The long-term effect of artificial destratification on phytoplankton species composition in a subtropical reservoir

1. The response of phytoplankton to the installation of an artificial destratification system in North Pine Dam, Brisbane (Australia) was investigated over an 18 year period (1984-2002); 11 years before and 7 years after installation. 2. An overall increase in phytoplankton abundance was revealed for some groups (in particular, diatoms, cyanobacteria and chlorophytes), but not for others (chlorophytes). Changes in the abundance of chlorophyte functional groups was attributed to eutrophication. 3. A strong spatial gradient in phytoplankton abundance and chlorophyll a was observed, with low abundance in the downstream regions affected by the destratification system which was likely because of light limitation induced by vertical mixing. The upstream region acted as a surrogate for the unaltered state of the reservoir, particularly as an indicator of eutrophication without direct influence from the destratification system. Despite the continuous trend in eutrophication of the reservoir, there has been a definite decrease in the rate of eutrophication (approximately 30%) since the installation of the destratification system at the downstream locations. 4. Correlations of the dominant cyanobacteria Cylindrospermopsis raciborskii with other genera changed after destratification, indicating that prior to destratification the dominance of Cylindrospermopsis was because of its ability to compete for phosphorus, whereas after destratification its dominance was because of its ability to compete for light.