Abstract :
The phytoplankton and zooplankton populations of a series of temperate region waste stabilization ponds treating dairy effluent were studied over a period of 17 months. The system was organically overloaded. The phytoplankton consisted almost entirely of small, round to oval Chlorococcales (dominated, most probably, by one or more species of Chlorella), with densities of up to ca. 8 million cells per ml. The metazoan zooplankton was dominated by the rotifer Brachionus calyciflorus, in high abundances (commonly ca. 25,000, and up to ca. 160,000 individuals per litre) during the summer months, in the later ponds. Daphnia magna, a zooplankter previously dominant in the system, was not recorded; this was most probably due to the high organic loading. Despite high rotifer densities, the phytoplankton populations and suspended solids were not reduced to low levels by grazing.
