Nitrate is one of the most important elements in marine chemistry, at least as far as sea life is concerned. Marine algae support virtually all life in the oceans, but in order to flourish, they require lots of sunlight and nutrients, especially nitrate. The In Situ Ultraviolet Spectrophotometer (ISUS), an instrument developed by marine chemists at MBARI, provides a quick and easy way for scientists and water-quality managers to measure nitrate concentrations in seawater or fresh water.
For decades, oceanographers have measured concentrations of nitrate using standardized laboratory techniques for chemical analysis. This involved filtering a water sample, then adding various indicator chemicals that caused the seawater to change color, depending on how much nitrate was present. Each water sample had to be processed individually, often by hand.
In the late 1990s, Ken Johnson began looking for an alternative to this time-consuming process. He knew that for decades chemists had been trying to measure nitrate concentrations in water by passing beams of ultraviolet light through the water. The idea was that nitrate in the water would adsorb a small portion of this light at very specific wavelengths—the greater the concentration of nitrate, the more light would be adsorbed. However, no one had been able to develop instruments that could measure enough different wavelengths of light to detect the subtle changes necessary for measuring typical nitrate concentrations in seawater.
By 1997, however, advances in ultraviolet lamps, spectrometers (which measure light levels at a variety of wavelengths), and computer software (for analyzing the data from the spectrometers) made such an approach more feasible. In March 1998, Johnson and Senior Research Technician Luke Coletti assembled a bench-top system for measuring nitrate, and tried it out during an early voyage on MBARI's newly acquired research vessel, the Western Flyer.
Based on encouraging results from these initial tests, Johnson received a grant from the National Science Foundation in 1999 to develop an ultraviolet nitrate-measuring instrument that would operate automatically underwater. By May 2000, he and his colleagues in the Chemical Sensor Lab had built the first ISUS instrument. During its first year, this instrument proved its worth during a multi-month stint on an oceanographic mooring in the middle of the equatorial Pacific Ocean. The following year, an ISUS was installed on one of MBARI's long-term monitoring buoys in Monterey Bay.
How the ISUS works
Like many high-tech oceanographic instruments, the ISUS looks simple on the outside—just a half-meter-long metal cylinder with a smaller, notched cylinder at one end. The guts of the instrument are housed inside the large cylinder—a waterproof pressure housing that protects the instrument from the corrosive effects of saltwater and (in some models) from the crushing pressure of the deep sea.
The smaller notched cylinder (the "optical probe") is the business end of the ISUS. Water passes through the notch, where it is exposed to a beam of ultraviolet light from inside the housing. Nitrate dissolved in the water adsorbs a small amount of the ultraviolet light. A mirror at the far end of the notch reflects the light beam back into the housing. Inside the housing, optical fibers carry the light to a spectrometer, which breaks the light up into its component wavelengths, just as a prism splits sunlight into a spectrum of different colors. A small computer then analyzes the light from the spectrometer to determine how much nitrate is present in the water.
"Building a new sensor is only half the battle--making it work is the other half," Johnson said. For example, when the instrument was deployed near the water surface, especially in estuaries such as Elkhorn Slough, marine organisms rapidly colonized the ISUS' tiny mirror and lens. As Johnson put it, "One day it's polychaetes. The next it's bryozoans. Then after a rain, it will be mud." To address this issue, Coletti developed a covering for the optical probe that filters out particles with a fine mesh and shields the optics with a copper screen (copper repels many microscopic algae and animals).
In 2002, Johnson and Coletti published a paper describing their new instrument. They already knew that the device would be popular not only with oceanographers but with anyone who wanted to make continuous measurements of nitrate in water samples. Johnson remembers, "Before the paper was even out, we started looking around for a company that could produce the ISUS for commercial use, and would be a good match for MBARI's development philosophy." They eventually partnered with Satlantic Instruments, which produced the first commercial instrument in 2003.
Even after years of deployments, the team is still discovering quirks of the instrument. For example, when suspended vertically underwater, the ISUS sometimes became less sensitive over time, but would work perfectly after being brought back to the surface. After years of puzzling, the researchers finally concluded that tiny bubbles were being trapped against the instruments' lens. The solution: always mount the instruments horizontally for near-surface measurements.
Johnson and his team continue to improve the ISUS, primarily by developing better computer software for interpreting the spectra. This has made the instrument useful in a wide range of temperatures and salinities. Measurements that used to require half an hour of lab time can now be made three times a second. Such rapid processing is possible in part because the ISUS contains no chemicals and no moving parts.
As of December 2007, Satlantic had sold about one hundred of the ISUS systems, as part one of MBARI's most successful commercial partnerships. Johnson is proud to point to the variety of web sites displaying real-time nitrate measurements from all over the country. ISUS instruments are being used on monitoring systems from the Equatorial Pacific to the Arctic Ocean and from deep-sea hydrothermal vents to coastal estuaries and fjords. They have also helped farmers determine when they are applying too much nitrate to their fields, which helps them reduce the environmental effects of agricultural runoff. In coming years, ISUS sensors may become part of the routine monitoring equipment used in rivers, lakes, and sewage-treatment plants.
Johnson is quick to point out that the principles used in ISUS can also be used to measure other chemicals in water, including sulfides that support life at deep-sea hydrothermal vents. Johnson's team is developing a new version of ISUS that can measure concentrations of carbonate ions in seawater. Such measurements are essential for understanding the rapidly increasing acidity of the world's oceans due to CO2 buildup in the atmosphere. Thus, the ISUS has proven its usefulness not just in oceanographic research, but in addressing environmental challenges on land and in the sea.
MBARI contributors to ISUS project: Luke Coletti, Hans Jannasch, Ken Johnson, Carole Sakamoto
- ISUS web site
- 2003 Conference poster describing some oceanographic applications of ISUS (pdf file)
- 2002 News release on MBARI-Satlantic partnership to produce ISUS for commercial use.
- 2002 technical paper describing the design of the original ISUS instrument (pdf file)