||Pb-210 and Po-210, a parent-daughter pair, are particle-reactive radionuclides. Pb-210 tends to be associated with inorganic particles but Po-210 prefers organic particles. In the context of these characteristics the purpose of this study is to determine Po-210 and Pb-210 in the surface water plankton of the northern South China Sea (SCS) and the Luzon Strait (LS) areas in order to understand their temporal and spatial distributions and the extent of their radioactive disequilibrium. As the LS has provided a pathway for the exchange between the Kuroshio and the SCS waters, the study area has the characteristics of an open ocean and a marginal sea.|
The plankton Po-210 activities in the study area are about 10~400 dpm/g, but may reach 1200dpm/g in an El Nino year, the effect of which on Po-210 is not clear at present. The Po-210 in the plankton is mainly affected by the surface water Po-210 , biomass concentration, and the Po-210 in the surface water as well as plankton transported from other area(s). If the biomass concentration (as indicated by Chlorophyll-a) increases, the Po-210 in the plankton decreases, i.e. they are inversely correlated. Based on the planktonic Po-210 distribution, the Kuroshio water which has both high Po-210 and high Po-210-bearing plankton has evidently intruded into the northern SCS. The Pb-210 activities of in the plankton vary from 5 to 25dpm/g; the variation trend is similar to that of Po-210 but with much lower activity, resulting in a Po-210/Pb-210 ratio much greater than unity. Compared to the suspended particles, the plankton is highly enriched in Po-210 but it strongly repels Pb-210. Based on earlier studies and this one, the extent of Po-210 enrichment (as indicated by the Po-210/Pb-210 activity ratio) in various organisms increases sequentially from plankton to mussel, large swimmer and then to marine fish, i.e. the higher the level of organism in the food chain, the higher the ratio becomes. However, the Po-210 and Pb-210 activities per unit mass of these organisms are generally lower than those of plankton. Organisms of higher food-chain level may accumulate Po-210 in the digestive system, but part of it may be excreted. Po-210 accumulation does not occur in other parts of these organisms. In terms of adsorption, the plankton with greater surface area to volume ratio, can adsorb more Po-210 resulting in a higher specific activity, whereas large organisms with smaller surface area to volume ratio can adsorb less Po-210 yielding a lower specific activity. The enrichment of Po-210 in various organisms must have been achieved by absorption and adsorption with unknown proportion.
Based on a simple box model calculation for the northern SCS water within the upper 100m layer, the excess Po-210 in the planktonic biomass can account for about 70% of the total deficit in this layer. But in the LS area, the excess and the deficit are balanced. This suggests that the Po-210 deficit in this surface water is due to absorption and adsorption by organisms as evidenced by their large Po-210 enrichment.