During the year 1991 an intensive study was made of parameters governing phytoplankton dominance and succession in wastewater oxidation ponds in relation to their photosynthetic properties under environmental stress. The most successful phytoplankton species was Chlorella vulgaris based on its high photosynthetic potential and its relatively modest contribution to suspended solids. It bloomed during spring after which euglenoids and cyanobacteria became increasingly important. In late summer the green alga Ankistrodesmus became increasingly dominant. This alga strongly contributed to a rise in total suspended solids and had a low photosynthetic capacity. Its dominance appeared to be linked to a relief of ammonium stress on the system following lower organic loading during summer months. A potential for primary productivity in the presence of sulfide was present throughout the year. The pattern of activity correlated with blooms of Chlorella and coccoid cyanobacteria. Two cell types of Chlorella vulgaris were observed occurring in a stable ratio throughout the year. One type is fully phototrophic and only slightly affected by presence of organic solutes like acetate. Its photosynthesis has a pronounced resistance to sulfide and ammonia stress. The second type is auxotrophic and capable of exploiting various organic carbon sources. The presence of organic carbon strongly reduces photosynthesis in this type. Both Chlorella types appeared to be highly adapted to the environmental stress imposed on photosynthetic phytoplankton in the oxidation pond inlet. This stress was defined as being derived from anoxic conditions lead to elevated concentration in volatile fatty acids (acetate, lactate etc.), ammonium and sulfide.

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