The Interactions Research Between Cyanobacteria And Large-bodied Cladocerans In The Spring-summer Chaohu Lake

Lake eutrophication and cyanobacterial blooms is one of the ubiquitous environmental problems in the world. Researchers at home and abroad began to pay close attention to biological method to control cyanobacterial blooms since "biological control method" was born in the1970s, Cladocerans are the important groups of lake zooplankton, and they can ingest cyanobacteria, therefore, to study their ecological relationship with algae is very necessary. Large-bodied cladocerans(such as Daphnia) have a higher efficiency of filter cyanobacteria, therefore, if we can maintain a certain number of large-bodied cladocerans(such as Daphnia), then control cyanobacterial blooms will become true in a certain extent.But the results tend to show some differences because of different cladocerans.Cyanobaterial blooms often occurred in warm seasons of summer-autumn. During the outbreak of cyanobacterial blooms, the dominance of large-bodied cladocerans (such as Daphnia) was often replaced by small-bodied cladocerans and copepods, even the midsummer decline or disappearence of large cladocerans (such as Daphnia) was observed.This research adopts the combinations of field experiment and laboratory experiments. The seasonal dynamic of large cladocerans and the rise-fall population mechanism of Daphnia during spring and summer (Febury-Jane) in Lake Chaohu were investigated using field investigations and experimental enclosure.The population ecology was used for laboratory experiments, on one hand, to evaluate the effects of colonial Microcystis aeruginosa on interspecific competition and sexual reproduction of two cladocerans. on the other hand, to evaluate the ingestion and utilization of large cladocerans using single-cell of Microcystis,As to reveal the interaction mechanism of cyanobacterial and large cladocerans.The main results and conclusions are as follows:1、Large cladocerans Daphnia hyaline and Daphnia pulex dominant in spring(March-May), the maximum density were79.5ind./L (in April) and30.5ind./L (in April). The density of D. carinata surpass D. hyaline and D. pulex in june and the maximum density was56ind./L and three large cladocerans almost disappeared in July. Perason correlation analysis showed that the density of D. pulex has a very significant positive correlation with total nitrogen and total phosphorus. However, the density of D. hyaline has no relevance with total nitrogen, but has a very significant positive correlation with the total phosphorus. In addition, the density of D. carinata also has very significant positive correlation with chlorophyll a. Perason correlation analysis showed that the density of Daphnia has significantly negatively correlation with microcystin biomass (P<0.05). The results suggested that colonial Microcystis aeruginosa, food quality and quantity are the main reasons for the weakening or disappearance of Daphnia2、The population density of M. irrasa was significantly higher at the higher biomass of M. aeruginosa than at the lower level under both culture types. D. carinata maintained higher population density at the biomass of0-10mg/L for M. aeruginosa under the single-species culture, but was strongly inhibited at higher biomass (20mg/L). M. irrasa dominated over D. carinata in their interspecific competition with increased M. aeruginosa biomass under the mixed culture. The influence of M. aeruginosa on the population density of D. carinata was significant (single-species culture, P=0.017; mixed culture, P<0.001).At0-10mg/L of M. aeruginosa biomass under the single-species culture, the cumulative ephippia number and the percentage of ephippia containing resting eggs of D. carinata increased with increased M. aeruginosa biomass, but gradually declined under the mixed culture. The effects of M. aeruginosa on the cumulative ephippia number of M. irrasa and D. carinata were significant under single-species and mixed cultures (P<0.01). Regardless of colonial M. aeruginosa, smaller-bodied M. irrasa dominated when competed with larger-bodied D. carinata, and this result can be attributed to their different reproductive strategies. These results suggested that interspecific competition inhibited ephippia production and resting egg formation in Daphnia under colonial M. aeruginosa.3、 Daphnia dominant in Spring, One-way ANOVA analysis showed no significant difference between groups of the density of Daphnia (P>0.05). But small Cladoceras, Sinodiaptomus sarsi and the Cyclops dominanted in summer. Even if there is no colonial or filamentous cyanobacteria, large-bodied cladocerans will be replaced by small Cladoceras and copepods. These results suggested that the interference of colonial cyanobacteria may not be the main reasons for the disappear of Daphnia, the small phytoplankton (food) may play an more important role in the disappearance of Daphnia.4、The Chaohu M. aeruginosa significantly inhibited the growth of D. pulex at the low concentrations of Scenedesmus obliquus while a high concentration of the algal diminish this inhibition. D. pulex can use a single M.aeruginosa cells to provide energy at the beginning of the experiment and their reproductive was less affected by the microcystin. The negative impact increased with the accumulation of toxic microcystin, thus the subsequent population growth of D. pulex was inhibited.