利用浮游动物摄食蓝藻水华是生物操纵控制藻类水华的基础理论。根据浮游植物-浮游动物相互关系的体积-效率假说（Size-efficiency hypothesis），浮游动物对尺寸＜100 µm的藻类均有摄食作用，但对藻类的摄食效率与浮游动物的体长有密切的关系。在以水华微囊藻为食物时，浮游动物的不同体型大小对浮游动物滤食率有超过40%的影响力。大型浮游动物Daphnia是藻类最有效的牧食者，因此为控制富营养化造成的藻类水华，就要保持大型浮游动物Daphnia种群的优势地位。针对建立大型浮游动物摄食种群的需要，与经典生物操纵理论的控制滤食性鱼类的方法不同，本研究发现了造成大型浮游动物种群衰败的两个新的因素：
大型浮游动物夏季种群衰败的生理原因-实验中测定一个世代时间内隆线溞(Daphnia carinata)体内CDNB, p-NB, p-NC, EA, Trans等五种不同底物的GST酶活力随生长变化的情况。GSTs酶活力随着生长的下降，如正常食物条件下，隆线溞体内CDNB为底物的GST酶活力由幼体的579.30nmol/min.(mg Protein)下降到成体的230.18 nmol/min.(mg Protein), 下降了60%. 而产毒铜绿微囊藻PCC7820的加入虽然短期内提高了GSTs酶活力水平，却大大降低了浮游动物的生长速度和繁殖能力（同期的隆线溞成体体长为2.17mm而加入微囊藻后体长为1.79mm，下降了17%；作为繁殖力代表的怀卵量下降也非常显著，正常条件下，成体隆线溞第八天怀卵量为9.50个/ind，而在12000cells/ml有毒铜绿微囊藻PCC7820的加入下，每个成体的第八天怀卵量为4.83个/ind）。 但成体GSTs酶活力仍然保持稍低或略等于幼体的水平，这种GSTs酶活力水平的下降可能预示着大型浮游动物Daphnia的“生理衰老（Senescence）”，这种“生理衰老”的后果是大型浮游动物的成体死亡率（Adult mortality）居高不下，种群受其影响而衰败。因此很多富营养化湖泊中夏季的大型浮游动物种群衰败，对藻类滤食率下降，藻类水华无法控制。
小型浮游动物取代大型浮游动物成为湖泊中优势种群的原因-为研究这种取代现象的存在，本研究设计了提升不同浮游动物对微囊藻毒素毒素耐受力的实验。实验中用到三种不同体型的枝角类浮游动物：隆线溞(Daphnia carinata，成体体长约2.7mm)；微型裸腹溞（Moina micrura，成体体长约1.0mm）；角突网纹溞（Ceriodaphnia cornuta，成体体长约0.5 mm）进行驯化实验，所用的蓝藻为产毒和不产毒的铜绿微囊藻PCC7820, 产毒微囊藻的产毒能力为：Microcystin-LR 1.34×10-10µg. µm-3。驯化过程持续四个星期：从第一周到第四周，食物中微囊藻的比例由10%提升到 40%。四周的驯化过程结束后，收集怀卵的浮游动物母体所产的幼体进行有毒微囊藻喂养的毒理实验。尽管产毒和不产毒的微囊藻均可造成微囊藻毒素毒素耐受能力的提高，但是小型浮游动物显然有更快的提高效率(角突网纹溞在100%有毒蓝藻条件下LT50 上升了100-194%，远大于微型裸腹溞的35-52%和隆线溞的49-69%)。这说明小型的枝角类浮游动物如网纹溞长期遭受蓝藻水华的过程中提升对微囊藻毒素耐受的能力较强，因而维持较高的存活率和较大的种群，从而取代大型浮游动物成为水体中的优势种群，进而造成了藻类水华的无法控制。
|Other Abstract||Zooplankton and its use in the control of algal blooms
Directed by Prof. Ping Xie
(Institute of Hydrobiology, Chinese Academy of Sciences, Wuhan, 430072)
During the past decades, eutrophication has resulted in a wide increase of cyanobacterial blooms in freshwater lakes，and many efforts have been done to alleviate the progress of eutrophication and the control cyanobacterial blooms.
