氮磷比对水华蓝藻优势形成的影响

Influence of nitrogen-phosphorus ratio on dominance of bloom-forming cyanobacteria(Microcystis aeruginosa)

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(0.240d-1),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/P比对生长速率已经没影响.可见,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.

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.