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| Influence of nitrogen-phosphorus ratio on dominance of bloom-forming cyanobacteria (Microcystis aeruginosa). |
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Abstract Effects of nitrogen to phosphorus molar ratios (N/P) on growth rates of the cyanobacterium Microcystis aeruginosa and chlorophyte Scendesmus obliquus under various initial phosphorus levels were investigated using bulk culture. The chlorophyll a (Chl a) concentrations and N/P ratios were monitored during a cyanobacterial boom in Meiliang Bay and central Lake Taihu, China. The results showed that the growth rates of M. aeruginosa and S. obliquus did not depend on a specific N/P ratio but on the absolute concentrations of inorganic N and Pin these water bodies. The growth rates of M. aeruginosa and S. obliquus were very low (0.067~0.074/d; 0.018~0.022/d) under N/P ratios ranging from 4:1~32:1 when initial inoganic P concentrations was 0.02mg/L, indicating nutrient limitation. However, when initial P concentration was 0.20 mg/L, M. aeruginosa and S. obliquus showed maximal growth rates at N/P ratios of 32:1 and 64:1 respectively, showing S. obliquus requires more N than M. aeruginosa did. Both phytoplanktons consistently showed maximal growth rates(0.24~0.25/d;0.378~0.381/d) when P concentration was 2.00mg/L at various N/P ratios, indicating N and P were enough for growths of both species. Therefore, N and P concentrations played a more significantl role in controlling growth of M. aeruginosa and S. obliquus than N/P ratios, and N/P ratio alone dcd not appear useful for predicting dominance of this cyanobacterial bloom former. Compared wich S. obliquus, M. aeruginosa required lower N concentration to achieve maxima growth rates. Furthermore, the maximal growth rate was lower than that of S. obliquus. M. aeruginosa was most likely a K-selected species, and tended to be dominate under lower N/P ratio. The cyanobacterial bloom in Meiliang Bay occurred during periods of low inorganic nitrogen (DIN) concentrations and high soluble reactive phosphorus (SRP) concentrations, with a N/P ratio of less than 20:1. Low N:P ratios in Taihu were therefore caused by Microcystis spp. blooms.
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Received: 12 January 2011
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