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

Area of Science:

• Environmental Science
• Geochemistry
• Hydrology

Background:

• Stable isotopes (δ²H, δ¹⁸O) in water are key tracers for hydrological processes like evaporation, precipitation, mixing, and residence time (τ).
• Estuarine residence time (τ) significantly influences biogeochemical cycling, trophic dynamics, and habitat suitability for fish spawning.
• Variability in estuarine τ is driven by complex interactions of river inflow, tides, wind, and water levels, making it challenging to predict.

Purpose of the Study:

• To investigate the relationship between estuarine residence time (τ) and biogeochemical processes using high-frequency, continuous stable isotope measurements.
• To generate high-resolution spatial maps of τ, nitrate, and other water quality parameters in a dynamic estuarine system.
• To calculate whole-ecosystem nitrate uptake rates based on measured spatial gradients of τ and nitrate.

Main Methods:

• Continuous measurement of δ²H and δ¹⁸O using cavity ring-down spectrometry with diffusion sample introduction at a 1 Hz sampling frequency.
• Simultaneous in-situ measurement of nitrate and water quality parameters aboard a high-speed boat equipped with a hull-mounted underwater intake.
• Calculation of residence time (τ) using established methods based on isotopic evaporation signals.

Main Results:

• Achieved high-resolution (∼10 m) spatial mapping of residence time (τ) and nitrate concentrations.
• Observed strong spatial gradients in τ and nitrate that correlated with the geomorphic features of estuarine channels.
• Calculated whole-ecosystem nitrate uptake rates ranging from 0.006 to 0.039 d⁻¹, with a mean measured τ of 30.5 days (range 0-50 days).

Conclusions:

• Continuous, high-frequency stable isotope analysis enables detailed mapping of estuarine residence times and biogeochemical parameters.
• The findings highlight the influence of estuarine geomorphology on spatial gradients of residence time and nutrient dynamics.
• This methodology provides a powerful tool for understanding drivers of phytoplankton abundance, water source mixing, and biogeochemical rates in complex estuarine environments.