Water transparency and dissolved oxygen concentration are fundamental water quality characteristics that are associated with aquatic life. Transparency is a measure of the degree to which light penetrates into water, and is affected by the presence of suspended particles in the water. Dissolved oxygen is produced by photosynthetic algae, and is consumed by fish, zooplankton, bacteria and other organisms.

We compared GLOBE transparency data to those from the U. S. Geological Survey (USGS) and a volunteer program sponsored by the U.S. Environmental Protection Agency, the Great American Dip-In. Next, we investigated time-series trends in transparency and dissolved oxygen for five GLOBE schools, and considered the relation of these properties to temperature, precipitation and other water quality measurements.

GLOBE schools measure transparency with two different methods: turbidity tube and Secchi disk. A number of volunteer water-quality monitoring groups throughout the United States and Canada collected transparency data using Secchi disk in early July 1997; as part of The Great American Dip-In. Only the geometric means of each group’s measurements were reported in the publicly available Dip-In data. The USGS reports turbidity rather than transparency, so we calculated the equivalent transparency depth using an empirical correlation determined in our laboratory.

GLOBE turbidity tube data were divided into two groups: transparency depths less than the length of the tube and depths greater than the length of the tube (i.e. after filling the tube completely the pattern at the bottom was still visible). Since there is a physical limit to the tube length, some schools may report only a greater than value.

Cumulative frequency distributions show that GLOBE Secchi disk data have a very similar distribution to that of the Great American Dip-In data, however the GLOBE data exhibit a greater range. This is most likely due to averaging of the Great American Dip-In data as described above. GLOBE turbidity tube measurements in which the transparency depth was less than the length of the tube also have a similar distribution. The plot of GLOBE turbidity tube data with transparency depths greater than the length of the tube shows that approximately 80% of those measurements indicated that the transparency was greater than one meter. The presence of some measurements significantly less than one meter in this category suggests a data entry error.

Time-series plots of precipitation, air and water temperature, dissolved oxygen, and transparency for the GLOBE schools show that transparency generally corresponds with measured dissolved oxygen. Precipitation events and associated changes in temperature drive some of these changes. While transparency corresponds well with measured dissolved oxygen levels, it does not correlate well with saturated dissolved oxygen levels (which are estimated from water temperature and elevation).