A CULTURE-BASED ASSESSMENT OF THE EFFECTS OF CHLORPYRIFOS ON MULTIPLEMEIOBENTHIC COPEPODS USING MICROCOSMS OF INTACT ESTUARINE SEDIMENTS

Meiobenthos occur at densities of >10(6)/m(2) in almost every uncontam inated estuarine sediment of the world, yet few studies have explored their utility for a multiple-species chronic bioassay of sediment cont aminant effects. A new approach using laboratory-reared, whole-sedimen t meiobenthic microcosms was developed to holistically test the sublet hal effects of chlorpyrifos on benthic copepod survival, age structure , and reproduction. Entire sediment-dwelling meiofaunal communities fr om a pristine estuarine preserve were cultured (i.e., net population g ains were achieved in controls) for at least a full generation to the known 96-h copepod adult 25% lethal concentration and larval 50% letha l concentration of sediment-associated chlorpyrifos (21-33 mu g/kg sed iment). As an internal bioassay control, we added and cultured 100 gra vid benthic copepods (Amphiascus tenuiremis) in each microcosm to asse ss the exact rather than relative effects of chlorpyrifos on populatio n growth and dynamics. All but one copepod species reproduced and was cultured successfully for at least 21 d using these techniques. We fou nd 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 significant ly reduced in some chlorpyrifos-spiked microcosms. Other naturally occ urring copepods were either unaffected or their growth was enhanced in the presence of chlorpyrifos. Amphiascus tenuiremis, known to be adve rsely affected at this concentration in 96-h static bioassays, increas ed dramatically in every microcosm. Species-specific responses of ende mic copepods were masked when combined into the more commonly used coa rse taxonomic category of total copepods.