About Data Interpretation:
Measuring Water Quality
The “quality” of water is determined by its chemical and biological characteristics. The quality of a body of water such as a stream affects the ecology integrity of that stream. In this way, water quality is a determination of the health of the ecosystem and its ability to be sustainable.
Numerous chemical, physical and biological measurements can be used to determine the quality of stream water. Traditionally, parameters such as suspended sediment (i.e. The amount of solids in a volume of water), the dissolved oxygen content, or DO, and the diversity of aquatic insects within a length of stream all have been important water quality parameters. However, natural variation in water quality measures due to climate, slope, stream type, etc. occur so there is no “universal” or absolute measure of water quality. Rather, water quality parameters can be compared to others to assess the impact of both human activities on streams, and also to evaluate the differences in climate, geology, water flow and even biological variation. Different environmental agencies used different measures of water quality. The various monitoring programs at your schools also do. In our work in the McMurdo Dry Valleys LTER, there are few animals in our streams, so we rely on chemical measurements to assess water quality changes through time.
Nitrogen is contained in all plant and animal tissue. It is an essential nutrient and is required for life. At high concentrations, dissolved nitrogen can become a pollutant.
Nitrogen can exist in many different chemical forms. When we collect water samples from the streams and lakes in the Taylor Valley, we measure dissolved inorganic nitrogen (DIN) in two forms, ammonium (NH4+) and nitrate (NO3-).Nitrate and ammonium occur naturally in the snow, glacier ice, meltwater streams, soils and lakes in the Taylor Valley, Antarctica. Low concentrations of DIN (in the form of nitrate or ammonium) might limit the amount of primary productivity of the organisms living there.
It should be noted that several other factors could also limit growth in the Taylor Valley streams, such as carbon limitation, availability of liquid water and light. In addition, we observed that organisms tend to grow in certain parts of the stream channels, such as downstream from boulders. The organisms need to be able to colonize and grow in an area. If there is too much sediment carried by the stream or if the channel is too steep, the organisms are unable to colonize and survive in these areas.
There are many natural and man-made sources for nitrogen compounds. In the Taylor Valley streams, the amount of man-made nitrogen compounds is very low compared to temperate streams in more populated areas. In Antarctica, we are especially careful to limit the amount of nitrogen and other nutrients that we release into the environment because we want to keep the area as pristine as possible. For example, human waste and the water we use for cooking, cleaning and bathing are removed from Taylor Valley.
During the austral summer of 1998-1999 we collected a series of samples from Andersen Creek and Canada Stream in Taylor Valley (http://huey.colorado.edu/LTER/datasets/streams/streambio/strmdesc.html). Since there is very little precipitation in Taylor Valley (<10 cm per year snowfall; Fountain et al., 1998), the major source of the stream water is glacial melting. Streams in Taylor Valley only flow for about six to ten weeks per year during the summer. Because both Canada Stream and Andersen Creek are "fed" by the same glacier (Canada Glacier), we expect that the initial chemistry (the sampling site farthest upstream) of the streams is similar. We wanted to see if the chemistry of the stream water changed as the streams flowed from the glacier to the lakes. Canada Stream flows along the eastern side of Canada Glacier and then it flows into Lake Fryxell. Andersen Creek flows along the western side of Canada Glacier and flows into Lake Hoare. Both streams are less than a few inches deep, and have very rocky stream beds. There are no plants or trees in Taylor Valley, but Canada Stream has microbial mats and mosses while Andersen Creek does not. We were particularly interested in comparing nitrate concentrations in these two streams.
Figures 1 and 2 show the concentration of chloride and nitrate at different points along the length of the streams. In Andersen Creek (Figure 1), the nitrate concentrations remain relatively constant along the length of the stream. Chloride concentrations increase slightly probably due to dissolution of salts. However in Canada Stream (Figure 2), nitrate concentrations drop as the stream flows over reaches with microbial mats and mosses. The algae and bacteria use the nitrate for growth. The nitrate concentrations in Canada Stream are consistently lower than those in Andersen Creek. If we assume that the source water (glacier melt) is the same for the two streams, we can hypothesize that some of the nitrate in Canada Stream is taken up by microbes, especially after site 5.
Figure 1. Chloride (Cl) and nitrate (NO3) concentrations in Andersen Creek. Gl3 and Gl2 are water samples taken from water flowing on top of Canada Glacier; sites 1 through 4 are stream water samples. Site 1 is farthest sample upstream and Site 4 is near the mouth of the stream.
Figure 2. Chloride (Cl) and nitrate (NO3) concentrations in Canada Stream. Sites 6 through 1 are stream water samples with Site 6 being the farthest sample upstream and Site 1 near the mouth of the stream.
It is possible that nitrate concentrations in Taylor Valley streams are influenced by the presence of microbial mats and that the distribution of microbial mats is influenced by the availability of nutrients.
Nitrate concentrations are also measured in the stream water at Tuscaloosa Academy in Tuscaloosa, AL and in Thornton Creek in Seattle, WA. In urban, suburban, industrial and agricultural areas, it is important to monitor dissolved nitrogen concentrations to study the health of stream ecosystems. High nitrate concentrations are usually more common in water impacted by human activities, but ammonium may be more common in poorly oxygenated waters with high organic loading. Nitrate is usually higher in agricultural runoff and ammonium is more common in urban sewage. (Reference: Berner and Berner, Global Environment.)