Technology▼Solutions In situ nutrient analyzer
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nutrient analyzers commercially available for field use, but they also typically rely on wet chemistry and can only be deployed for a limited duration. Nitrate-specific electrodes are available, but they need to be calibrated frequently, which is difficult to KEN JOHNSON © 2002 MBARI
A joint effort between scientists at the Monterey Bay Aquarium Research Institute (MBARI) and a Nova Scotiabased oceanographic instrument company called Satlantic, is leading to the commercialization of a new in situ nutrient analyzer. The instrument, which is expected to hit the market this fall, requires no chemical reagents and is capable of monitoring nitrate autonomously in extreme environments like the deep sea and the Arctic. Nitrate has probably attracted the most attention for contaminating drinking-water wells and causing algal blooms that deplete water bodies of oxygen. But scientists are also monitoring nitrate to better understand how changes in nutrient levels affect ecosystems, as well as climate. Nitrate is essentially the base of the food chain in the ocean, where it is consumed by phytoplankton, which take up carbon dioxide. Changes in phytoplankton productivity due to changes in nutrient concentrations will affect carbon dioxide uptake and therefore climate. Nitrate levels have been increasing in marine environments, largely because of fertilizer runoff from agricultural fields, excess manure from large-scale animal feeding operations, and overburdened sewage treatment facilities. In addition, burning fossil fuels, which release nitrogen oxides into the air, plays a role because some of that nitrogen ends up being deposited onto water bodies as nitrate. Historically, nitrate has been monitored by collecting water samples and bringing them back to the laboratory for wet chemical analysis. The most common approach is a colorimetric analysis, called the hydrazine reduction method, in which reagents are mixed with the sample to form a highly colored azo dye. The absorbance at a particular wavelength is then measured to determine the amount of nitrate. There are a few
An in situ nitrate analyzer is deployed on a conductivity, temperature, and depth rosette sampler to generate profiles of nitrate concentration as a function of depth.
do in the field, and they suffer from interferences due to the high chloride levels in seawater. “We have a big problem with sampling and monitoring nitrate autonomously, without somebody being there,” says MBARI’s Kenneth Johnson, who helped develop the new in situ nutrient analyzer. “We don’t have enough graduate students to keep [monitoring a] lake, river, or ocean 365 days a year.” The new analyzer, which is essentially a small, battery-powered UV spectrophotometer with a photodiode array detector, can be deployed for up to six months at a time, in even the harshest of conditions. Every hour, it measures the full-spectrum absorbance over the 200–400 nm wavelength range. Real-time data can be sent via radio transmission, or
ENVIRONMENTAL SCIENCE & TECHNOLOGY / SEPTEMBER 1, 2002
data can be stored in the instrument and retrieved at a later time. Nitrate is determined based on the fact that it absorbs UV light around 210–220 nm. Although the absorbance is weak, the instrument is sensitive enough to measure nitrate levels down to 0.1 micromoles per liter. A few chemicals in seawater do absorb light at the same wavelengths as nitrate. “Bromide in the ocean absorbs in the UV, so we have to deconvolve the bromide signal,” Johnson says. Sulfide also interferes with the nitrate signal. “The good news is that places that are typically anoxic don’t have nitrate in them anyway.” The nitrate analyzer has already attracted quite a bit of interest, according to representatives of Satlantic. Johnson too has gotten several inquiries about it, including one from the U.S. Coast Guard, which is looking to equip a large ship dedicated to Arctic science research with the analyzer. The instrument is very rugged, and so far has fared well in extreme environments. It’s been tested in the deep ocean, thousands of meters down, and was successfully deployed last October in the Arctic, Johnson says. Scientists who have had a chance to use the new nitrate analyzer welcome the development. “Being able to deploy it on towed vehicles, autonomous underwater vehicles, and moorings will bring us a wealth of new data on the spatial and temporal distributions of the nutrients that fuel coastal productivity,” says Jack Barth, a physical oceanographer at Oregon State University who hopes to purchase the analyzer for work on coastal ocean ecosystems. Barth and his colleagues tried out the new instrument on his towed undulating vehicle, over a large region of the Oregon and northern California coastal ocean. He says they obtained “an unprecedented three-dimensional picture of the nitrate distribution in this region.” They expect to present their results at the American Geophysical Union meeting in December. —BRITT E. ERICKSON © 2002 American Chemical Society
