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 ofte n decline through time, such that the beginning of a test yields disproport ionately higher exposures than the end. Thus, those life stages initiating a test often are exposed to the highest concentrations, and for rapidly mat uring test fauna, this may lead to varying conclusions regarding compound t oxicities depending on the initial life stage chosen. This problem can be a ddressed 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 tenui remis to assess the importance of developmental stage at the onset of sedim ent toxicant exposure relative to reproduction, net population growth, and sex and age structure. Tests were conducted with a model spiked-sediment in secticide, chlorpyrifos, for each of the major life stages (P-1) of A. tenu iremis (nauplius, copepodite, and adult). Each P-1 stage was allowed to mat ure and reproduce in low chlorpyrifos concentrations (6-33% of stage-specif ic 96-h LC50s; 4-22 ng chlorpyrifos/g dry sediment) for 26 d. Test endpoint s 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 de clined 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 sensit ive, with F-1 production being 33-47% of that in controls. However, on a re alized production per female basis, both the copepodite and naupliar P-1 yi elded significantly reduced F(1)s of 23 and 40% of controls at 11 and 22 ng /g.