Ingestion rates of Daphnia magna Straus (Crustacea : Branchiopoda : Anomopoda) on bacterioplankton and phytoplankton in an aerated waste stabilisation pond

The impact of zooplankton on waste stabilisation pond (WSP) performance has been poorly studied until now Zooplankton grazing activity is, however, worth considering as it can control the bacterioplankton and phytoplankton, which are the cornerstone organisms of the WSP treatment technology. The aim of the present study was to determine whether the grazing activity of the dominant zooplankter, Daphnia magna, can significantly control phytoplankton and bacterioplankton in a WSP (Differdange, Grand-duchy of Luxembourg). The biomass of phytoplankton (B-phyto) bacterioplankton (B-bact) and Daphnia magna (B-Daphnia) were estimated fortnightly from January to July 1998. At four key moments during this period, the ingestion rates of phytoplankton (IRphyto) and bacterioplankton (IRbact) by D. magna were determined and compared to B-Phyto and primary production (P-prim), and to Bbact and bacterial production (P-bact), respectively. From January to June, IRPhyto varied between 66 and 92 \% of B-phyto.day(-1) and between 2 and 90 \% of P-prim. In July, the grazing impact on phytoplankton increased and reached 171 \% of B-phyto.day(-1) and 310 \% of P-prim, causing a significant drop in B-phyto The grazing impact of D. magna on bacterioplankton vaned between 0.1 and 18 \% of B-bact.day(-1) and between 0.8 and 226\% of P-bact. Even when higher than 100\% of P-bact, the grazing impact did not lead to a significant decrease of B-bact, because of a significant input of allochtonous bacterial biomass with the influent.