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Modeling the trophic transfer of beta radioactivity in the marine food web of Enewetak atoll, Micronesia Dalsgaard, Anne Johanne Tang


An approach for modeling the trophic transfer of beta radioactivity within the marine food web of Enewetak Atoll, Micronesia, Central Pacific is described. From 1948 to 1958 this atoll was used by the US military for testing of nuclear weapons while monitoring the impact on the ecosystem. In parallel to these military operations, a marine laboratory was operating on the atoll, hosting a wealth of scientists performing basic research. Probably the most renowned study was carried out by H.T. Odum and E.P. Odum in 1954, who examined the trophic structure of the windward reef community and its productivity per unit area. Based on this study and on the vast amount of scientific literature on the atoll, a mass-balance trophic model of the windward reef was constructed, based on the Ecopath modeling software. Ecopath uses as its basic inputs the biomass, production/biomass, and food consumption rates of the various functional groups in the ecosystem, along with a diet matrix. Based on these inputs it estimates the flow of biomass between the functional groups and presents the corresponding predation mortalities in a matrix where the columns represent the intake of, and the rows the losses of, biomass from the groups. A set of first-order differential equations, relating the intake and loss of biomass to the amounts of radioactivity in the groups, was then set up. The equations were integrated over time and calibrated by minimizing the sum of squared deviations between the observed and predicted levels of radioactivity, thus mapping the transfer of radioactivity onto the transfer of biomass. The original food web / mass-balance model, which was constructed without reference to the data on radioactivity, was subsequently re-calibrated to achieve a match between the food web and the radioactivity data. The results predict that there is a time lag between the observed maximum o f radioactivity and the trophic position of the groups, and that beta radioactivity is not bioaccumulated up through the food web. Finally, suggestions on how to incorporate the approach as a general routine into the Ecopath software are given.

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