| Planktonic algae have the considerable periodicity in species composition andabundance, and this seasonal succession pattern is the important characteristic of algaefluctuations. The mechanisms by which environmental factors affect algae successionare complex. Besides, algae succession varies in lakes or reservoirs of differentlimnological features and trophic state. Although hypotheses of algae succession havebeen stated in some research, there is uncertainity for specific lake and specific species.Lake Qiandaohu is a deep and mountain-valley type reservoir, experiment of AquaticEnvironment Protection Oriented Fishery (AEPO) was carried out in this lake using thesilver carp and bighead carp to promote the water quality since2000. What is the algaesuccession pattern in Lake Qiandahu which has the unique hydrologic condition? Whatis the relationship between algae succession and nutrients in Lake Qiandaoh which hasthe silver carp and bighead carp dominated? To resolve these problems, in this research,we investigated the relationship between the planktonic algae succession and theenvironmental factors in Lake Qiandaohu during2009-2010, and try to know about thesuccession process of planktonic algae structure and the top-down or bottom-up effectson the succession. Besides, we studied the effect of nitrogen and phosphorus ratios onthe growth of cyanobacteria and chlorophyta under the laboratory conditions in order tofurther verify the impact of nutrients on the succession pattern.The main results and conclusions are as follows:1. Through the two years research, a total of251taxa of planktonic algae wasrecorded, which belonged to91genera of8phyla. The primary phyla were Chlorophyta(41genera and115species), Bacillariophyta (22genera and73species), Cyanophyta(16genera and40species). Chlorophyta species composition was abundant in every season, Bacillariophyta was abundant in winter and spring and Cyanophyta wasabundant in summer and autumn. The mean values of algae density and biomass were189.92±60.37×104cell/L and0.6044±0.1513mg/L (for2009:175.45±38.42×104cell/Land0.6839±0.1054mg/L, for2010:204.40±78.75×104cell/L and0.5250±0.1568mg/L).The total dominant species of the two years were18species (6species in Cyanophyta,3species in Chlorophyta,7species in Bacillariophyta,1species in Xanthophyta and1species in Cryptophyta), and the species with relative high donimnace in2009wereFragilaria crotomensis and Cyclotella meneghiniana, in2010were Anabaenaflos-aquae and Cyclotella atomus. From the view of seasonal variation of algae densityand biomass, S1was dominated in Bacillariophyta, S3and S4were dominated inCyanophyta, and the proportion of chrysophyceae rose in S8and S9. For each samplingstations, the blue-green algae peak occurred in late spring (May-June), and changed tothe chlorophyta peak in early spring (July-August), when the autumn come(September-October) blue-green algae peak occurred again, and the rest seasons weredominated in Bacillariophyta. The algae density and biomass were abundant in0.5-8mof the water column and can be stratified from May to October.2. During December and April, the water temperature was found to be almosthomogeneous at different depth and can be stratified form May, and the thermoclinewas about8-16m. Secchi depth (SD) was lowest in riverine zone S1(1.136±0.196mg/L)which was influenced by surface runoff, and SD was found to be much higher in thedownstream. TN concentration in S1(1.136±0.196mg/L) showed the eutrophic state,and the other four sampling stations showed meso-eutrophic state. TP concentration inthe five sampling stations appeared as mesotrophic, and the oligo-mesotrophic state forS9(0.010±0.003mg/L). Correlation analysis of algae and environmental factorsindicated that seasonal variation of algae density and biomass were significantlypositively correlated with water temperature (R=0.567, P<0.01, N=120, R=0.454,P<0.01, N=120), and also had the significant correlation with SD, pH, DO and CODMn.Besides, the density ratio of blue-green algae and chlorophyta had the significantnegative correlation with TN (R=-0.189, P<0.05, N=120), bacillariophyta weresignificantly positively correlated with SiO32-(R=0.209, P<0.05, N=120). During themain growing season (May-October) for algae, chlorophyta density and biomassproportion had the significant positive correlation with TN: TP (R=0.280, P<0.05, N=60, R=0.302, P<0.05, N=60). For the horizontal distribution, a significant positivecorrelation was found between blue-green algae and TN: TP from S4to S9. For thevertical distribution, total biomass were significantly correlated with water temperature(R=0.588, P<0.01, N=324), TP (R=0.113, P<0.05, N=324) and SiO32-(R=0.110, P<0.05,N=324).3. Under the laboratory conditions, we set five N/P ratio treating groups (N/0.25P,N/0.5P, N/P, N/5P and N/20P) based on the BG-11cultures to explore the effects ofdifferent N/P ratio on blue-green algae (Microcystis flos-aquae, Dactylococcopsis sp.)and chlorophyta (Chlorella sp., Scenedesmus Quadricauda) growth. The resultsindicated that the logistic model can well describe the growth of four kinds of algae.Microcystis flos-aquae grew better with the decrease of N/P ratio (the maximumbiomass K and the specific growth rate r increased significantly). ScenedesmusQuadricauda had the higher value of K and r with the increase of N/P ratio.Dactylococcopsis sp. favored the relative high N/P ratio cultures and the highest valueof r at N/5P group which was close to Redfield ratio. Chlorella sp. presented algalbloom under N/5P and N/20P group. At the end of the experiment, the accumulated TNconcentration of Microcystis flos-aquae were the significant higher in N/0.25P andN/0.5P group than others (P<0.05), Scenedesmus Quadricauda had the lower TN underN/20P group than others (P<0.05). TP concentration in four kinds of algal bodysignificantly increased with the decrease of N/P ratio (P<0.05). |
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