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 zooplanlcton biomass and the environmental kinetics of the natural radionuclide Po-210 during a one-year period (October 1995 to November 1996) in northwestern Mediterranean coastal waters, a modelling analysis was applied. Using Po-210 concentrations in seawater and zooplankton, the Po-210 uptake rate constant from food for zooplankton was evaluated using a biokinetics calculation involving the uptake and the excretion rate constants between seawater and zooplanlcton. Using the transfer constants obtained, the Po-210 concentrations in zooplankton were reconstructed and validated by observed concentrations. The simulation results were in good agreement with the measured Po-210 concentrations in zooplanlcton. Assuming that Po-210 fecal excretion represents the majority of the excretion of Po-210 from zooplanlcton, the fecal matter associated Po-210 vertical flux was calculated, and compared with the observed vertical fluxes of Po-210 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 Po-210. The relationship between Po-210 concentration in seawater and that in rain and dry fallout and their potential effect on Po-210 concentrations in zooplankton at this location were also examined. A similar, but diphased trend between Po-210 in zooplankton and Po-210 in rain and dry fallout deposition rate was demonstrated. Po-210 concentrations in the dissolved phase of seawater tended to diminish as mean daily rainfall increased suggesting that rain inputs serve as a Po-210 dilution mechanism in seawater at this location. (C) 2016 Elsevier Ltd. All rights reserved.