An experiment with high rate algal ponds (HRAP) was carried out in the Mediterranean region. Plankton populations and physicochemical parameters were measured weekly over a two year period in a high rate algal pond treating domestic wastewater. The residence time was kept at 8 days. The pond was mixed with a paddle wheel. The objective of the work presented here was to characterise the seasonal successions of phytoplankton and zooplankton in this hypereutrophic ecosystem. There was intensive photosynthetic activity in the pond. Chlorophyll concentrations approached 5 mg/l. Because of the extremely eutrophic environment, phytoplanktonic diversity was restricted (3-12 taxa). Chlorella and Scenedesmus were the predominant taxonomic units. Scenedesmus dominated in Spring whilst Chlorella, with its more rapid reproduction rate, dominated during the months with lower irradiance (winter and autumn) when there was less predation. The influence of seasonal factors on the zooplankton translated into alternating populations dominated by protozoa with a short generation time in autumn and winter and metazoans (with a longer generation time) in spring and summer. A multivariate factor analysis of the variables sampled showed the effect of the physico-chemical variables on the major plankton successions in this ecosystem. A comparison of the successions in this hypereutrophic ecosystem with those observed and modelled by Sommer (1989) in the natural environment showed a good correlation between these two ecosystems despite their differences in hydrodynamic characteristics.

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