Seasonal structure of zooplankton in high rate ponds and residence time influence
High rate ponds for wastewater treatment have been the focus of much attention since their creation by W.J. OSWALD in the early 1960's. These aquatic ecosystems are hypereutrophic because of massive nutrient introduction by wastewater influents. These kinds of ponds are very different front natural biological ponds because of short residence times (2 to 12 days), shallow depths (<1 m) and constant mechanical mixing which improve alga : growth. This study presents the following objectives : 1) to characterize seasonal successions of zooplankton in this ecosystem where they are unknown and 2) to study residence time influence on zooplanktonic successions in function of seasons. Our investigation consists in studying two similar high rate ponds of 48 m2 surface area and 35 cm depth constantly stirred by paddle wheels. Only residence time is different. The first pond has eight days residence time and in the second one, it changes with solar radiations and organic matter concentrations. The influents come from a primary pond of eight days residence time, fed continuously with domestic influents front a small town, Mèze. The aim of this experiment is to compare seasonal successions in these two high rate ponds (called A and B) with different residence times. Zooplanktonic investigation has been carried out for two years. One sample was collected twice a week in primary pond effluent and two others samples collected once a week into the two high rate ponds near the outflow areas. The results of chronological clustering take inter account the discontinuities of zooplanktonic structure. We obtain six sequences in B pond and ten groups in A pond. Stepwise discriminant analysis shows that Crustaceans and Rotifers are respectively responsible for chronological clustering in B and in A pond. During the sterling period of wastewater treatment and in autumn and in winter, photosynthesis is low and zooplankton is composed mainly of Protozoa : free-living or fixed Ciliates in great concentrations (until 2,7.107 ind. l-1) and Rhizopods (Amæba sp.). These organisms have important food needs (bacteria and algal detritus). During spring and summer, as solar radiations become important, Protozoa are replaced by metazoan grazers. Spring or summer with short residence times increase Rotifers or Daphnia magna production white summer with longer residence times increase Cyclops sp and cypris ovum production. All these organisms are active components of wastewater treatment : the consumption of particulates by zooplankton Increases TSS and COD removal efficiencies (PIZAY-PARENTY, 1985). Therefore, because of the filtering activity of Crustaceans (especially of Daphnia magna), it seems possible to conceive a new pond managment, with residence time changes, favourable to exponential algal growth and consequently to crustacean biomass production. It is easier to harvest this biomass (with nets) than to harvest algal biomass, which still presents problems.