ABSTRACT
Various micro- and mesocosms simulating the natural environment have been used to study the fate and effects of the insecticide chlorpyrifos. Literature was reviewed to observe the influence of scale, test design and meteorological conditions on the fate and effects of chlorpyrifos.
The disappearance of chlorpyrifos from water is consistent in all studies, despite variation in system dimensions (9 to 450 m3) and in physico-chemical and biological properties. Chlorpyrifos concentrations were variable during the first hours after introduction of the pesticide into micro- and mesocosms. Therefore exposure prediction during the first day is likely to be relatively inaccurate. The decrease in chlorpyrifos concentration in water from approximately one to ten days post-application is probably governed especially by partitioning processes and therefore depends on physical, chemical and biological characteristics of the system. After approximately two weeks, the chlorpyrifos concentration in water becomes more dependent on degradation and redistribution processes. Extensive but temporary structural changes in invertebrate composition follow the application of the higher rates of chlorpyrifos. In most studies however, the product has no effect on the physico-chemical characteristics of the water. It is possible that intermesocosm variability, especially that due to the macrophyte biomass at the time of application of the pesticide, obscures subtle effects.
The primary effects of chlorpyrifos were consistent in all studies, even though wide differences were apparent in the composition of the main taxonomic groups at the time of application of the pesticide. Direct effects on susceptible species were often concentration-related, and did not depend on the system scale or geographical locations. Given the knowledge of field exposure concentrations and species-susceptibility as determined in the laboratory, it is possible to predict the direct effects that occur at the species or generic level in micro- or mesocosms. At higher taxonomic levels less quantitative conclusions can be drawn. For example, within the zooplankton, although the severity of impact varies substantially from study to study, the effect was almost always most severe for Cladocera and Copepoda, and least severe for Ostracoda and Rotifera. For the macroinvertebrate groups the Chironomidae, Ceratopogonidae, Chaoboridae, Ephemeroptera, Heteroptera, Collembola, and Amphipoda have all been shown to be susceptible to chlorpyrifos. Variation in susceptibility within taxonomic groups higher than species level can be considerable, as shown for some Copepoda, Trichoptera, and Odonata.
Indirect effects of chlorpyrifos in micro- and mesocosms are much more variable, in both direction and magnitude. In some, but not all studies, phytoplankton, periphyton, rotifers, oligochaetes, some mollusc taxa and the isopod Asellus have shown a tendency to increase in biomass or abundance. Reductions in chlorpyrifos-sensitive invertebrate forage species resulted in transient reduced growth of endemic larval fathead minnows. The complexity of natural ecosystems and the lack of qualitative and quantitative a priori information on trophic structure can make prediction of indirect effects very difficult. In the reviewed literature there were no indications of direct or indirect effects on macrophytes, Coelenterata or Arachnida. No mention was made of other taxa.218 219
