We propose to apply newly developed method of determining estuarine water-mass ages based on radium isotopes to 1) verify estimates of water residence times (Tr) used by more conventional calculations based on water transport; 2) address the paradoxical occurrence of local plankton blooms in estuaries with water residence times shorter than or on the same time frame of phytoplankton division times, and 3) see if water age interacts with nitrogen supply in control of phytoplankton stocks and composition in estuaries.
       One of the critical variables in basic and applied knowledge about estuaries has always been the issue of water residence times (Tr), which are not easily determined.  Historically, a variety of methods have been used, often furnishing widely different values.  In recent years, most studies have relied on estimates from two-dimensional finite-element hydrodynamic models as an improvement on tidal excursion or salinity-based calculations.  A recently developed method for reliable determination of short-lived radium isotopes (²²³Ra: t1/2 =11.4 d; ²²⁴Ra: t1/2 =3.66 d) has made it possible to compare model-based estimates against direct measurements.

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