Derivation of ecotoxicity thresholds for uranium

Assessment of the risk of impact from most radionuclides is based on the total radiological dose rate to the organism of concern. However, for uranium (U) there can be greater risk from chemical toxicity than radiological toxicity (depending on the isotopic composition). Chemical ecotoxicity of U is dependent on several environmental parameters. The most important are carbonate content, because of the formation of soluble carbonate complexes, and divalent cation content (Ca++ and Mg++), because of their competitive interaction with the uranyl ion (UO2++). This study summarizes the literature available to set PNECs (predicted no-effect concentrations) for chemical toxicity of U to non-human biota. The corresponding radiological doses were estimated, and as expected chemical toxicity proved to be the greater concern. There were limited data from some types of biota; however, PNECs for the types of biota of interest were as follows: terrestrial plants--250 mg U kg(-1) dry soil; other soil biota--100 mg U kg(-1) dry soil; freshwater plants--0.005 mg U L(-1) water; freshwater invertebrates--0.005 mg U L(-1) water; freshwater benthos--100 mg U kg(-1) dry sediment; freshwater fish at water hardnesses of: <10 mg CaCO3 L(-1) (very soft water)--0.4 mg U L(-1) water; 10-100 mg CaCO3 L(-1) (soft water)--2.8 mg U L(-1) water; and >100 mg CaCO3 L(-1) (hard water)--23 mg U L(-1) water; or as a function of hardness--0.26 (hardness as mg CaCO3 L(-1); mammals--0.1 mg U kg(-1) body weight d(-1).     

Asymmetric sibling rivalry and nestling growth in red-winged blackbirds (Agelaius phoeniceus)

Many birds hatch their offspring asynchronously, and the adaptive significance of this trait, if any, is controversial. David Lack suggested long ago that by facilitating brood reduction when resources are scarce, hatching asynchrony provides relief from the effects of overcrowding. Some field workers interpret this to mean that the growth and survival of survivors should rise following partial brood loss. Here we show in a 6-year study of red-winged blackbirds (Agelaius phoeniceus) that the presence or absence of marginal offspring in experimentally manipulated broods had virtually no effect upon the growth of core offspring, whereas alterations of the size of core brood had strong and significant effects. Nestling growth was, not surprisingly, slower in broods with partial brood loss. Intriguingly, marginal offspring showed significantly greater variation in mass. Core offspring are less sensitive to, but not exempt from, the inimical effects of resource shortfall than are marginal offspring. The phenotypic handicap appears to marginal offspring a caste of high-variance progeny whose fitness prospects rest upon levels of parental input (stochastic resources) and the size of the core brood (stochastic development). Received: 21 June 1999 / Revised: 5 June 2000 / Accepted: 25 June 2000     

The secondary adjustment of clutch size in red-winged blackbirds (Agelaius phoeniceus)

More than a half century ago, the British ornithologist David Lack suggested that parent birds may use brood reduction to track uncertain food, a process facilitated by the asynchronous hatching of their young. Lack sketched the logic of asymmetric sibling rivalry: the phenotypic handicap imposed upon last-hatched marginal offspring renders their growth and survival conditional upon uncertain ecological conditions while buffering first-hatched core offspring from the inimical effects of overcrowding during periods of stringency. Though subjected to numerous indirect tests in short-term studies, the central prediction of Lack's hypothesis - that parents use marginal offspring to track unpredictable brood-rearing conditions and thus achieve a secondary adjustment of clutch size - has never been tested directly. Here we present the results of a 7-year study of marsh-nesting red-winged blackbirds (Agelaius phoeniceus) showing that (1) brood size tracks interannual variability in growth and survival of nestlings, (2) the growth and mortality of marginal but not core offspring is contingent upon stochastic environmental conditions (mean air temperature) during brood rearing, (3) the mortality of marginal but not core offspring is strongly affected by developmental uncertainty in the form of both experimental and natural alterations of brood size, (4) the phenotypic handicap of hatching asynchrony buffers core offspring from poor growth conditions, but (5) its effects upon marginal nestlings are reversible when growth conditions are favourable and especially when brood size is reduced either experimentally or via hatching failure. The presence of marginal offspring ensures that blackbird parents are not left with a too small brood when brood-rearing conditions are favourable. Parents create two castes of progeny: marginal offspring that are strongly affected by both ecological and developmental stochasticity, and core offspring that are not.