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 5. 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 significant 1 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.%通过批量培养实验研究了不同磷水平下N/P比对铜绿微囊藻(蓝藻)和斜生栅藻(绿藻)生长速率的影响,并在太湖蓝藻水华暴发期间,监测了梅梁湾和湖心区水体叶绿素a浓度和氮磷营养盐结构变化,以探讨N/P比对蓝藻优势形成的影响.结果表明,N/P比对铜绿微囊藻和斜生栅藻生长的影响并不表现在一个确定值上,而与水体氮磷的绝对浓度有关,在0.02mg/L磷浓度下,铜绿微囊藻和斜生栅藻在N/P比为4:1～32:1范围内生长速率均较低(0.067～0.074,0.018～0.022d-1),说明受到营养盐的限制；当磷浓度达到0.20mg/L时,铜绿微囊藻在N/P比为32:1时生长速率达到最大值(0.240d1),斜生栅藻在N/P比为64:1时生长速率达到最大值(0.380 d 1)；而在磷浓度升高到2.00mg/L时,不同N/P.比下铜绿微囊藻和斜生栅藻均达到最大生长速率(0.24～0.25,0.378～0.381d 1),说明氮磷浓度均比较充足,N伊比对生长速率已经没影响.町见,氮磷浓度比N/P比对两种藻的生长影响更大.与斜生栅藻相比,铜绿微囊藻对氮磷营养的生理需求和最大生长速率均相对较低,属K策略物种,易在低氮磷浓度下形成优势.梅梁湾在水华暴发期间]氮浓度一直远低于水华较轻的湖心区,而磷浓度远高于湖心区,进而导致梅梁湾N/P质量比(低于20:1)在水华期间一直低于湖心区(124:1),低N/P比是蓝藻水华暴发导致氮浓度下降,磷浓度升高的结果.
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