Correlation of210Po and210Pb enrichments in the sea-surface microlayer with neuston biomass

Samples of the surface microlayer, of bulk seawater from 20-cm depth and of the 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 the naturally occurring radionuclides²¹⁰Po and²¹⁰Pb. Enrichment of²¹⁰Po in the microlayer compared with the bulk seawater was always observed, and the degree of enrichment was found to be correlated significantly with the neuston biomass per unit volume. Enrichment of²¹⁰Pb in the microlayer was also observed, but only under the higher neuston biomass conditions. The²¹⁰Po:²¹⁰Pb ratio was always higher in the microlayer than in the bulk seawater. Additional information was obtained from²¹⁰Po 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 that the neuston lost²¹⁰Po to the water at a rate of about 1 pCi g^?1 dry biomass h^?1. A significant flux of²¹⁰Po from bulk seawater to the surface microlayer, and thence possibly to the atmosphere, is estimated. This flux is mediated by the biota, and will vary seasonally with the planktonic biomass. Under high biomass conditions a similar flux for²¹⁰Pb may also be significant. An association of²¹⁰Po with the organic cycle at the top of the sea, and with marine bacteria in particular, is suggested.