Application of subfossil cladocerans(water fleas) in assessing ecological resilience of shallow Yangtze River floodplain lake systems(China)

Majority of shallow floodplain lake ecosystems of the middle and lower reaches of the Yangtze River(China) have gone through serious eutrophication problems over the recent past. The severe environmental deterioration accompanied by cyanobacterial blooms have become major water resource management challenges in the region. An advanced research method is urgently needed to tackle these challenges. The concept of ecological resilience address pressing questions of non-linear dynamics, threshold effects and regime shifts in shallow floodplain lakes, and help manage the ecosystem effectively. Palaeolimnological techniques are important for assessing long term resilience and associated thresholds effects of shallow lake ecosystems. However, the lack of reliable proxy methods available, the assessment of long term ecological resilience of shallow Yangtze River lake systems has become increasingly difficult. Cladocerans(water fleas) play a central role in lacustrine food webs by responding to external drivers and internal ecosystem processes in lakes. Their subfossils are well preserved and becoming one of potential proxy indicators of lake ecosystems change for a longer time scale. This study explores the potential application of subfossil cladocerans and their ephippia in assessing a long term ecological resilience and help better management strategies of lake ecosystems and water resources of the middle and lower reaches of the Yangtze River in China.

Negative effects of Microcystis blooms on the crustacean plankton in an enclosure experiment in the subtropical China

Effects of Microcystis blooms on the crustacean plankton were studied using enclosure experiments during July-September, 2000. Eight enclosures were set in the hypereutrophic Donghu Lake. Different nutrient concentrations through additional nutrient and sediment in enclosures were expected to result in different abundance of Microcystis. From July to early August, the phytoplankton community was dominated by Chlorophyta, Cryptophyta, Bacillariophyta and Cyanophyta other than Microcystis aeruginosa. M. aeruginosa showed a rapid increase during early August in all enclosures and predominated. Crustacean plankton was dominated by the herbivorous Moina micrura, Diaphanosoma brachyurum and Ceriodaphnia cornuta, and the predaceous Mesocyclops sp. and Thermocyclops taihokuensis. During the pre-bloom period, the dynamics of M. micrura population appeared to be mainly affected by the predaceous cyclopoids. With the development of Microcystis blooms, such interaction between M. micrura and cyclopoids seemed weakened, especially when the Microcystis biomass was high. But there was no apparent influence on the interaction between Leptodora kindti and its zooplanktonic prey. The density of two cyclopoids decreased with the enhancement of Microcystis. The density decline of M. micrura was caused by both predation and inhibition by Microcystis. The low food availability of other edible phytoplankton during the blooms led to low densities of both C. cornuta and D. brachyurum by late August. It appears that dense Microcystis blooms exert strong negative effects on the herbivorous cladocerans and the predaceous cyclopoids.

Dynamics of Cladocera Community in a Tropical Hypereutrophic Environment (Garças Reservoir, São Paulo, Brazil)

This study was performed at Garcas Reservoir, a hypereutrophic urban lake within Sao Paulo metropolitan area, Southeast Brazil. This reservoir underwent to a marked limnological change after the harvest of free floating macrophytes, shifting to a hypereutrophic state. Present purposes were to characterize the Cladocera community and verify its association with the water deterioration after the macrophytes removal period as well as to compare it to previous studies, before macrophytes harvesting. Samplings were collected from October 2007 until September 2008 and the results were compared to data obtained during pre-removal period (1997). Principal Component Analysis (PCA) showed that the highest values for water transparency were associated with the macrophytes pre-removal period while the highest values of electrical conductivity, chlorophyll a and total phosphorus were associated with the macrophytes post-removal period, indicating the degradation of the water quality. During this period, large cladocerans disappeared and the small sized species of Bosmina predominated. Male organisms of this genus were found as well as elevated densities of Bosmina huaroensis, suggesting overcrowding. Furthermore, it was detected a reduction of 82% in the number of species compared to data from 1997. The shifts to the hypereutrophic state led to aggressive alterations in Cladocera community, suggesting a strong stress on populations.