Search

In many large European rivers, the number of typical riverine insect species, such as mayflies, stoneflies, and caddisflies, is greatly reduced compared to historic records. This can no longer be explained by high concentrations of a relatively small number of dominant toxicants since many rivers have changed from heavily contaminated systems with a few selected key toxicants to systems with a complex contamination. This contamination consists of many substances in low concentrations coinciding with other unfavorable conditions, such as low oxygen concentrations. It was hypothesized that the joint adverse effects of such multiple stressors may be a steering factor in the distribution of riverine insect species. The aim of this study was therefore to determine the combined effects of toxicants and oxygen depletion. To this purpose, larvae of the indigenous riverine mayfly Ephoron virgo were exposed to two different model toxicants, copper and diazinon, under normoxia and hypoxia (50% air saturation) conditions. The median effective concentrations for mortality for copper were significantly lower in the hypoxia treatments than in the normoxia treatments. For diazinon, no differences were observed between two treatments differing in dissolved oxygen levels, and therefore we argue that interactions between multiple stressors may be compound specific. It is concluded that the combination of toxicants and lowered oxygen may have a stronger impact than can be expected based on the adverse effects of the individual factors and that standard toxicity tests may be insufficient alone to determine the impact of human activities on the ecological state of riverine communities. Instead, attention needs to be paid to more environmentally realistic nonoptimal conditions in toxicity testing to adequately fulfill the needs of ecological recovery programs.

The toxic unit concept is commonly applied for determining the effects of a mixture of toxicants. Use of this concept is, however, limited to compounds with similarly shaped dose-response relationships. In the present study, a method is proposed to gain insight regarding the influence of differences in the shapes of dose-response relationships on the applicability of the concentration-addition model by judging additivity at different effect levels. To this purpose, two model toxicants with different modes of action and dose-response relationships were selected: copper, and diazinon. Using mortality of the mayfly (Ephoron virgo) as the endpoint, it was demonstrated that the two compounds act in a less-than-concentration-additive manner. Application of the proposed calculation method revealed that the less-than-concentration-addition effect was independent of the effect level on which the mixture was judged. [ABSTRACT FROM AUTHOR]

Several variables determine the impact of a pesticide on aquatic invertebrates. In this study, aquatic insects were subjected to the common insecticide diazinon; we analyzed the variables taxon, developmental stage, and exposure time. Effects of diazinon on the caddis fly Hydropsyche angustipennis and the midge Chironomus riparius were determined in the laboratory during different exposure times (48 and 96 h) using mortality, activity, and growth as end points. Last instars of both species displayed a clear behavioral response at concentrations much lower than those affecting survival. Doubling the exposure time from 2 to 4 d decreased survival of midges and caddis flies by a factor 1.4 to 8.4. The 96-h 50% lethal concentrations were: 1.3 mug/L (first instar of the caddis fly), 29 mug/L (fifth instar of the caddis fly), 23 mug/L (first instar of the midge) and 167 mug/L (fourth instar of the midge). Within the spectrum of tested insects (nine species for which 48-h 50% lethal concentrations have been reported in the literature), H. angustipennis is the second most sensitive, and C. riparius the most tolerant species. However, the ranking of species strongly depends on the developmental stage; differences between species are often smaller than differences between instars of one species. Thelarge difference in sensitivities between young and old larvae implythat the impact of a pesticide strongly depends on the season of occurrence. Runoff from pesticide applications on crops is more likely to occur in spring and summer and may have a relatively greater impacton insect communities since young larvae prevail in these seasons. In addition, recovery of typical riverine insects such as H. angustipennis from incidental exposure will be slow, considering their relatively long life cycle. [ABSTRACT FROM AUTHOR]