In situ versus laboratory estimations of length-weight regression and growth rate of Daphnia magna (Branchiopoda, Anomopoda) from an aerated waste stabilization pond

The present paper questions the adequacy of using length-weight regressions and growth rates calculated in the laboratory under constant physico-chemical and food conditions for the estimation of biomass and secondary production of animals living in a variable environment from the physico-chemistry and food availability point of view. Length-weight regressions (LWR) and growth rate of Daphnia magna were determined in situ at five key periods of the year. In parallel, LWR and growth rate were determined in laboratory incubators at temperature adjusted to the mean temperature measured during the in situ experiments. LWR estimated from pond daphnids collected during the in situ experiments were, on the whole, not significantly different from LWR established during laboratory experiments, indicating that the food availability was globally similar in the laboratory and in situ experiments, even though food items were substantially different between the experiments. In situ algal biomass was indeed low compared to the algal biomass in laboratory experiments, but high biomasses of bacteria, protozoa and detritus were available for daphnid feeding in the tubes incubated in situ. Growth rate of D. magna was monitored in situ using 50-ml tubes closed with Nylon net (mesh size = 80 mu m) and in the laboratory using 50-ml glass flasks. The physico-chemical, bacteriological and algological variables were checked to be similar in the tubes and in the pond. Growth rates varied according to the size of the animal and according to the water temperature. The maximum growth rates were observed for juveniles at 20.2 degrees C. Growth rates were also determined in the laboratory at temperature corresponding to the mean temperature recorded in the pond during the in situ growth experiments. Differences between in situ and laboratory body length-growth rate regressions (LgR) were observed for the experiments conducted at 15.6 degrees C and 23.6 degrees C. Due to differences in LWR and LgR between in situ and laboratory experiments, biomass and daily production estimated from laboratory cultures were found to be significantly, but not severely, higher than biomass and daily production estimated on the basis of in situ experiments. It has been, therefore, concluded that, when the constraints linked to the realization of in situ growth experiments are too strong, the laboratory approach is fully justified.