polluted and a pristine stream that feed the Mississippi River watershed, and found an unexpected source of change -- reactions due to differences not only in nutrient composition, but in light exposure. Ziegler's new way of looking at the ecology of microorganisms in streams recently won her a three-year, $307,000 grant from the National Science Foundation to study the function of pristine and polluted streams based on her initial findings.
To minimize the impact of agricultural and industrial runoff, scientists must understand how healthy streams function and how pollution changes the way they function. However, little is known about the microscopic organisms that populate stream headwaters -- the unseen 'factories' that process nutrients in ways that are important to ecosystems far downstream.
"Small streams haven't really been on the radar screen in a large way until the past 10 years," Ziegler said. "It's the headwater streams that seem to be hammered by land use on a global scale, yet they are typically where a lot of the nutrient processing within watersheds occurs."
Ziegler and her students use stable isotopes, atoms of slightly different atomic weights, to look at how microorganisms process organic and inorganic materials. They will examine two small streams in the Buffalo National River watershed and two small streams in the Illinois River watershed, both of which feed into the Mississippi River watershed. Each pair of study streams consists of one relatively low in nutrients and one elevated in nutrient concentrations.
In addition to excess nitrogen flushed into the system through agricultural practices and other land-use activities, many headwater streams have had trees and brush cleared from their banks, creating changes in the amount of light the streams receive.
"These are big changes to these ecosystems," Ziegler said. "Changes in light exposure can lead to another suite of biological and chemical mechanisms that further denigrate stream systems."
In a normal stream, the microscopic plants and bacteria in the water use nitrogen and carbon from dissolved organic matter for biological processes. Trees and brush grow along the banks, creating cycles of shade and light that affect the growth of microbes and the processing of organic matter. As the microscopic organisms process the dissolved organic matter, the by-products of that process get sent downstream, some in the form of nitrogen that is released as part of the nitrogen cycle.
Today, however, most streams also contain excess nutrients from man-made sources, such as fertilizer or effluent, which can greatly impact the composition and cycling of dissolved organic matter. Ziegler and her students have found that in streams with large amounts of nitrogen from man-made sources, chemical reactions supported by light appear to facilitate the attachment of the nitrogen to other organic materials, making a complex material that microorganisms can't break down. This may affect the natural nitrogen cycle and may explain in part fish kills and algal blooms that occur downstream where that nitrogen is re-released. So it appears that ultraviolet radiation from increased sunlight in the streams may play a role in the breakdown of the nitrogen cycling.
"Basically, light is playing a role in tying up the nitrogen so that it bypasses the biological processing that typically occurs in the stream," Ziegler said.
To characterize the processes that take place in undisturbed and polluted streams, the researchers set up small, self-contained systems in the water using plastic enclosures filled with rocks from the streambed. These enclosures include a pump on top that moves the water at the pace of the stream water. The water in the contained environment is 'spiked' with various organic compounds, such as sugars, containing an isotope of carbon that allows researchers to see how microorganisms in the system are using the dissolved organic matter during the cycling process. The self-contained systems are placed in the stream and monitored hourly over an entire day. Multiple light and dark systems are incubated during several days over the course of each season at each study site.
The researchers use this technique to look at how microorganisms use and release nutrients into the stream. They will repeat the experiment with a more complex carbon compound and also with nitrogen isotope-labeled ammonia to see how the microbial community responds to these different substances in varying amounts of light and darkness. The more complex carbon isotope represents plant-like material that might become dissolved in streams. The isotope-labeled ammonia represents by-products of man-made processes from industry and agriculture. The ammonia will be used to test if light fuels chemical processes that cause the excess nitrogen to create a complex with organic matter, making it unavailable for biological processing within these streams.
"We're trying to understand how those headwater stream processes critical to the Mississippi River watershed and the Gulf of Mexico are impacted by major sources of environmental change," said Ziegler.
This article originally appeared in the 04/01/2005 issue of Environmental Protection.