Correlation of 210Po and 210Pb enrichments in the sea-surface microlayer with neuston biomass

Samples of surface microlayer, bulk seawater from 20 cm depth and neustonic organisms inhabiting the top 5 cm of the sea, were collected at regular intervals over a period of 17 months at a site 3 km off Monaco and analysed for naturally occurring radionuclides 210Po and 210Pb. Enrichment of 210Po in the microlayer compared with bulk seawater was observed, and found to be correlated significantly with the neuston biomass per unit volume. Enrichment of 210Pb in the microlayer was also observed, but only under higher neuston biomass conditions. The 210Po: 210Pb ratio was always higher in the microlayer than in bulk seawater. Additional information was obtained from 210Po measurements made on the bulk seawater in which the neuston had been collected and in which it had stood for periods of 2 to 4 h. These showed the neuston lost 210Po to the water at a rate of about 1 pCi g-1 dry biomass h-1. A significant flux of 210Po from bulk seawater to the surface microlayer, and thence possibly to the atmosphere, is estimated. This flux will vary seasonally with the planktonic biomass. Under high biomass conditions a similar flux for 210Pb may also be significant. (author)