Long-term trends in inflowing chlorophyll a and nutrients and their relation to dissolved oxygen in a large western reservoir

Abstract Naymik J, Larsen CA, Myers R, Hoovestol C, Gastelecutto N, Bates D. 2023. Long-term trends in inflowing chlorophyll a and nutrients and their relation to dissolved oxygen in a large western reservoir. Lake Reserv Manage. 39:53–71. Anoxia in Brownlee Reservoir is one of the numerous water quality issues associated with eutrophic conditions in the Snake River as it flows through southern Idaho and parts of eastern Oregon. The states of Idaho and Oregon have developed total maximum daily loads (TMDLs) for multiple reaches of the Snake River and its tributaries upstream of Brownlee Reservoir intended to address poor water quality. Despite the emphasis on developing TMDLs throughout the Snake River and its tributaries, published long-term trend monitoring to evaluate the results of the TMDLs is lacking. Trends in Snake River concentrations and loads summarized using weighted regressions on time, discharge, and season show that combined efforts to improve water quality upstream of Brownlee Reservoir have realized decreasing trends in concentrations of chlorophyll a, total phosphorus, and suspended solids (80%, 46%, and 61% reductions, respectively) from 1995 to 2021. Brownlee Reservoir, a large mainstem reservoir with short residence time, has responded quickly to inflowing reductions of chlorophyll a and total phosphorus. Since 2005, dissolved oxygen (DO) has improved in the reservoir, with a 33% reduction in the volume of the reservoir having DO less than 1 mg/L. This supports the primary premise of upstream TMDLs and demonstrates that inflowing water quality improvements are effective at improving in-reservoir dissolved oxygen.