Responses of a mixed phytoplankton population to nutrient enrichments of ammonia and phosphate, and some associated ecological implications

Observations suggested that nitrogen and probably phosphorus were major elements limiting the growth of phytoplankton in tropical Lake George (lat. equator, long. 30° 20' E). To test this hypothesis enrichment experiments were carried out, in which ammonia-nitrogen and phosphate-phosphorus were added to freshly sampled mixed populations of phytoplankton. Nitrogen starvation has been inferred from the very high uptake rate of ammonia in the light and also substantial uptake in the dark, implying a reserve of photosynthate unused for growth purposes. Phosphate enhanced these responses. The addition of both ammonia and phosphate increased the respiratory rate of the algae, but not the photosynthetic rate until after a period when growth of the phytoplankton must have occurred. There is some evidence that such growth did not occur until after the second day of experimentation and that responses before this time reflected the demands of the algae in the lake at the time of sampling. Repeated experiments over a period of 6 months showed similar patterns of response to enrichment although quantitatively they were different. A number of ecological implications of the results are discussed.