A coupled, time-dependent, system of diffusion/advection equations was used as the basis for estimating values of the eddy diffusion coefficient Kx from simulated vertical distributions of four state variables, phytoplankton, nitrate, zooplanlcton and detritus. All concentrations were expressed in mmol m−3 of nitrogen, and a conceptual 100 m water column was considered closed at top and bottom. The method was tested first with model output, then applied to quasi-steady-state distributions of chlorophyll a and nitrate concentrations observed in the Celtic Sea on July 7, 1982. Values of Kx in the simulated system were set at 30 m2 day−1 above and below the thermoclme. Estimates for the Celtic Sea were higher near the surface, lower just above the thermodine, and similar at depth. The model value within the thermocline was 0.2 m day−1, whereas the estimate for the Celtic Sea was ∼3 m2 day−1 Finally, equations were modified to allow for non-steady-state conditions noted during an upwelling period in the Tasman Sea between July 10 and July 11, 1987, one tidal cycle apart. The estimated value of K, approached 4700 m2 day−1 at 65 m