THE SIGNIFICANCE OF GROWTH IN CHIRONOMUS-TENTANS SEDIMENT TOXICITY TESTS - RELATIONSHIP TO REPRODUCTION AND DEMOGRAPHIC END-POINTS

In the Chironomus tentans 10-d growth test, changes in larval growth r elative to sediment contamination are often ascribed ecological releva nce by assuming that such changes become manifest at the population le vel through effects on reproductive output even though the relationshi p between growth and reproduction is poorly understood. The objective of this study was to evaluate the relationship between growth and repr oduction in C. tentans and to use these data in a demographic model to predict the growth and size of a theoretical population. Growth was m anipulated 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. Larv al growth and survival were determined at 20 d, and reproduction was m onitored daily during emergence. Food supply did not significantly aff ect 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 si gnificantly with a reduction in food supply. Total emergence was reduc ed at the lowest feeding level only, whereas the rate of emergence dec lined at food supplies below 0.42 mg/individual per d. Based on the re lationship between larval and adult dry weight, a minimum larval tissu e mass of between 0.5 and 0.6 mg dry weight/individual appears to be n ecessary before emergence can take place. The number of eggs/female de clined significantly with a decrease in food supply below 0.42 mg/indi vidual per d. Above this level (corresponding to larval dry weight of similar to 1.5 mg/individual), the addition of more food had no effect on reproductive output. Fecundity (number of daughters/female) and ex pected number of progeny declined linearly with reduced food supply. A pplication of the data in a demographic model showed that the growth a nd predicted size of a population would decline significantly with a d ecline in larval growth and reproductive output. These results demonst rate that changes in growth, positive or negative, can be used to make meaningful predictions regarding reproduction and population dynamics in the midge C. tentans.