Responses of phytoplankton to in-situ nutrient enrichment: Potential Influences on Species Dominance in a River

Wiley- VCH Verlag GMBH
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Journal Article
International Review of Hydrobiology, 2001, 86 (3), pp. 283 - 296
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The Hawkesbury River at Sackville, New South Wales, Australia is fresh and vigorously mixed by tidal movement. The location has frequent blooms of Microcystis aeruginosa, which have been recorded occurring throughout the year, including winter temperatures as low as 13 °C. Nutrient enrichment tests were performed in-situ on the natural phytoplankton population in 1997 and 1998 while Microcystis aeruginosa dominated (covering both summer and winter periods). These experiments compared population changes under the ambient nutrient regime with those after additions of ortho-phosphate, nitrate, ammonia and various combinations of these nutrients. Under ambient conditions, the Microcystis population was able to grow significantly (P < 0.05) while most non-cyanobacterial phytoplankton did not. Nutrient additions induced a variety of nutrient limitation responses that often varied between genera of major groups i.e. in the Chlorophyceae (Actinastrum sp. responded to phosphorus while Psephonema sp. responded to nitrogen). The possibility that shifts in population dominance from Chlorophyceae to the Cyanobacteria (M. aeruginosa) at Sackville are in response to competition for limiting nutrients is discussed
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