Long and short term investigations have revealed dramatic changes in fish communities after impoundments by reservoirs. However, the process of these changes during impoundment remains poorly studied. In 2005 and 2006, before and during the second stage of impoundment of the Three Gorges Reservoir (TGR), in the upper reaches of the Yangtze, China, we investigated fish catches to assess the impacts of reservoir filling on fish community structure. We conducted sampling at 2 sites, Wanzhou reach, which had been inundated by the first water filling in 2003, and Fuling reach, which still remained a riverine site before this water filling. The results revealed an evident and immediate shift of fish communities from lotic to lentic components. In the Wanzhou reach, during the filling, the abundance percentage of lentic fish increased to 94% compared to 69% before the filling. In the Fuling reach, after the filling, lotic fish declined in abundance from 99% to 85%, while lentic fish increased from less than 1% to 12% in abundance. Based on our investigation and the regulation scheme of the TGR, we predicted that after the completion of the Three Gorges Dam (TGD), there would be typical lacustrine, transitional and riverine zones in TGR, with their corresponding fish assemblages. We suggest that long term monitoring should be conducted to evaluate the future ecological effects, and different strategies implemented in these three zones for conservation purposes.
Infrared absorption spectra associated with the CO 2 asymmetric stretch vibration have been recorded for weakly bonded gas-phase complexes of CO 2 with HF, DF, HCl, DCl, and HBr, using tunable diode laser spectroscopy and a pulsed slit expansion (0.15 X 38 mm 2 ) that provides > 20 MHz overall resolution. Results obtained with C0 2 -HF are in agreement with earlier studies, in which the HF-stretch region near 3900 cm -1 was examined. In both cases, broad linewidths suggest subnanosecond predissociation. With CO 2 -DF, the natural linewidths are markedly narrower than with C0 2 -HF (e.g., 28 vs 182 MHz), and this difference is attributed to slower predissociation, possibly implicating resonances in the case of CO 2 -HF. Both CO 2 -HF and CO 2 -DF exhibited overlapping features: simple P and R branches associated with a linear rotor, and P and R branches containing doublets. As in earlier studies, the second feature can be assigncid to either a slightly asymmetric rotor with Ka = 1, or a hot band involving a low-frequency intermolecular bend mode. Results obtained with CO 2 -HCl are in excellent agreement with earlier microwave measurements on the ground vibrational state, and the ~ibrationally excited state is almost identical to the lower state. Like CO 2 -DF, linewidths ofC0 2 -HCI and CO 2 -DCl are much sharper than those ofC0 2 -HF, and in addition, C0 2 -HCl and CO 2 -DCl exhibited weak hot bands, as were also evident with C0 2 -HF and CO 2 -DF. Upon forming complexes with either HF or HCl, the asymmetric stretch mode of CO 2 underwent a blue shift relative to uncomplexed CO 2 , This can be understood in terms of the nature of the hydrogen bonds, and ab initio calculations are surprisingly good at predicting these shifts. Deqteration of both HF and HCl resulted in further blue shifts of the band origins. These additional shifts are attributed to stronger intermolecular interactions, i.e., deuteration lowers the zero-point energy, and in a highly anharmonic field this results in a more compact average structure. While both HF and HCl complexes exhibit nearly linear geometries,C0 2 -HBr is asymmetric, with the Br-C symmetry line essentially perpendicular to the CO 2 axis, and the H atom probably localized near one of the oxygens. Although the moments of inertia are insensitive. to the location of the H atom in CO 2 -HBr, Bose-Einstein statistics require that odd K = states are missing for C 2. symmetry, as is observed with Tshaped CO 2 -(rare gas) complexes. However, we observe a full complement of odd and even Ka states, indicating that the H atom is not located symmetrically about the C 2. axis on the time scale of the measurement. With CO 2 -HBr, the low gas-phase acidity ofHBr and the high Br-atom polarizability encourage a qualitative change in the geometry relative to CO 2 -HCl and CO 2 -HF. This has valuable implications for photoinitiated reactions in such complexes.
A high resolution rovibrational absorption spectrum of the weakly bonded CO2–DBr complex has been recorded in the 2350 cm−1 region by exciting the CO2 asymmetric stretch vibration with a tunable diode laser. The CO2–DBr band origin associated with this mode is 2348.2710 cm−1, red-shifted by 0.87 cm−1 from uncomplexed CO2. The position of the hydrogen atom is determined from differences in moments-of-inertia between CO2–DBr and CO2–HBr, i.e., by using the Kraitchman method. From this, we conclude that ground state CO2–H(D)Br has an average geometry that is planar and inertially T-shaped, with essentially parallel HBr and CO2 axes. Average values of intermolecular parameters are: Rcm=3.58 Å, θBrCO=79.8°, and θHBrC=93.1°. The validity of using the Kraitchman method, which was designed for use with rigid molecules, with a floppy complex like CO2–HBr is discussed. The experimental structure is corroborated qualitatively by results from Mo/ller–Plesset second-order perturbation calculations, corrected for basis set superposition errors. The theoretical equilibrium geometry for the inertially T-shaped complex is planar with structural parameters: RCBr=3.62 Å, θBrCO=89°, and θHBrC=86°. A number of cuts on the four dimensional intermolecular potential surface confirm large zero-point amplitudes, which are known to be characteristic of such systems, and these cuts are used to estimate tunneling splittings. Tunneling is shown to occur by out-of-plane rotation of the H atom, in accord with the experimental observations of Rice et al. There is no significant in-plane tunneling. A quasilinear hingelike isomer (OCO–HBr) with ROH=2.35 Å at equilibrium is calculated to be as stable as the T-shaped complex; however, this species has yet to be observed experimentally. Photoinitiated reactions in CO2–HX complexes are discussed.
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