Potential For Future Studies

The results from this study could be the foundation for further research into the relations between aquatic plant growth, nutrients, and dissolved oxygen and pH conditions in the lower Yakima River. In addition to the results from this study, three general areas of study might provide water-quality managers with enough understanding of the processes occurring in the river to improve dissolved oxygen and pH conditions.

Water-Quality Models

Models could be developed that can simulate the physical, chemical, and biological complexity of the lower Yakima River. Modeling may be able to estimate the effect of different streamflows on algal metabolism in the Zillah reach (for example, changes in habitat compared to dilution). Modeling also could examine the effect that reduced light availability from high streamflow or high turbidity would have on aquatic plant growth. Models developed to test hypotheses regarding the distribution and abundance of macrophytes throughout the lower river would benefit from an assessment of peak flows (timing and magnitude) and sediment flux to and through the river and the role of other factors (for example, light, biological complexities) on macrophyte growth.

The hydraulic model and water temperature model that were recently developed for the lower Yakima River (Hilldale and Mooney, 2007; Voss and others, 2008) could be used to estimate water temperature, depth, and velocity in the Zillah reach under different streamflow conditions and to relate those conditions to periphytic algal biomass.

Model predictions of dissolved oxygen in the lower Yakima River may be improved by adding the relation between dissolved oxygen and temperature data determined during this study to the water temperature model that was recently developed for the Yakima River between RM 129 and the Prosser Dam at RM 46 (Voss and others, 2008). This model could be extended from Prosser Dam to near the mouth of the river, which would allow for the prediction of daily minimum concentrations throughout the irrigation season in the Zillah, Mabton, and Kiona reaches.

Continuous Water-Quality Monitoring

The monitoring results from this study clearly showed the spatial (longitudinally through the lower river) and temporal (seasonally and yearly) variation in water-quality conditions and provided the data needed to estimate GPP. Continuous water-quality monitoring could provide the data necessary for further insight into the important factors controlling aquatic plant growth in the river and the effect of aquatic plant growth on dissolved oxygen and pH conditions. Data from the monitoring could help in model development and in assessing the effectiveness of future water-quality management actions.

Isotope Studies

An analysis of the isotopic composition of nitrate, organic matter, inorganic carbon, dissolved oxygen, and aquatic plants can provide fundamental information about the processes that influence the growth of algae and macrophytes in rivers. The results from this type of analysis might help determine the sources of nitrogen and carbon (such as wastewater treatment plants, fertilizer, manure, or atmospheric deposition) to the algae and macrophytes, how much of the productivity in the river is due to different forms of plant life (phytoplankton, attached algae, and macrophytes), and the relative importance of different stream organisms (plants, microbes, and insects) in respiration and oxygen depletion.