Responses of phytoplankton to in-situ nutrient enrichment; Potential influences on species dominance in a river

The Hawkesbury River at Sackville, New South Wales, Australia is fresh and vigorously mixed by tidal movement. The location has frequent blooms of Mic rocystis aeruginosa, which have been recorded occurring throughout the year , including winter temperatures as low as 13 degreesC. Nutrient enrichment tests were performed in-situ on the natural phytoplankton population in 199 7 and 1998 while Microcystis aeruginosa dominated (covering both summer and winter periods). These experiments compared population changes under the a mbient nutrient regime with those after additions of ortho-phosphate, nitra te, ammonia and various combinations of these nutrients. Under ambient cond itions, the Microcystis population was able to grow significantly (P < 0.05 ) while most non-cyanobacterial phytoplankton did not. Nutrient additions i nduced a variety of nutrient limitation responses that often varied between genera of major groups i.e. in the Chlorophyceae (Actinastrum sp. responde d to phosphorus while Psephonema sp. responded to nitrogen). The possibilit y that shifts in population dominance from Chlorophyceae to the Cyanobacter ia (M. aeruginosa) at Sackville are in response to competition for limiting nutrients is discussed.