Metazooplankton dynamics and secondary production of Daphnia magna (Crustacea) in an aerated waste stabilization pond

The seasonal dynamics of metazooplankton biomass was monitored in an aerate d waste stabilization pond during three consecutive years (1994-1996). The pond showed a low diversity of planktonic metazoans because of elevated pH, relatively high concentration of free dissolved ammonia and low oxygen con centration. The planktonic community was composed of the anomopod branchiop od Daphnia magna, and the cyclopoid copepods Cyclops vicinus and Cyclops st renuus. Both predation by cyclopoids and competition with D.magna excluded rotifers from the pond, except during a short period in spring 1996. Daphni a magna was the dominant organism from a biomass point of view. In parallel with biomass, demographic parameters, secondary production and the spatial distribution of D.magna were studied. A significant seasonal and interannu al Variation in the density, biomass and production of D.magna was observed . The maximum density of daphnids varied from 264 x 10(3) to 686 x 10(3) in dividuals m(-2) and the maximum biomass from 4 to 30 g dry weight (DW) m(-2 ). The annual net production was high compared with the production of Daphn ia in natural environments, ranging from 288 to 593 g DW m(-2) year(-1). Th e annual net production of exuviae accounted for similar to 25% of the tota l annual net production. Harvesting of daphnids for commercial applications that took place during the productive period did not have any discernible effect on the population dynamics of D.magna. Sexual reproduction was not o bserved during the three studied years. Negative mortality rates, occurring during early spring, however, indicated that recruitment from ephippia was effective in the pond of Differdange and that sexual reproduction took pla ce before 1994. Swarming was regularly observed in relation to high densiti es.