The significance of growth in Chironomus tentans sediment toxicity tests: relationship to reproduction and demographic endpoints

In the Chironomus tentans 10-d growth test, changes in larval growth relative to sediment contamination are often ascribed ecological relevance by assuming that such changes become manifest at the population level through effects on reproductive output even though the relationship between growth and reproduction is poorly understood. The objective of this study was to evaluate the relationship between growth and reproduction in C. tentans and to use these data in a demographic model to predict the growth and size of a theoretical population. Growth was manipulated by varying food supply (0.29, 0.33, 0.38, 0.42, 0.46, and 0.5 mg/individual per d). The test was initiated with 12 newly hatched larvae per replicate and carried through one complete generation. Larval growth and survival were determined at 20 d. and reproduction was monitored daily during emergence. Food supply did not significantly affect survivorship at any life stage; survival of larvae at 20 d, pupae, and adults exceeded 83%, while survival of larvae in the reproduction replicates exceeded 65%. Both larval and adult dry weight declined significantly with a reduction in food supply. Total emergence was reduced at the lowest feeding level only, whereas the rate of emergence declined at food supplies below 0.42 mg/individual per d. Based on the relationship between larval and adult dry weight, a minimum larval tissue mass of between 0.5 and 0.6 mg dry weight/individual appears to be necessary before emergence can take place. The number of eggs/female declined significantly with a decrease in food supply below 0.42 mg/individual per d. Above this level (corresponding to larval dry weight of approximately 1.5 mg/individual). the addition of more food had no effect on reproductive output. Fecundity (number of daughters/female) and expected number of progeny declined linearly with reduced food supply. Application of the data in a demographic model showed that the growth and predicted size of a population would decline significantly with a decline in larval growth and reproductive output. These results demonstrate that changes in growth, positive or negative, can be used to make meaningful predictions regarding reproduction and population dynamics in the midge C. tentans.