Modelling the relationship between zooplankton biomass and environmental variations in the distribution of (210)Po during a one year cycle in northwestern Mediterranean coastal waters.

To clarify the relationship between zooplankton biomass and the environmental kinetics of the natural radionuclide (210)Po during a one-year period (October 1995 to November 1996) in northwestern Mediterranean coastal waters, a modelling analysis was applied. Using (210)Po concentrations in seawater and zooplankton, the (210)Po uptake rate constant from food for zooplankton was evaluated using a biokinetics calculation involving the uptake and the excretion rate constants between seawater and zooplankton. Using the transfer constants obtained, the (210)Po concentrations in zooplankton were reconstructed and validated by observed concentrations. The simulation results were in good agreement with the measured (210)Po concentrations in zooplankton. Assuming that (210)Po fecal excretion represents the majority of the excretion of (210)Po from zooplankton, the fecal matter associated (210)Po vertical flux was calculated, and compared with the observed vertical fluxes of (210)Po measured in sediment traps. The modelling evaluation showed that fecal pellet vertical transport could not fully explain the observed sinking fluxes of particulate organic matter at 150 m depth, suggesting that other sinking biodetrital aggregates are also important components of the plankton-derived vertical flux of (210)Po. The relationship between (210)Po concentration in seawater and that in rain and dry fallout and their potential effect on (210)Po concentrations in zooplankton at this location were also examined. A similar, but diphased trend between (210)Po in zooplankton and (210)Po in rain and dry fallout deposition rate was demonstrated. (210)Po concentrations in the dissolved phase of seawater tended to diminish as mean daily rainfall increased suggesting that rain inputs serve as a (210)Po dilution mechanism in seawater at this location.