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Meiobenthos occur at densities of >106/m2 in almost every uncontaminated estuarine sediment of the world, yet few studies have explored their utility for a multiple-species chronic bioassay of sediment contaminant effects. A new approach using laboratory-reared, whole-sediment meiobenthic microcosms was developed to holistically test the sublethal effects of chlorpyrifos on benthic copepod survival, age structure, and reproduction. Entire sediment-dwellingmeiofaunal communities from a pristine estuarine preserve were cultured (i.e., net population gains were achieved in controls) for at least a full generation to the known 96-h copepod adult 25% lethal concentration and larval 50% lethal concentration of sediment-associated chlorpyrifos (21-33 Mu g/kg sediment). As an internal bioassay control, we added and cultured 100 gravid benthic copepods (Amphiascus tenuiremis) in each microcosm to assess the exact rather than relative effects of chlorpyrifos on population growth and dynamics. All but one copepod species reproduced and was cultured successfully for at least 21 d using these techniques. We found no chlorpyrifos effects on total meiobenthic copepod densities, but the predominant naturally occurring copepod Microarthridion littorale, known to be a major prey item of many juvenile fish, was significantly reduced in some chlorpyrifos-spiked microcosms. Other naturally occurring copepods were either unaffected or their growth was enhanced in the presence of chlorpyrifos. Amphiascus tenuiremis, known to be adversely affected at this concentration in 96-h static bioassays, increased dramatically in every microcosm. Species-specific responses of endemic copepods were masked when combined into the more commonly used coarse taxonomic category oftotal copepods. [ABSTRACT FROM AUTHOR]