Abstract
SUMMARY 1. An image analysis technique was developed for the semiautomatic determination of abundance, size distribution and biomass in Daphnia cultures. This allowed detailed observations of growth, demography and biomass accumulation in live populations, avoiding artifacts caused by subsampling and sampling losses.2. The image analysis method gave fast, non‐destructive and reliable individual counts, even in cultures with high density and a large fraction of juveniles.3. In Daphnia, animal width changes with nutritional status and growth within instar, while length changes only at the moult. Thus, estimation of individual biomass using an ellipsoidal model based on animal width gave improved biomass calculations compared to manual counting, sizing, and length : weight regressions.4. The power of the image analysis technique for assessing population growth and size structure was demonstrated in two 40‐day experiments, with Daphnia magna feeding on the green algae Selenastrum capricornutum in a two‐stage chemostat system.
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