| Previous studies have suggested that extreme climatic events play a decisive role to the human survival. With the aggravation of human impact on the environment, the research on the impact of natural climate and manmade environment to future development of human beings has become to one of the hot research topics. To understand the knowledge of the past environment is one of the most important approaches for predictions of future climate. Geochemistry and biological indicator have established a wide application in paleoenvironmental reconstruction. Especially the biological indicator, which has a direct link to the biosphere, has increasingly displayed its special research value. The micro-organisms from lake sediment have a sensitive response to environmental change. Ostrocods, one of the micro-organisms, have been widely studied as an important index.Ostracod, which is bivalved micro-crustaceans, can inhabit almost all varieties of aquatic habitats, such as lakes, ponds, streams, rivers, oceans and so on. It is the same to the other crustaceans which moult 8 to 9 times during the life circle. It has good potential as palaeoenvironmental indicators due to their low-magnesium calcite shells which are commonly preserved well in lake sediments. Ostracods assemblages can indicate the specific ecological environment because of its sensitive response to the temperature, salinity (conductivity), elementary composition, nutritional condition and the water depth, size, water residence time, illumination, aquatic plants coverage and the energy level in niches.With the continuous development of global change research, studies on ostracods from the Tibetan Plateau have drawn attention and made great progresses. Some palaeoenvironment reconstruction has been done by using ostracod assemblages from northeastern Qaidam Basin, Qinghai Lake, Qilian Mountain, inland lake basin of northwest and lake basins from south. However, the knowledge on its significance of indicator and the response mechanism is still lacking. Therefore, having a better understanding on it is specially imperative and urgent.This study has come to the conclusions by the analysis on ostracod distribution and assemblage from surface sediment from different water types, including lakes from Heihai, Kusai, Haiding Nuur, Yanhu and lakes from northern Tibetan Plateau (Naqu and Ali regions), river, streams and ponds from their surrounding areas.1) 18 species belonging to 11 genera were identified from 112 surface samples from northern Tibetan Plateau, Heihai Lake in north Kunlun Mountain on the northern margin of the Tibetan Plateau and Kusai Lake, Haiding Nuur, Yanhu and surrounding areas from Hoh Xil. Species of Tonnacypris cf. estonica and Tonnacypris tonnensis are recorded first time on the Tibetan Plateau; Potamocypris spp., Pseudocandona sp., Fabaeformiscandona rawsoni, Fabaeformiscandona gyirongensis, Heterocypris salina, Ilyocypris echinata and Cypris pubera are only found from nothern Tibetan Plateau. According to the study on samples from Heihai and Hoh Xil region, Leucocythere sp. and Ilyocypris cf. bradyi are dominant species, and Leucocythere sp. is recorded widely in study region with the full specific conductivities range (0.6-53.0 mS cm-1); Eucypris mareotica as a typical brackish and saline species distributed in samples with higher specific conductivities (2.8-53.0 mS cm-1); whilst Leucocythere dorsotuberosa, Candona candida and Eucypris afghanistanensis mostly inhabit freshwater to slightly oligohaline water with specific conductivities less than 1.8 mS cm-1. Additionally, the tolerance range and optimum of specific conductivities for each species were determined and made comparisons with the previous record from other regions of Central Asia. The results indicate that the data of species assemblage from fossil ostracod valves have a great potential to provide important information on the past specific conductivity level and more general climate-determined moisture conditions.2) The ratio of total carbonate alkalinity to calcium (alk/Ca, alkalinity-depletion), the ostracod occurrence pattern respecting to alk/Ca and salinity (TDS) and the ostracod distribution pattern with respect to the major cations and anions are determined:Cypris pubera, Pseudocandona sp., Candona candida, Potamocypris spp., Heterocypris salina occur in waters with alk/Ca> 1 and low salinities. Eucypris afganistanensis, Eucypris cf. dulcifons, Fabaeformiscandona gyirongensis, Fabaeformiscandona rawsoni, Leucocythere dorsotuberosa occupy a low to moderate waters with alk/Ca> 1. Ilyocypris cf. bradyi and Limnocythere inopinata occupy waters with alk/Ca> 1 and with alomost the entire salinity gradient. Tonnacypris spp., Eucypris mareotica and Leucocythere sp. occupy the full range of covered alk/Ca ratios, however, they have different preference to salinities:Tonnacypris spp. prefer waters with moderate salinities; Eucypris mareotica prefer waters with moderate to high salinities; Leucocythere sp. occupy the full range of covered salinities.Species of Leucocythere sp., Limnocythere inopinata, Eucypris mareotica, Tonnacypris spp. and Fabaeformiscandona rawsoni are mostly recorded from Na++K+dominated waters, whilst the other species don not display significant distribution pattern with respect to the major cations. Candona candida, Eucypris afganistanensis, Cypris pubera, Pseudocandona sp., Potamocypris spp. and Heterocypris salina distribute in HCO3- dominated waters; Eucypris mareotica and Tonnacypris spp. have a preference to Cl" dominated waters; Leucocythere sp., Leucocythere dorsotuberosa, Limnocythere inopinata, Ilyocypris cf. bradyi, Fabaeformiscandona gyirogensis and Eucypris cf. dulcifons don not show any preference to specific HCO3-, Cl- and SO42-; Fabaeformiscandona rawsoni has a tendency to distribute in either HCO3- or SO42- dominated waters.3) Quite abundant male valves of Limnocythere inopinata with a high ratio of male to female are recorded in lakes from the interior region of the Tibetan Plateau. It is the first report from the Tibetan Plateau and Central Asia with altitute> 3500 m. Valves of Limnocythere inopinata were recorded from 31 lake surface sediment samples. Valves of male were recorded in 17 samples. Valve abundances for male specimens range from 2 to 90 per 100 gram sediment. The ratio between the number of male valves and the sum of male and female valves (♂/(♂+♀)) is in the range from 0.02 to 0.55 (mean 0.35). The tolerance range and optimum of each ecological parameter is determined. The results indicate that males can tolerant higher SO42- concentration compared to females; meanwhile, male specimens have higher optima of CO32-and HCO3-. Since the concentrations of CO32- and HCO3- are the mean contributions to the water alkalinity, their higher optima in male specimens support the assumption that the occurrence of male L. inopinata is related to relatively alkaline water bodies. However, the relationship between the portion of males and each ecological parameter is not significant. This may indicate that the occurrence of abundant males might be impacted by multiple environmental factors. By measuring the length and height of female and male valve, the result indicates that there are no clear boundaries between size groups for the last juvenile (A-l) and adult stages (A). |
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