Summary

The results from this study showed that the concentrations of nutrients in the entire lower Yakima River were high enough at certain times and places to support the abundant growth of free-floating algae (phytoplankton), attached algae (periphyton), and vascular aquatic plants (macrophytes) and that the metabolism associated with this plant growth led to dissolved oxygen concentrations and pH levels that exceeded the Washington State water-quality standards. The abundance and distribution of these aquatic plants, however, varied greatly throughout the lower river and, in the case of macrophytes, between years because of substantial differences in spring streamflow.

Aquatic Plant Conditions

Aquatic plants in the lower Yakima River consisted of phytoplankton, periphytic algae, and several species of rooted macrophytes. Periphytic algae were especially abundant in the Zillah reach (RM 116–RM 72) and moderately abundant in the Kiona reach (downstream of RM 47). Periphytic algae were much less abundant in the Mabton reach (RM 72–RM 47) where, because of greater depths and longer residence times, conditions were more favorable for phytoplankton growth. The abundance of macrophytes was greatest in the Kiona reach (where water stargrass was the dominant species) compared to the other two reaches.

Effect of Aquatic Plants on Dissolved Oxygen and pH Conditions

Aquatic-plant assemblages and other organisms in the lower Yakima River had pronounced effects on the dissolved oxygen concentrations and pH levels. The effects of aquatic plant metabolism were observed during the growing season in all three reaches of the lower Yakima River, producing large daily swings in dissolved oxygen and pH during low-flow periods. The daily swings in dissolved oxygen and pH were not as extreme during the high-streamflow snowmelt periods in 2006 and 2007, however, because of the combined effects of dilution from higher streamflow and the decrease in available light for photosynthesis due to deep water and high turbidity.

The daily minimum dissolved oxygen concentrations were less than the Washington State standard of 8.0 mg/L during much of the irrigation season at Kiona (RM 30) during 2004–07, at Mabton (RM 55) and at Zillah (RM 87) during 2005, and at Zillah during 2006, but the period when the dissolved oxygen standard was exceeded at Kiona began earlier in 2004 and 2005 compared to 2006 and 2007 due to high spring streamflows in 2006 and 2007. The daily minimum dissolved oxygen concentrations at Kiona, Mabton, and Zillah were strongly related to maximum water temperature, and this relation could prove useful if a dissolved oxygen predictive model is developed for the lower Yakima River.

The daily maximum pH levels were greater than the Washington State standard of 8.5 at Kiona during almost all of the irrigation seasons in 2004 and 2005 and after the spring runoff periods in 2006 and 2007. Although the pH conditions at Mabton and Zillah were not as severe as those at Kiona, there were extended periods during the irrigation season when the pH standard was exceeded at both locations, especially at Zillah in 2005. The pH standard also was exceeded at Kiona during some periods between October and February during all years of the study.

Nutrient Conditions

The lowest nutrient concentrations generally were in the Zillah reach upstream of the major agricultural returns. Nutrient concentrations generally increased through the lower sections of the Zillah reach and the entire Mabton reach, which contained the major agricultural returns, and remained unchanged or decreased slightly in the Kiona reach. The nutrient concentrations in the Mabton and Kiona reaches during the irrigation seasons in 2004–07 almost always were greater than the reference conditions suggested by the U.S. Environmental Protection Agency to protect water bodies from the negative effects of nutrient enrichment. In contrast to the Kiona and Mabton reaches, the nutrient concentrations measured in the Zillah reach during the 2004–07 irrigation seasons often were less than the suggested USEPA reference conditions. The largest contributors of nutrient load to the Zillah reach were sources that enter upstream of the study area. About equal amounts of the nutrient load in the Mabton reach came from upstream of the reach and from nutrient-rich agricultural return drains and other tributaries, and almost all the nutrient load in the Kiona reach came from upstream of that reach.

Factors Related to Periphyton Biomass

In addition to adequate dissolved nutrient concentrations in the Zillah reach, favorable habitat conditions help explain the abundant periphytic algae in this reach during summer. Light limitation and poor substrate conditions might explain the relatively low abundance of periphytic algae that was observed in the Mabton reach. The clear water and abundant macrophyte growth in the Kiona reach provided favorable conditions for epiphytic algal growth during summer.

Results from this study indicated that the longitudinal decrease in nutrient concentrations measured in the Zillah reach in summer was due to uptake by the abundant periphyton in the reach. Algal growth at some locations in the Zillah reach also might have been limited by the supply of nitrogen during summer. The lack of a positive relation between surface-water nutrient concentrations and algal biomass in the Zillah reach could mean that nutrients were being removed from the water by algae, that some combination of physical factors and (or) grazing by invertebrates were controlling algal growth, or that ground water was providing supplemental nutrients to the algae

Factors Related to Macrophyte Biomass

Differences in light availability appeared to be the reason for the substantially greater macrophyte biomass measured in the Kiona reach in 2005 compared to 2006 and 2007—higher turbidity and deeper water between March and June during 2006 and 2007 led to conditions that were not as favorable for macrophyte growth. These results were supported by the strong negative relation between high turbidity and low spring gross primary productivity (an estimate of the rate of plant growth) in the Kiona reach. Differences in substrate stability, rather than reach-scale differences in nutrient availability, light availability, velocity, or water temperature, may explain the substantially greater macrophyte biomass measured in the Kiona reach in 2005 compared to the Zillah and Mabton reaches. The substrate in the Kiona reach is more stable than the substrate in the Zillah and Mabton reaches, providing conditions that are more favorable to macrophyte growth.

Macrophyte growth in 2005 was not limited by either nitrogen or phosphorus in any of the three reaches. The results from this study indicated that the macrophytes in the Kiona reach were obtaining sufficient nutrients from the sediment in the river bed, which was consistent with the results from other studies that showed that macrophytes with extensive root systems (for example, water stargrass) are able to meet their nutrient needs completely from the bed sediment.

Conclusion

This study evaluated the key factors that controlled aquatic plant growth in the lower Yakima River and the effect that the plant growth had on dissolved oxygen and pH conditions. These complex relations, however, will need further investigation to develop a strategy for improving water-quality conditions in the lower Yakima River.