Assessing Spatial and Temporal Variability in Aquatic Photosynthesis and Respiration with Oxygen and Carbon Stable Isotopes

Periodic low dissolved oxygen (DO) conditions in the deep water shipping channel (DWSC) of the San Joaquin River at Stockton, CA are an issue of concern because they inhibit the upstream migration of salmon and other anadromous fish. Improved understanding of chemical and biological reactions that contribute to low DO conditions will enable calibration of water quality models geared toward aiding managerial decisions for ameliorating this environmental problem. We apply novel analytical techniques to obtain the isotopic composition of dissolved oxygen (\delta¹⁸O-DO) and of dissolved inorganic carbon (\delta¹³C-DIC) in an effort to quantify photosynthesis and respiration taking place diurnally at different depths in the DWSC. We collected samples from five depths (1m, 3m, 5m, 7m, 9m) at a stationary point in the DWSC every two hours across a thirty-hour period in the late summer, when low DO conditions tend to develop; surface samples were also collected at 9 adjacent sites. This sampling strategy was repeated twice: during a spring tide and during a neap tide. Samples for nitrate and seston isotopes were also collected but have not yet been analyzed. Values of \delta¹⁸O-DO range from +19\permil to +25\permil. Shifts in \delta¹⁸O-DO on the order of 6\permil are evident in the shallow (1m) samples across a diurnal cycle, but are less pronounced at greater depths where values varied between +22\permil and +25\permil. Values of \delta¹⁸O-DO strongly decrease at 1m depth during the late afternoon when photosynthetic activity would have been at an expected maximum, while remaining nearly constant at 9m depth. Values of \delta¹³C-DIC ranged from -9\permil to -11\permil, with more negative values generally occurring with increasing depth; however, the greatest variability in \delta¹³C-DIC was also observed in the deepest (9m) samples. \delta¹³C-DIC values were strongly negatively correlated with DIC concentrations at 5m, indicating more pronounced heterotrophic respiration at that depth, particularly during the daylight hours. These preliminary data suggest depth-dependent correlations between the \delta¹⁸O-DO and \delta¹³C-DIC of these samples. More positive \delta¹³C-DIC values correlate with lower \delta¹⁸O-DO values at the shallow 1m depth, consistent with a photosynthetic signature. At mid-depth in the channel (5m), a correlation between \delta¹⁸O-DO and \delta¹³C-DIC is still apparent yet weaker, and at 9m the correlation breaks down. These preliminary data illustrate the potential of a multi-isotope approach for understanding sources and sinks of DO in hypoxic environments.