In situ and laboratory experiments were performed on species of the waste stabilization pond algae Chlorella, Scenedesmus, Euglena and Chlamydomonas to investigate the utilization of various organic acids and sugars, light limitation on species dominance and the toxicity of ammonia and sulphide. Euglena showed the most flexible metabolism and was able to utilise butyrate. Scenedesmus did not grow on any of the substrates in the dark. Pond populations of Euglena and Chlamydomonas were able to incorporate acetate in situ in competition with the heterotrophic bacterial population but Chlamydomonas was the more successful. Light availability affected speciation in facultative ponds where rapid motility by flagellates was a clear advantage. All the algal species were sensitive to sulphide at µM concentrations in a tolerance sequence ChlamydomonasChlorellaScenedesmusEuglena. Ammonia was less toxic than sulphide and the tolerance sequence was ChlorellaScenedesmusChlamydomonasEuglena. It was the undissociated forms of both sulphide and ammonia which were toxic, Toxicity was influenced by pH and this would be a critical factor controlling ammonia inhibition of algal photosynthesis in ponds. The effects of these findings are related to species dominance and pond performance.

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