富营养水体大型溞(Daphnia magna)的种群数量对浮游植物的控制效应

The control effects of Daphnia magna population on the phytoplankton in eutrophic water body

"非经典生物操纵"治理水体富营养化的手段在我国大量尝试,结果有成有败,原因聚焦于水溞作为关键种在水体中的种群数量问题。为研究不同密度溞类对包括水华蓝藻在内的浮游植物群落及其生物多样性的影响,以蓝藻占优势的富营养型水体作为研究对象,在实验室可控条件下进行了大型溞的种群密度操纵实验,设置三个处理组:对照组0个·L–1,低密度溞37.5个·L–1,高密度溞125个·L–1。结果显示,与对照组相比,低密度溞和高密度溞添加对浮游植物丰度和生物量具有显著的抑制效应,表明大型溞对浮游植物的直接滤食效力高于营养盐再循环所产生的间接促进效力,这暗示着Daphnia的添加对保持水体的清水态具有较好的效果。此外,高密度溞添加组同时降低了浮游植物群落中蓝藻的比例,并显著降低了浮游植物生物多样性,使一些可以逃避Daphnia捕食的大个体藻类(针杆藻)得以发展,这暗示着Daphnia对水华蓝藻的发生具有一定的抑制效应的同时,过高密度的Daphnia对浮游植物生物多样性的保持不利。

‘Nonclassical biomanipulation＇ is a useful tool for controlling water eutrophication and was widely applied in China. The results of the application are uncertain. Some were succeed, while others were failed. The situation attributed to the population of daphnia in water body. To investigate the effects of Daphnia in different density on phytoplankton community structure and biodiversity, an enclosure experiment was designed with three treatments in 8 days. The three treatments included different Daphnia density （control group： 0 ind·L-1, low density of Daphnia： ind·L-1, high density of Daphnia： 125 ind·L-1）. Our results showed that the low and high densities of Daphnia could significantly suppressed phytoplankton abundance and biomass, which indicated that the directly filter-feeding of Daphnia was more important than the indirectly stimulation for phytoplankton. It suggests that keeping enough population of Daphnia is useful to maintain clear-water state. In addition, high density of Daphnia reduced the relative biomass of cyanobacteria and biodiversity of phytoplankton community. Instead, large phytoplankton developed （such as Synedra sp.） for them escaping from Daphnia predation. The results suggest that Daphnia could suppress bloom of cyanobacteria, meanwhile it is disadvantageous for phytoplankton biodiversity conservation when Daphnia is excessive.