A food web model for fate and direct and indirect effects of Dursban 4E (active ingredient chlorpyrifos) in freshwater microcosms

Abstract

An integrated fate and effects model was constructed to simulate direct and indirect effects of chlorpyrifos in experimental indoor ecosystems (microcosms) without macrophytes. These microcosms resembled the ubiquitous drainage ditches in the Netherlands. Observed densities of algae, zooplankton and macrofauna were converted to biomass (g DW m−2) for calibration of a bio-energetics model based on functional groups. The fate of chlorpyrifos could be simulated well by calibrating the sorption coefficient for organic carbon, diffusion rate to the sediment and degradation rates in water and sediment. Direct effects were simulated by integrating a sigmoidal concentration-effect relationship with the population growth model for functional groups. It was hypothesized that indirect effects resulted mostly from resource competition. Functional groups were divided in sensitive and insensitive subgroups to allow such model behaviour. Direct effects of chlorpyrifos affected all arthropods in the microcosms which the model could replicate with reasonable accuracy. Recovery of sensitive zooplankton (consisting of cladocerans and copepods) occurred faster in the microcosms than in the simulation. Indirect effects on algae, rotifers, shredders and tubificids resulted from the direct effects as incorporated in the model. Except for algae, simulations showed good qualitative agreement with observed indirect effects, but no exact quantitative match could be obtained.