| The Lake Qiandaohu, a large reservoir (artificial deep lake) in the lower reaches of the Yangtze River, locates in Chun’an County, Hangzhou City, Zhejiang Province. This artificial lake was formed in 1959 when Xin’anjiang hydropower Station, China’s first self-designed and homemade device-equipped power Station, was built. Because of few point source pollution, the water quality of Lake Qiandaohu is better than other lakes. But wide attention was drawn in years 1998 and 1999 when large algal blooms occurred. So understanding the Lake Qiandaohu entire ecosystems’main ecological processes (material circulation, energy flow, etc.) may provide a new thought for the study of lake eutrophication.In this paper, by using the ecological stoichiometry, we studied the temporal and spatial patterns of nutrient composition(C, N, P content) and element ratios (C:N, N:P, C:P) in the pelagic food weds (waterbody-phytoplankton-zooplankton) of Lake Qiandaohu. Based on the quantitative analysis of stoichiometric aspect, this work also focused on the trophic relaotionship of the ecosystem, biogeochemical cycle of main nutrients in the lake and consumer (zooplankton) driven nutrient recycling. The results are shown as follows:1. The concentration of TC, TN and TP were, in decreasing order, S1>S4>S9. For the three sampling stations, the annual mean concentration of TC were 8.23mg/L,7.30mg/L,7.11mg/L, with values of TN (1.06mg/L, 0.94mg/L, 0.87mg/L) and TP (0.030mg/L, 0.013mg/L, 0.009mg/L) calculated likewise. On the whole, TC content were higher in Autumn and Winter than Spring and Summer, and values were higher in Spring and Summer (comparing with Autumn and Winter) for TN and TP content in each sampling station.TC concentration were higher in the surface layer and also peaked in the water depth of 20-30m.TN content basically increased with the deeper depth from 0.5m to 25 or 30m and TP concentration showed little difference among each water depth.The annual mean C: N ratio differed little among the three sampling stations (7.90, 7.84 and 8.19, respectively). However, the N:P ratio (57.59, 85.37, 101.35 ) and C:P ratio (490, 671, 822) varied a lot with the highest values in S9 and lowest values in S1.2. The C% and N% of seston varied little among the three sampling stations, and the P% were the highest in S1 than the other two sampling stations. Among different seasons, the C% of seston were higher in Autumn and Winter, N% in Spring were significantly more than the other seasons and P% displayed high values in Spring and Summer. In the three sampling stations, the ratios of C:N (5.61, 5.63, 5.53), N: P (14.00, 15.11, 15.62) and C: P (78.22, 84.54, 86.31) did not differ a lot. Unlike the water body, the element ratios in the seston were relatively stable.3. No special pattern was found in the C% of zooplankton, which were stable throughout the whole year. For N%, the values were higher in Autumn and Winter and P% were higher in Spring and Summer.4. The storage capacity of C, N, P showed different proportions in water body, seston and zooplankton: TC (water body 96.84%, seston 1.18%, zooplankton 1.98%), TN (water body 94.76%, seston 1.59%, zooplankton 3.65%), TP (water body 80.05%, seston 5.07%, zooplankton 14.88%). It can be seen that zooplankton had the potential in the storage of P.5. The change of C:N:P (mainly of N:P)of the water body induced the corresponding change in the seston so that cause the changes of zooplankton community structure. Besides, copepods were found to be dominated when the N:P ratio was higher in water body and seston, and cladocerans were dominated when the N:P ratio was lower.6. The N:P ratios presented variation between zooplankton and their prey- seston with higher values in zooplankton though the difference was not high. This difference resulted the consumer-driven nutrient recycling. Zooplankton contributed a lot in the recycling of N:P and exacerbated the limitation of P in the seston, with the higher values in Autumn and Winter. |
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