Abstract: Plants influence biotic and abiotic conditions in most ecosystems, although their effects in rivers are not yet well-documented. Plants photosynthesize during the day, releasing oxygen, but their nighttime respiration can lower dissolved oxygen (DO) in rivers. In the lower Yakima River (Washington State, USA), water stargrass often grows across the full width of the river, especially in years where spring peak flows are dampened. At the same time, nighttime DO in this reach is often insufficient for migrating salmon, and preliminary data suggest that water stargrass respiration causes the low DO. Under climate change, low nighttime DO concentrations are likely to become worse as the effect of water stargrass respiration is magnified with lower flows and increased biomass. Given the highly regulated nature of the Yakima River, there may be an opportunity to time flow releases in the spring to stall early-season water stargrass growth, potentially resulting in higher DO. We tested this hypothesis by comparing DO concentrations in the lower Yakima River during a high-flow/low-water stargrass year (2022) to a low-flow/high-water stargrass year (2021). In each year, we deployed DO sensors in the lower Yakima River during the summer and fall. We paired the data by Julian date and compared DO concentrations during 6 time periods, with each period spanning 2 weeks. In the low-water stargrass year, peak spring discharge was 1.55 times higher than the spring peak in the low-flow year, and occurred about 5 months later in the year. During the first two time periods (mid-July to mid-August), minimum daily DO was 0.8 and 0.9 mg/L higher, respectively, and diel DO swings (i.e., daily DO maximum minus daily DO minimum) were 4.3 and 2.8 mg/L lower. Despite the higher DO, the site still regularly dipped below Washington State standards for salmonids (8 mg/L). During the last 4 time periods, when water stargrass biomass was similar between the two years, minimum daily DO and daily diel swings were consistent. High spring flows thus do appear to attenuate early-season water stargrass biomass, with resultant higher DO concentrations. However, the biomass attenuation achieved through increased spring flow was not sufficient to meet water quality standards. Overall, our results demonstrate that plants influence oxygen conditions in rivers and that plant biomass can be resilient to early-season disturbances.
