Lower Yakima River Eutrophication Study: Year Three Update Presentation to Yakima Basin Science and Management Conference

June 14, 2007

Marie Zuroske, Water Quality Specialist

South Yakima Conservation District

Study partners, objectives
2004 & 2005 recap
2006 highlights
- Rooted aquatic plant biomass
- Relationship between algae and nutrients
2007 questions

3-way partnership: U.S. Geological Survey, South Yakima Conservation District, and Benton Conservation District.
- Multi-agency support: U.S. Bureau of Reclamation, EPA, USDA Agricultural Research Service, Yakama Nation, Dept. of Ecology, Washington State University, Yakima Joint Board, Roza Irrigation District, Sunnyside Valley Irrigation District, etc.
5-year study, objectives include:
- Characterize pH and dissolved oxygen (DO) conditions, nutrient concentrations, type of substrate, and aquatic plant and algal communities.
- Assess relationships between nutrient concentrations, plant growth, substrate, stream flow, pH, and DO for the conditions observed.
Why? Wanted to learn reason for sudden increase in plant growth, was it causing problems, and what could we do about it. Because excess aquatic plants/algae can cause:
- low DO due to nighttime plant respiration and
- high pH due to daytime photosynthesis.

Caveat:
My understanding of nutrient-related processes occurring in the Yakima River has completely changed after each year of this study. I fully expect it to change again after this year.

Water quality: similar results to 04 but much stronger data set (continuous data at 3 sites from March to Oct):
- Very poor DO at Kiona (down to 3 mg/L near dawn on a few days);
- Severe pH at Zillah and Kiona (> 9.5);
- Least impaired reach was Mabton;
- Generally low turbidity. Light generally not limiting plant growth.
Differences in plant biomass did not seem to correspond to differences in DO & pH conditions.
At end of 2005, my biggest concern: had improved water clarity due to improved irrigation practices irreparably harmed the river? Was excessive water star grass here to stay?

Decreased light availability. From Mar 1 to Jun 30:
- Increased turbidity. Median turbidity 1 FNU in 2005 and 16 FNU in 2006.
- Increased depth. Median daily mean gage height at Kiona was 3.7 ft in 2005 and 6.1 ft in 2006.
Increased velocity (personal observation; no data yet).
Excellent timing? In one study in Michigan, water star grass began growing when water temperature exceeded 8ºC. At Kiona, temperatures began to consistently exceed 8ºC in early March 2005 and mid-March 2006.

Hypothesis: Nutrient concentrations at the bottom of the reach might be low enough to limit algal growth because so much of the nutrients have been taken up by algae throughout the rest of the reach.

Abundant water star grass & algae in the Kiona reach resulted in DO concentrations and pH levels of concern in 2004 and 2005. In 2006, water star grass biomass was significantly reduced thanks to runoff from an abundant snowpack. While DO and pH conditions improved, they still did not meet state standards.
Relationship between algae and nutrients in the Zillah reach were more complex than anticipated.
- There was no relationship between nutrient concentrations and algal biomass, even at fairly low nutrient concentrations.
- Nutrient concentrations in porewater were generally higher than in surface water.
- Nitrogen may be the nutrient closest to limiting concentrations.