HR: 1340h
AN: H13F-1377    [Abstracts]
TI: Diel Stable Isotopic Variation of Dissolved Oxygen, Carbon, Nitrate and Particulate Organic Matter in the San Joaquin River, California
AU: * Doctor, D H
EM: dhdoctor@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd, MS434, Menlo Park, CA 94025
AU: Rollog, M E
EM: merollog@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd, MS434, Menlo Park, CA 94025
AU: Silva, S R
EM: srsilva@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd, MS434, Menlo Park, CA 94025
AU: Kendall, C
EM: ckendall@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd, MS434, Menlo Park, CA 94025
AB: In productive aquatic systems, biological activity produces pronounced diel changes in nutrients and chemistry. We applied stable isotope techniques in order to elucidate the mechanisms responsible for low dissolved oxygen conditions in the San Joaquin River, CA. Measurements of δ¹⁸O of dissolved oxygen (DO), δ¹³C of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC), δ¹⁵N of nitrate, and δ¹³C and δ¹⁵N of particulate organic matter (POM) were made during a diel sampling campaign at a shallow upstream site in 2005 and compared to a similar study in 2004 at a deeper downstream site where low DO conditions occur. At the shallow upstream river site, we observed strong, repetitive, smooth waveform diel variations in DO concentration (between 7.5 to 9.9 mg/L) and δ¹⁸O of DO (+13 to +23 per mil), with the two being directly out of phase. Diel variability is also evident in δ¹³C and C:N ratios of POM. Variations in DO concentration, δ¹⁸O of DO, δ¹³C of POM, and C:N ratios of POM are independent of flow variability. In contrast, NO3, DIC, and major ion concentrations are affected by changes in river discharge as well as biogeochemical processes. Once normalized with Cl- concentrations to remove the effects of flow variability, DIC concentrations also exhibit damped diurnal fluctuation with DIC increasing at night and dropping during the day over a maximum variation of 0.4 mmol C/L. Diel change in DIC concentration is out of phase with shifts in δ¹³C of DIC (maximum change of 2.0 per mil). At the downstream site, DO concentrations and isotopic compositions varied diurnally as well as with depth, however DO concentrations were 5 mg/L lower and δ¹⁸O of DO were 3 per mil higher on average at the downstream site than at the upstream site, indicating a shift toward higher δ¹⁸O of DO with the development of persistently low DO concentrations. Our results indicate that short-term diel changes of DO in this river system are independent of discharge and other chemical variability, and that results of DO monitoring are highly dependent on the time of day of sampling. We also find that stable isotopic ratios of dissolved oxygen, carbon, and C and N of organic matter reveal the rapid dynamics of nutrient cycling in this river.
DE: 0438 Diel, seasonal, and annual cycles (4227)
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 0470 Nutrients and nutrient cycling (4845, 4850)
DE: 0496 Water quality
SC: Hydrology [H]
MN: Fall Meeting 2005