Modelling the relationship between zooplankton biomass and environmental variations in the distribution of ²¹⁰Po 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 ²¹⁰Po during a one-year period (October 1995 to November 1996) in northwestern Mediterranean coastal waters, a modelling analysis was applied. Using ²¹⁰Po concentrations in seawater and zooplankton, the ²¹⁰Po 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 ²¹⁰Po concentrations in zooplankton were reconstructed and validated by observed concentrations. The simulation results were in good agreement with the measured ²¹⁰Po concentrations in zooplankton. Assuming that ²¹⁰Po fecal excretion represents the majority of the excretion of ²¹⁰Po from zooplankton, the fecal matter associated ²¹⁰Po vertical flux was calculated, and compared with the observed vertical fluxes of ²¹⁰Po 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 ²¹⁰Po. The relationship between ²¹⁰Po concentration in seawater and that in rain and dry fallout and their potential effect on ²¹⁰Po concentrations in zooplankton at this location were also examined. A similar, but diphased trend between ²¹⁰Po in zooplankton and ²¹⁰Po in rain and dry fallout deposition rate was demonstrated. ²¹⁰Po concentrations in the dissolved phase of seawater tended to diminish as mean daily rainfall increased suggesting that rain inputs serve as a ²¹⁰Po dilution mechanism in seawater at this location. - Highlights: • Biokinetics calculation used to estimate ²¹⁰Po in zooplankton. • Estimates were in agreement with the measured ²¹⁰Po concentrations. • Fecal pellet vertical transport could not explain observed fluxes at 150 m depth. • Other sinking aggregated biodetritus are also important. • ²¹⁰Po in seawater and that in rain and dry fallout were also examined.