Plankton as monitors of radionuclides in the South Pacific
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Date
1994-11-16
Authors
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Publisher
Australian Institute of Nuclear Science and Engineering
Abstract
Aims and rationale Zooplankton are known for their ability to accumulate a variety of elements to levels up to 104 times higher than their concentration in associated water. Thus, they have been sampled for use as biomonitors of radionuclides in the waters surrounding the French nuclear test sites at Mururoa and Fangataufa as well as in the more populated areas of French Polynesia (Figure 1). Given the global distribution of fallout from atmospheric testing in past decades it was important to determine background levels within similar latitudes. For this reason, sampling was also undertaken in Australian coastal waters. These organisms also have a fundamental importance in marine foodchains, particularly in relation to biomagnification of isotopes such as polonium to humans, and are also significant factors in the biogeochemistry of radionuclides in the marine environment. This paper summarises our results to date on the concentrations of natural and artificial radionuclides in zooplankton from the two regions, and discusses the implications of the findings to dose assessment in areas of low planktonic productivity. Methods Samples were collected between 18/8/90 and 29/6/92 from waters of the Maquesas (n = 12), Society (10), Tuamotu-Gambier (6) and Austral (2) island groups in French Polynesia as well as from the Gulf of Carpentaria (27) and the waters off Sydney (3) (Figure 1). Depths between the surface and 100m were sampled by trawled plankton nets and the filtered volumes determined using meters positioned at the opening of the nets. Samples were dried at 60°C for 7 days to give an average FW/DW ratio of 7.5. The dried samples were split between laboratories for replicant analyses and quality control. Gamma analyses were performed using shielded, HP Ge, coaxial detector systems and efficiencies determined using mixed nuclide standards in similar geometry to the samples. For the alpha emitters, 239/240Pu and 2l0Po, extra replicates were also analysed by the Service Centrale de Protection Centre les Rayonnments lonisants, France as part of our quality assurance program. Detection efficiency was determined using 238Pu or 208Po yield tracers. Results and Discussion Average zooplankton densities varied from 0.47 to 50.0 mg (DW)/m3. The lowest densities were recorded in samples from French Polynesian waters which were between 1 and 2 orders of magnitude lower than those measured in Australian samples. In all regions Copepods (primarily Calanoid then Podoploid) numerically dominated the taxonomic composition of the samples. As part of the quality assurance program, the results for replicate sample analyses for individual isotopes were linearly regressed between each laboratory. In all comparisons the regression coefficients were highly significant (p < 0.005, r= 0.54 to 0.95). These results imply good quality control despite the errors implicit in such low level determinations and the fact that split, not homogenised, samples were used. The results for specific activities detected by each laboratory showed that:
• I3i>Cs, 137Cs, 60Co and 90Sr were not detected. This in part reflects the low concentration factor for some elements, but also indicates that levels of these isotopes in French Polynesian waters were not high enough to generate significant concentrations in the plankton;
• 239/240Pu was detected in some samples from each region except the Austral Archipelago and the waters off Sydney. The measured low levels ranged from 0.05 to 2.15 Bq/kg DW with the highest levels consistently found in the Tuamotu-Gambier group. Subsequent observations showed that these high values were from samples collected during a single trip in 1991;
• 228Ac was only detected in samples from the Gulf of Carpentaria which is consistent with their collection from a shallow coastal environment. Its progenitor, 232Th, is derived from erosion of continental masses but is quickly adsorbed onto particles and drops out of solution once it enters the marine environment; and
• Average 210Po concentrations were inversely correlated with plankton density (figure 2). This result was consistent with the greater removal rate for Po by adsorption onto sinking biogenic particulates (eg faecal pellets) which are produced more frequently in higher productivity regions. This is combined with the major source of 210Po being decay of atmospheric 222Rn and aerial deposition of its progeny 2l0Pb directly into the oceans. Ingrowth of 210Po then occurs with a 138 day half-life. It is proposed that plankton in lower productivity regions are exposed to and thus accumulate higher concentrations of 210Po. One hypothesis arising from the higher concentration of 2l0Po in plankton is that secondary consumers, eg fish, could be expected to have similarly increased 2l0Po concentrations in lower productivity areas. It follows that humans consuming these fish could therefore be expected to be exposed to higher dose from 210Po due to consumption of these fish. Some support for this hypothesis can be found in the limited data bases for edible fish in low productivity regions around the Marshall Islands, located in the mid North Pacific Ocean. The values from two studies, when compared with global averages for commercial fisheries, indicate higher concentrations in both pelagic and reef fish from the low productivity area. When this result is compounded by the higher intake of fish as a proportion of total diet in these environments and by the proposed changes in the transfer factors for ingested polonium in the latest ICRP models, then the dose from 210Po may become radiologically significant.
Description
Physical copy held by ANSTO Library at DDC 541.382/16.
Keywords
Cesium, Coastal waters, Cobalt, Isotope ratio, Pacific Ocean, Polonium, Radiation monitoring, Strontium, Zooplankton, Food chains, Fallout deposits, Radioisotopes, Australia
Citation
Twining, J. R., Poletiko, C., & Jeffree, R. A. (1994). Plankton as monitors of radionuclides in the South Pacific. Paper presented to the AINSE Conference on Radiation Biology and Chemistry, Cuming Theatre, Chemistry Department, The University of Melbourne, Melbourne, Victoria,16-18 November 1994, (pp. 25-26).