The use of zooplankton to control cyanobacterial blooms is a basic mechanism of the so-called “traditional biomanpulation”. There is a relationship between the body length of zooplankton and its food items (called as size-efficiency hypothesis): zooplankton can ingest algae smaller than 100 m, but the ingest rate is dependent on the body length of zooplankton. When Microcystis was use as food, the function of anmial length explained 40% of the variance in the fitration rate of cladocerans, Large-sized Daphnia was an efficient grazer of algae. In eutrophic lakes, we need to build a high population of Daphnia to control cyanobacterial blooms. There are two main reasons leading to the decline of Daphnia population.
“Senescence” leaded to the decline of Daphnia population: we measured changes in GSTs activity of Daphnia carinata during their growths. Five different substrates were used: CDNB, p-NB, p-NC, EA and Trans. GSTs activity of Daphnia declined as the animal grew, i.e., when Daphnia carinata was fed with Senedesmus, GST activity toward CDNB was 579.30nmol/min.(mg Protein) for juvenile Daphnia, but declined to 230.18 nmol/min.(mg Protein) for adult Daphnia, a 60% decline. When MC-containing Microcystis aergunosa PCC7820 was added to the food, GSTs activity of Daphnia showed a little rise, but followed by a quick decline in body length and reproductive capacity. When fed with Senedesmus, adult D. carinata had a body length of 2.17 mm, while fed with Microcystis, adult D. carinata had a body length of only 1.79 mm, a 17% decline. Brood size also decreased significantly: in the control, each adult Daphnia produced 9.50 individuals during a period of eight days, while when fed with 12000 cells/ml MC-containing Microcystis aergunosa PCC7820, each adult Daphnia produced only 4.83 individuals. When fed with MC-containing Microcystis, the adult Daphnia showed similatr or lower GSTs activities compared with the juvenile Daphnia, suggesting that GST was a likely source of tolerance. Decrease of GSTs activity in Daphnia may mean “Senescence” during growth. Such “Senescence” leads to higher adult mortality in Daphnia, which causes decline of Daphnia population. As Daphnia population declines, the ingest rate of algae decreases, and consequently cyanobacterial blooms go out of control.
In order to clarify why large-sized Daphnia are usually replaced by small-sized zooplanlton, we conducted an experiment to compare the ability of different zooplankton to develop tolerance to MC-containing Microcystis. Three different cladocerans were used in the experiments: Daphnia carinata (mean adult size 2.7mm), Moina micrura (mean adult size 1.0mm) and Ceriodaphnia cornuta (mean adult size 0.5mm). All cladocerans were pre-exposed to Microcystis strains for four weeks, with a proportion of Microsytis ranging from 10 to 40% in the food. Two Microsytis strains were used: MC-containing Microcystis aergunosa PCC7820 and MC-free Microcystis aergunosa PCC7820. Their newborns were collected for experiments. A pre-exposure to MC-containing or MC-free Microcystis increased tolerance against toxic Microcystis. The marked increases in survival rate and median lethal time (LT50, 100-194% increase, 35-52% in Moina micrura and 49-69% in Ceriodaphnia cornuta) in the M-C population of Ceriodaphnia suggest that small-sized cladocerans may develop stronger tolerance against Microcystis than large-sized ones when both groups are exposed to toxic Microcystis. This leads to a higher survivship of the small sized cladocerans when cyanobateria blooms. In this way, dominance of large-sized Daphnia is replaced by small-sized species, and a lowered filtering rate of algae is followed by burst of cyanobaterial blooms.
To maintain a sufficiently large population of Daphnia in eutrophic lakes is very important for the control of cyanobacteria. In “traditional biomanilation”, the control of zooplanktontivore is a chief measure for the enhancement of Daphnia population, nevertheless, this measure fails to control cyanobacterial blooms in many etrophic lakes. In this study, we proposed two key factors responsible for the decline of large-sized Daphnia population, and it is recommended that measures overcoming the effects of these factors are important for a successful control of cyanobacterial blooms.|