Developmental stage-specific life-cycle bioassay for assessment of sediment-associated toxicant effects on benthic copepod production

In chronic bioassays of sediment organic compounds, toxicant exposures often decline through time, such that the beginning of a test yields disproportionately higher exposures than the end. Thus, those life stages initiating a test often are exposed to the highest concentrations, and for rapidly maturing test fauna, this may lead to varying conclusions regarding compound toxicities depending on the initial life stage chosen. This problem can be addressed by comparative full life-cycle tests initiated with different test-organism life stages. Thus, a full life stage-to-life stage toxicity test was developed for the rapidly maturing meiobenthic copepod Amphiascus tenuiremis to assess the importance of developmental stage at the onset of sediment toxicant exposure relative to reproduction, net population growth, and sex and age structure. Tests were conducted with a model spiked-sediment insecticide, chlorpyrifos, for each of the major life stages (P-1) of A. tenuiremis (nauplius, copepodite, and adult). Each P-1 stage was allowed to mature and reproduce in low chlorpyrifos concentrations (6-33% of stage-specific 96-h LC50s; 4-22 ng chlorpyrifos/g dry sediment) for 26 d. Test endpoints were numbers of surviving adult females, males, eggs per female (clutch), first generation (F-1) nauplii, F-1 copepodites, F-1 total production, and realized F-1 production per surviving female. Only the copepodite P-1 test showed a significant decline in survival of an adult age class: females declined by 28% at 22 ng/g. Reductions in total production ranged from 33-96% of controls from nauplius to adult. The P-1 naupliar stage was most sensitive, with F-1 production being 33-47% of that in controls. However, on a realized production per female basis, both the copepodite and naupliar P-1 yielded significantly reduced F(1)s of 23 and 40% of controls at 11 and 22 ng/g.