WATERWAYS
Published 10:33 pm Thursday, August 28, 2008
Got oxygen?
The Stillwaters Environmental Education Center stream-monitoring crews measure dissolved oxygen levels in Carpenter Creek every month. Aquatic organisms need oxygen to live. Microscopic bubbles of oxygen gas, called dissolved oxygen (DO), transferred from the water to the blood of organisms as they pass across gills or skin. Organisms need a certain minimum amount of oxygen to survive. Salmon eggs, incubating in stream gravels, need even more DO than adult fish. Oxygen is also needed for chemical and biological processes going on in the stream and sediments – such as the bacterial breakdown of organic matter like leaves – and for the transformation of nitrogen sources to forms animals and plants can use.
How is it measured?
For many years, monitors would perform a chemical test called the Winkler titration method, which had many steps and required careful precision and a steady hand (oops, too much – start over!). Then in 2007, they began to use a DO probe, a sensitive rod immersed in the water to electronically measure DO. Having a new instrument like this is always great but you have to verify it is working well so it took several months of comparing readings from the Winkler and the probe to be sure the data was correct. An important thing to remember with monitoring – the data is only useful if it is accurate!
How is Carpenter Creek fairing?
In general, all four stations have DO levels good enough to support adult fish and benthic (bottom dwelling) organisms. Site 1 (on the Stillwaters property), Site 2 (near Kingston Hill), Site 4 (just south of Highway 104) and Site 3 (just north of Highway 104) have averaged DO concentrations of 7 to 10 micrograms per liter (mg/l) from 2001 to early 2008 with maximums of 14 mg/l but also as low as 5 mg/l.
We are lucky to have Coho reds (spawning gravel nests) in the lower watershed of Carpenter Creek. Washington state levels of the DO concentration necessary to assure the health of incubating salmon eggs and maturing alevins (immature fish) are determined to be an average maximum of 9 mg/l and a daily minimum of 8 mg/l. Our data show, however, during the time fish might be spawning and eggs maturing, we have periods near Site 2 that have dipped frequently below 8 and as low as 6.2 to 6.8 in the in the last three winters.
Why do oxygen levels vary?
Oxygen is in the water because of photosynthesis by aquatic plants and physical diffusion – oxygen mixed in from the air during turbulence. The water’s ability to hold it depends on water temperature. The warmer the water, the less oxygen it can hold. DO is also stripped from the water by breathing organisms and the decomposition or oxidation of natural organic matter or stormwater pollutants. Also, changes in oxygen levels might not occur at the point where pollutants or extra organic matter enter the stream, but rather downstream as the oxygen is used, making it harder to determine why levels change. Water chemistry in every stream is a complex set of exquisite processes!
Next month, I’ll explore nutrients in Carpenter Creek and Appletree Cove.
Betsy Cooper is a board member and stream monitor at Stillwaters Environmental Education Center. She also serves on the Kingston Citizens’ Advisory Council.
