Using Continuous Underway Isotope Measurements To Map Water Residence Time in Hydrodynamically Complex Tidal Environments

Stable isotopes present in water (delta H-2, delta O-18) have been used extensively to evaluate hydrological processes on the basis of parameters such as evaporation, precipitation, mixing, and residence time. In estuarine aquatic habitats, residence time (tau) is a major driver of biogeochemical processes, affecting trophic subsidies and conditions in fish-spawning habitats. But tau is highly variable in estuaries, owing to constant changes in river inflows, tides, wind, and water height, all of which combine to affect tau in unpredictable ways. It recently became feasible to measure delta H-2 and delta O-18 continuously, at a high sampling frequency (1 Hz), using diffusion sample introduction into a cavity ring-down spectrometer. To better understand the relationship of tau to biogeochemical processes in a dynamic estuarine system, we continuously measured delta H-2 and delta O-18, nitrate and water quality parameters, on board a small, high-speed boat (5 to >10 m s(-1)) fitted with a hull-mounted underwater intake. We then calculated tau as is,classically done using the isotopic signals of evaporation. The result was high-resolution (similar to 10 m) maps of residence time, nitrate, and other parameters that showed strong spatial gradients corresponding to geomorphic attributes of the different channels in the area. The mean measured value of tau was 30.5 d, with a range of 0-50 d. We used the measured spatial gradients in both tau and nitrate to calculate whole-ecosystem uptake rates, and the values ranged from 0.006 to 0.039 d(-1). The capability to measure residence time over single tidal cycles in estuaries will be useful for evaluating and further understanding drivers of phytoplankton abundance, resolving differences attributable to mixing and water sources, explicitly calculating biogeochemical rates, and exploring the complex linkages among time-dependent biogeochemical processes in hydrodynamically complex environments such as estuaries.