The frequency, duration, and spatial extent of hypoxia have been increasing in Hood Canal since the 1990s. Those conditions have stressed marine life and periodically caused fish kills. Multiple complex interacting factors contribute to low DO conditions in the canal. Likely causes include the following:
- Increased nitrogen loading from septic field and agricultural runoff as well as forests changing from conifers to nitrogen-fixing alders stimulate increased phytoplankton production and corresponding increases in oxygen demand as they sink and decompose.
- Weather-related changes resulting in lower density of incoming ocean water decrease flushing and increase residence time, which allows more time for phytoplankton to grow, reproduce, and senesce.
- The flow of the Skokomish River, the main tributary to Hood Canal, is controlled by a dam. The river’s releases of less dense freshwater in summer enhances stratification in the canal and minimizes vertical mixing, which increases the likelihood of hypoxia in the bottom waters.
- A combination of hypoxia in canal bottom waters, ocean intrusion into the canal of high-density water that displaces the bottom water, and a shift from northerly to southerly winds causes upwelling of low DO bottom waters that causes fish kills in upper waters.
- Climate changes resulting in higher water temperatures, increasing incidence of drought, sunnier summers, and increased winds influence aquatic conditions.
A focused study effort spearheaded by the University of Washington has been launched to determine the sources of low DO in Hood Canal and its interaction with marine life. Researchers from federal, state, local, and tribal agencies are involved in the Program Integrated Assessment and Modeling Study. Results will be used to evaluate and implement corrective actions to restore and maintain a level of DO that will reduce stress on marine life (Bricker et al. 2007).
Reference:
Bricker, S., B. Longstaff, W. Dennison, A. Jones, K. Boicourt, C. Wicks, and J. Woerner. 2007. Effects of Nutrient Enrichment In the Nation’s Estuaries: A Decade of Change. NOAA Coastal Ocean Program Decision Analysis Series No. 26. National Oceanic and Atmospheric Administration, National Centers for Coastal Ocean Science, Silver Spring, MD. Accessed October 2016. https://yosemite.epa.gov/oa/EAB_Web_Docket.nsf/(Filings)/3BE82A42C7ED8C3585257B120059CB8A/$File/Opposition%20to%20Petition%20for%20Review%20–%20Ex.%2010%20Part1…23.53.pdf.