A study on environmental changes based upon cladoceran assemblages from the core sediments in Chen Co, southern Tibet

摘要

A consecutive sediment core (CC2) was drilled in Chen Co (lake) of southern Tibet using a PISTON sampler In 1998 AD. Cladoceran remains including their species, amount richness and ecological features are analyzed for the top 117 cm sections of the core, upon which 5 cladoceran assemblages are distinguished. CLO is an assemblage without any cladocera in the depth between 117-105 cm. CL1 assemblage (depths between 101-77 cm, ca. 1407-1533 AD) possesses total 9 cladoceran species that appear in the core, and performs the greatest richness in the whole sequence. This assemblage reflected that there was plenty of exotic cold water and organic debrises inputing to the sedimentary site under the warmer conditions. The lake was wide and shallow that satisfied the big hydrophyte living. Cladoceran species and richnesses of CL2 assemblage (depths between 77-29 cm, ca. 1533 - 1831 AD) perform much more decreasing than those of CL1. There was only less Chydorus sphaericus which was acclimatized to wide environmental ranges. This implied that the sedimentary environment is so execrable that most of cladoceran lost their living abilities. Compared with CL1 assemblage, CL3 assemblage (depths between 29 - 10 cm, ca. 1831 - 1941 AD) possesses nearly the same dominant species but lower richnesses. Especially, the cladoceran preferring organic debris also decrease in their amounts. This meant that the surface vegetation in the drainage basin was still poor though the environment turned to be better. Except the 2 zooplankton species, the other 7 species of cladoceran in this core are found in CL4 assemblage (depths between 19- 0 cm, ca. 1941 - 1998 AD), in which the eurythermal species had not performed their competition ability comparing with narrow-temperatures adapted species. It implied that water temperatures were stable and much more influenced by air temperatures tinder the shallow-water conditions. The environmental features reflected by cladocera assemblages of CC2 core are fully supported by the evidences from ostra-coda assemblages, diatom-salinity transfunction, environmental magnetism and relative proxies of the parallel CC1 core. Cladoceran assemblages in the lake sediments of Ti betan Plateau are sensitive to sedimentary environment ant very significant to-the studies of environmental changes.