Zooplankton And Its Use In The Control Of Algal Blooms

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.