Short-Term Biological Response to Hypolimnetic Oxygenation in North Twin Lake on the Colville Reservation

Summer dissolved oxygen (DO) depletion in deep waters (hypolimnia) of thermally stratified lakes is widely recognized as one of the most significant and intractable problems associated with water pollution. Oxygen depletion has severe consequences, especially enhanced internal nutrient cycling, driving cascading ecologic impacts such as increased primary productivity, shifts to cyanobacteria dominance of the phytoplankton, reduced transparency, and reduced fish habitat. Line-diffuser oxygenation was implemented in 2009 for trout habitat enhancement in North Twin on the Colville Confederated Tribal (CCT) Reservation, near Inchelium, WA. This is the first application of this technology to a natural lake. In the short-term, we have documented increased fish utilization of summer hypolimnetic habitats through net captures, hydroacoustic monitoring, and active fish tracking. Changes in zooplankton and benthic invertebrate populations were also quantified. Responses of the diurnal-migratory phantom midge (Chaoborus) are particularly interesting. Overall, the technology shows great promise, and the CCT plans to extend the oxygenation system to South Twin Lake. Long-term fish community responses, such as improved growth and survival, will be key to evaluating the ultimate success of oxygenation.