Water Quality and Aquatic Insects: A Story of Change in the lower Deschutes River

Do you ever think about the diversity and abundance of life that lives within the rocks and banks of a river? Aquatic entomologists study the organisms that inhabit the substrate, or benthic zone, in lakes and streams. The lower Deschutes River is historically well known for its abundant and diverse insect population that provide food for trout, birds, and other animals. Recent changes in the benthic zone’s appearance, including prolific algae growth, prompted concern for the aquatic insects that require clean and suitable substrate to thrive. In 2015 and 2016 the Deschutes River Alliance Science Team, with guidance from our trained aquatic entomologist Rick Hafele, collected benthic samples for analysis. We used methods developed by the Oregon Department of Environmental Quality and U.S. Environmental Protection Agency to collect and assess the macroinvertebrates at two sites in the lower Deschutes River. The 2015/2016 Lower Deschutes River Benthic Study (DRA 2019) can be found on our website: www.deschutesriveralliance.org.Our study results are consistent with responses to eutrophic conditions (meaning excess nutrients). In this case the problem is from high loads of nutrients, primarily nitrogen from agricultural sources, and the result is poor water quality. First, our results showed low abundance of pollution sensitive species of caddisflies, mayflies, and stoneflies relative to the abundance of pollution tolerant species like worms and snails. In fact, the abundance of non-insect pollution tolerant species comprised more than 50% of all invertebrates collected from all samples during the study. The results from September 2016 are summarized below.In addition, a polychaete worm was observed in high abundance.  This worm is the intermediate host of the Ceratonova (syn: Ceratomyxa) shasta parasite and ranged from 252 to over 8,000 individuals per square meter during the study. C. shasta is particularly concerning for the health of spring Chinook salmon in the Deschutes River. The figure below summarizes the complex life cycle of C. shasta. Concurrent studies in 2015 and 2016 by Oregon State University, Oregon Department of Fish & Wildlife, and the US Fish & Wildlife Service detected high concentrations of the C. shasta spores in water samples collected in the lower Deschutes River (see “ODFW Fellowship Report 2015-2016”). Enteronecrosis (aka “gut rot”) is caused by infection of the C. shasta parasite and has caused losses of adult and juvenile Chinook salmon in the Deschutes Basin. The overall effect of this disease on the spring Chinook salmon population is currently unknown but warrants further investigation.Low numbers of pollution-sensitive insects coupled with high abundance of parasite-hosting non-insect species continue to raise questions about water quality in the lower Deschutes River. Multiple lines of evidence now point toward the Selective Water Withdrawal Tower as the reason for the shift to eutrophic conditions in the lower Deschutes River. The Deschutes River Alliance continues to advocate for the return of cold and clean water to the lower Deschutes River in addition to monitoring the impacts of these changes.