Partitioning Snake River organic matter and modeling aerobic oxidation in Brownlee Reservoir, Idaho

A practical approach for estimating organic matter boundary conditions for ecological water quality models was applied to Snake River, Idaho, data collected at 7 river and 3 tributary locations during 2002. Total organic carbon was partitioned into detritus and biomass (bacterial and algal). Median algal biomass, ranged up to 0.8 mg C L-1 with highest levels in the lower eutrophic Snake River. Median bacterial biomass, estimated using heterotrophic plate counts, ranged from 0.03 to 0.48 mg C L-1 , with acridine orange direct counts producing medians ranging from 0.06 to 4.8 mg C L-1. Detritus was divided into labile and refractory fractions using 5-day biochemical oxygen demand and chemical oxygen demand, referred to as a labile ratio. Consistent with primary production, the median particulate labile ratio ranged from 12% to 45%. Though algal levels were relatively high, the dissolved labile levels were relatively low, with median values ranging from 8% to 27%. A dissolved labile ratio of 10% is recommended when ratios cannot be estimated.; Heterotrophic bacterial growth and respiration were added to an ecological water quality model, CE-QUAL-W2, to simulate aerobic oxidation of labile organic matter. Aerobic oxidation was limited by the availability of labile organic matter, dissolved oxygen, and temperature, while loss of bacterial biomass was simulated as settling and mortality. The bacterial model was applied to Brownlee Reservoir, located on the Snake River between Idaho and Oregon. When model oxidation rates were set to 0.5 and 1.0 day-1 for dissolved and particulate organic matter, respectively, the average absolute mean error was 1.3 mg-DO L-1. This was slightly higher than the error (1.21 mg-DO L-1) produced by the standard model with first-order organic matter decay rates. Monte Carlo analysis indicated that bacterial growth and loss rates contributed to model uncertainty, but the standard model produces similar levels of uncertainty.; The partitioning procedure was also used to estimate available organic phosphorus, which was found to be approximately half the available phosphorus in the Snake River above Brownlee Reservoir. This approach is recommended for water quality studies designed to address eutrophication.