Long-term change of the assemblages and abundance of cladocerans in different ecotypes of Lake Taihu

Abstract

枝角类作为淡水湖泊生态系统中的初级消费者之一，对生存环境的改变极为敏感.本文分析了太湖西、中和东部等湖区的钻孔沉积物、表层沉积物以及春夏秋季活体枝角类的组成与丰度.结果表明：活体枝角类组成以象鼻溞（Bosmina spp.）为优势种，秋季枝角类属种数量最多.太湖不同生态型湖区表层沉积物枝角类组成均以象鼻溞为优势种，其中西部与中心湖区的枝角类组成与丰度较为相似，种类单一，枝角类绝对丰度高；东部湖区枝角类属种较为丰富，绝对丰度低，优势种由浮游种象鼻溞以及沿岸种圆形盘肠溞（Chydorus sphaericus sl）和西方笔纹溞（Graptoleberis testudinaria）等种属构成.百年以来，太湖枝角类组成与丰度随着营养水平增加而改变，富营养指示种（Bosmina longirostris）丰度的增加与贫营养指示种（Bosmina longispina）丰度的下降，响应了湖区生态环境的演变过程.1970s末期，太湖西部与中心湖区在进入富营养化阶段，枝角类组成单一，象鼻溞占有绝对优势，与东部湖区相比，沿岸种、底栖种稀少.东部湖区在1960s以后，枝角类属种数量增加，但丰度下降，响应了1960s以来该区域营养水平提高、沉水植被生物量增加以及沼泽化加剧的环境过程.;Cladoceran, as one of the primary consumers in freshwater lake ecosystem, is sensitive to environmental change. So far, fruitful researches of subfossil cladocerans have been applied in many fields, such as paleoclimate reconstruction, food web, and nutritional changes and so on. However, most of studies focused on the lakes in boreal, temperate and subarctic zones, especially the alpine and deep lakes. Few works paid attention to freshwater shallow lakes in the middle and low latitudes of subtropics in Asia. Lake Taihu, located in the south of the Yangtze River Delta, is a large shallow lake covering over 2000 km² area which is also the third largest freshwater lake in China. Lake Taihu is ecologically divided into different basins. The west and north parts of Lake Taihu are observed increase of nutrient which has led to the emergence of non-N₂ fixing Microcystis sp. cyanobacterium causing serious cyanobacterial blooms and reduced biodiversity. The eastern part of Lake Taihu is also a mild eutrophic lake under the condition of growing large number of submerge plants. Those changes had influence on cladoceran composition and abundance. In this paper, we analysed the composition and abundance of cladoceran assemblages from cores and surface sediments and modern waters in spring, summer and autumn of the western, central and eastern parts of Lake Taihu. The results showed that Bosmina spp. was the dominant cladoceran of the sediments and water in different ecotypes of Lake Taihu. The composition of modern cladoceran was basically the largest number of cladoceran species in autumn. The cladoceran composition was single but with a high-level abundance in the western and central Lake Taihu. The cladoceran species in the eastern Lake Taihu were relatively rich but with a low-level abundance. The dominant species in the east part of Lake Taihu were composed of several species such as Bosmina spp., Chydorus sphaericus sl and Graptoleberis testudinaria. Over the past century, the composition and abundance of cladoceran in Lake Taihu had changed with the change of ecological environment. Especially, the change of the relative proportion of the eutrophic species (Bosmina longirostris) and the poor nutrient species (Bosmina longispina) indicated the evolution process of the eutrophication in Lake Taihu. At the end of 1970s, the western and central regions of Lake Taihu entered the eutrophication stage. The composition of cladoceran assemblages was single with a high-level abundance of Bosmina spp. and scarce littoral and benthic species compared with the eastern Lake Taihu. After 1960s, the cladoceran community in eastern Lake Taihu became relatively abundant but with a decrease of the abundance of cladocerans, which responded to the expansion of submerged vegetation biomass, swampiness and the aggravating eutrophication since 1960 AD.