Accumulation of 210Po in coastal waters (Gulf of Trieste, northern Adriatic Sea).

The total activity of 210Po was determined by alpha-spectrometry in various samples (matrices) collected in the Gulf of Trieste (northern Adriatic Sea) where fresh water inflows, especially from the Isonzo River in the northern part, affect water quality. Observed 210Po levels were: 1) 0.56-3.75 mBq/L in the dissolved phase (<0.45 µm) in the seawater column and local rivers, 2) 0.35-3.11 mBq/L (400-2300 Bq/kg, dry weight, dw) in suspended particulate matter (SPM, 0.45-20 µm) in the seawater column and local rivers, 3) 40 (Isonzo River) -158 Bq/kg (in a surface sediment cores collected in a NS transect in the gulf and sectioned to the depth of 20 cm) and 4) 239 (autumn) - 415 to 1800 (spring) Bq/kg (dw) in meso(zoo)plankton (>200 µm). In seawater and tributaries, up to 80% (mean 49%) of total 210Po was found in particulate form. In sediments, slightly higher levels were encountered in the Isonzo prodelta and in the central (depocenter) part of the gulf. KD (L/kg) calculated between seawater and SPM, and seawater and sediment amounted to about 5 × 106 and 6 × 104, respectively. Lower autumn 210Po levels can be a consequence of biological dilution by higher mesozooplankton biomass in the autumn compared to spring. Plankton fractionation revealed in general the highest levels in the >200 µm mesoplankton fraction (239-1800 Bq/kg) followed by 50-200 µm (388-996 Bq/kg) and 20-50 µm (318-810 Bq/kg) microplankton fractions. Obtained data show higher 210Po levels in all matrices analyzed in the Gulf of Trieste compared to other Adriatic (central Adriatic) and western Mediterranean areas. The 210Po/210Pb ratios in water, plankton and sediments were mostly below or around 1, while this ratio was much higher at higher trophic levels (up to about 50), reflecting a preferential bioaccumulation of 210Po over 210Pb. 210Po accumulation between seawater and SPM and seawater and mesozooplankton amounted to 3.7 × 104 and 1.1 × 104, respectively, similar to other Adriatic areas. Comparison of the relative importance of pelagic and benthic bioaccumulation pathways, excluding the filter feeder bivalves, suggests greater accumulation in pelagic-feeding species.