1. Temperature‐ and time‐dependent mortalities were studied and modelled in insects exposed in regimes with constant and alternating temperatures. In these experiments, freezing was not a cause of death. 2. Survival rates at a range of constant low temperatures (– 5 to + 1 °C) and for different exposure periods (1–14 days) were measured in the summer acclimated springtail Orchesella cincta. 3. Daily interruptions of the cold exposure with short intervals at high temperature reduced mortality or slowed the increase of mortality. This effect was stronger at higher temperature (19 vs 5 and 12 °C) and increased with the duration of the interruption (0·25–2 h). 4. The injury was reversible when the cold exposure was limited to 2 days. 5. Survival in desiccated animals (14% water loss) was reduced. 6. It is suggested that the mortality of summer acclimated springtails is caused by a complex metabolic disorder and membrane changes at low temperatures.

The effects of equitoxic mixtures of CuCI,, ZnCI,, and CdCI, on the filtration rate of the freshwater mussel Dreissena polymorpha were determined. The amounts of added metals were expressed in toxic units, and a range of toxic units was tested. C u + Zn was less than concentration additive, Zn + Cd was concentration additive, and Cu + Cd was more than concentration additive in their effects on the filtratioil rate of Dreissenapolymorpha. An equitoxic mixture of all three metals was concentration additive. The effect of a mixture could not be predicted from the effects of the single metals. The concentrations at which Cu and Cd contributed to the toxicity of a mixture of Cu + Zn + Cd were at or below the NOECs for these metals, determined in single-metal toxicity tests. At low but elevated Cu and Zn concentrations in water, no accumulation of these metals in the mussels took place; Cd, on the contrary, was accumulated at all Cd concentrations in the water.

In order to study the short-term ecotoxicity of metals to the freshwater mussel Dreissena polymorpha, the effects of Cu, Zn and Cd on the filtration rate of this mussel were determined in laboratory experiments. Filtration rate was chosen as the endpoint, because it is a sensitive sublethal parameter compared to mortality and it is an important parameter given the ecological role D. polymorpha fulfills. The filtration rate was calculated from the decrease in algal concentration, fed to mussels in aquaria, containing different metal concentrations. The EC50 for Cu (41 microg litre(-1)) was lower than for Cd (388 microg litre(-1)) and Zn (1350 microg litre(-1)). The NOEC(accumulation) for the essential metal Zn was higher than for the essential metal Cu. Cadmium, a non-essential metal, was accumulated at all elevated water concentrations, so the NOEC(accumulation) was the concentration in the control water (<0.2 microg litre(-1)). All (no) effect concentrations found in this study were above the quality criteria set for metal concentrations in Dutch surface water, suggesting that the zebra mussel is sufficiently protected by these quality criteria.

In order to evaluate ecological consequences of the long-term presence of metals in aquatic ecosystems, we investigated the filtration rate and survival of zebra mussels (Dreissena polymorpha) during chronic exposure to Cu and Cd. The filtration rate was measured once a week in laboratory experiments lasting 9-11 weeks. The lowest Cu concentration tested (13 micrograms/L) did not affect the filtration rate and survival of D. polymorpha, but the lowest Cd concentration (9 micrograms/L) did affect the filtration rate, but had no effect on survival. The EC50 for Cd decreased markedly from 388 micrograms/L to 27 micrograms/L when the exposure time was lengthened from 48 hours to 10 weeks. The largest decrease in EC50 for Cd was observed during the first week of exposure. In contrast, the EC50 for Cu did not decrease with increasing exposure time (chronic EC50: 43 micrograms/L). Since the chronic LC50 for Cd was 130 micrograms/L, the filtration rate appeared to be a far more sensitive endpoint for ecotoxicological laboratory experiments than mortality. D. polymorpha was capable of regulating the body concentration of the essential metal Cu at low concentrations in the water (13 micrograms/L). Cd was accumulated at every Cd concentration in the water, suggesting that Cd could not be regulated by D. polymorpha. It is concluded that the relation between short-term and long-term ecotoxicity was different for each metal and could not be predicted from the results of the short-term experiments.

Abstract. The effects of food quality on growth and body composition of the collembolan Orchesella cincta (L.) (Entomobryidae) were investigated to determine optimal values for dietary nitrogen.Hyphae of Cladosporium cladosporioides containing 2.2%, 2.7%, 4.0% and 6.2% nitrogen were used as food sources.These different nitrogen contents were obtained by adjusting the medium on which the fungi were grown.Collembola were fed on one of these four food sources for 8 weeks. O.cincta grew faster and reached a higher asymptotic body mass when fed on fungi with 4% or 6.2% nitrogen content: this effect decreased with higher nitrogen content.The lipid concentration in the animals was negatively correlated with dietary nitrogen content, whereas protein concentration was positively correlated.Glycogen concentration was not found to correlate with the composition of the food. A mechanism is proposed by which O.cincta compensates for low dietary nitrogen by consuming more to maintain a relatively constant amount of protein in its body and, as a result, accumulates more fat than animals fed on fungi with a higher nitrogen content.

The effect of differences in food composition on growth, body composition and cold tolerance of the isopods Porcellio scaber (Latreille) and Oniscus asellus (L.) has been studied. The effect on reproduction of P. scaber was included. Total lipid and protein content remained the same in O. asellus, given food with a nitrogen content of 2.4, 2.5, 2.8 or 3.1% for 8 weeks. Those fed on 3.1% N gained signi®cantly more weight than those fed on 2.4, 2.5 or 2.8% N. Cold tolerance after a period of low temperature acclimation could not be tested, due to high mortality at the subzero treatment. Haemolymph osmolality increased after the winter acclimation for all food treatments. Juvenile P. scaber, given food of 1.8, 2.5 or 3.1% N, showed no difference in growth over 18 weeks, nor in body composition over 14 weeks. Diet did not in¯uence cold tolerance ability and haemolymph osmolality remained the same with changing cold tolerance of the animals. Gravid females of P. scaber were separated to examine the effect of diet on reproduction. Diet did not in¯uence the number of juveniles, nor their weight. In all food treatments, a signi®cant correlation was found between the weight of the female and the number and weight of her offspring.

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