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741.
Mashhor Mansor 《Hydrobiologia》1996,340(1-3):121-125
After ten years of field surveys on various water bodies ranging from stagnant water ponds, pools and man-made lakes to flowing waters such as rivers, streams and canals, there is a clear evidence of four problematic weeds in Malaysia. These species are Eichhornia crassipes, Salvinia molesta, Lemna perpusilla, and Pistia stratiotes. Among these weeds, E. crassipes and S. molesta are widely distributed througout Malaysia. E. crassipes generally dominates canals and rivers although, recently, this species has spread to man-made lakes. The favourable tropical climate and conducive environmental factors help to trigger the massive growth of these weeds. The high nutrient concentrations, notably phosphate which has a soluble reactive concentration greater than 0.1 mg l–1, initiate a high productivity. Manual control methods are generally used and several herbicides including 2,4-D and glyphosate are frequently employed to eradicate these weeds. 相似文献
742.
As part of the Russian-Finnish research studies on Lake Ladoga, joint expeditions were organized in 1992 and 1993. Water samples were collected for intercalibration of chemical analysis methods and to monitor the chemical quality of the lake water.In August of 1992 water samples were taken from northern Lake Ladoga for intercalibration of Russian and Finnish analysis methods. In August 1993 water samples were collected from 23 sampling stations in all parts of the lake; some of these were also used for intercalibration purposes.The oxygen, colour and CODMn results were at the same level in the intercalibration. In 1993, the Ptot results obtained were acceptable. In Ntot, Fe and Mn analysis there seemed to be systematic and random errors between some results.The Secchi depth ranged from 1.5 m to 3.3 m. The average concentrations for the total phosphorus ranged from 15 µg 1–1 to 29 µg 1–1. The total nitrogen values were from 620 µg 1–1 to 690 µg 1–1. The N:P ratio varied from 24 to 40. The concentration of phosphorus indicated mesotrophic or even eutrophic conditions in the lake. Phosphorus seemed to be the limiting nutrient to bacteria and algae. 相似文献
743.
We studied a recently described method for the determination of total nitrogen in natural waters involving sample oxidation with persulfate and subsequent determination of nitrate-nitrogen with second derivative spectroscopy and compared it to the USEPA approved method involving the sum of nitrate-nitrogen and Kjeldahl-nitrogen as measured with an automated analyzer. The overall objective was to determine if the two methods gave the same estimates of total nitrogen and if the detection limits, precision and accuracy of the new method were as good as those of the USEPA method. The new method was used to make measurements on replicated blanks, standards and lake water samples covering a range of concentrations. We also collaborated with certified laboratories to make comparative measurements on 5 standards and 21 lake water samples that were run by us with the new method and by them with the USEPA method. The new method had an instrument detection limit of 0.07 mg 1–1, and the standard deviation of 20 sets of lake water samples averaged 0.03 mg 1–1. The new method gave concentrations equivalent to those found with the USEPA method, was more sensitive, and had a higher degree of precision. We concluded that the new method is suitable as a substitute for the USEPA method. We also found that the addition of acid to lake water samples stored under refrigeration was not necessary to preserve them for later determinations of nitrate-nitrogen and total nitrogen and that freezing was an effective means of sample preservation for 90 days. 相似文献
744.
A. Vandelannoote H. Robberecht H. Deelstra F. Vyumvuhore L. Bitetera F. Ollevier 《Hydrobiologia》1996,328(2):161-171
Sources of pollution at the north end of Lake Tanganyika are concentrated around Bujumbura, Burundi. The impact of River Ntahangwa on the water quality of this part of Lake Tanganyika was compared with that of an unpolluted river, the Mugere. A shallow bay, not influenced by a river, was used as a reference station. Chemical analyses were carried out at four week intervals during nearly a year. Samples were taken at different distances from the shore and at different depths. River impact was detected up to 100 m from both river mouths. Even at 50 m from the mouth of the polluted River Ntahangwa, no important decrease of dissolved oxygen was detected. Chlorophyll-a maxima were found 100 m from the Ntahangwa mouth, but this was also the case 100 m from shore in the unpolluted bay. Lake water at 60 m depth was saturated with oxygen during the dry season, as it used to be in 1956. The rivers studied descent initially within the lake due to greater sediment load and lower water temperature. Nevertheless, it appears that these rivers mix completely in the lake within 100–300 m from the river mouths. The direct effects from organic pollution of the Ntahangwa on the lake seem to be rather negligible. Appropriate anti-erosion measures should reduce massive sediment discharge into the lake. 相似文献
745.
Luísa M. D. R. S. Martins Armando J. L. Pombeiro Richard A. Henderson 《Inorganica chimica acta》1996,250(1-2):311-315
The kinetics of the displacement reactions of the bromide ligands of trans-[FeBr2(depe)2] (depe = Et2PCH2CH2PEt2) by the organonitrile NCCH2C6H4OMe-4, in tetrahydrofuran (either in the absence or in the presence of added Br−), to give the corresponding mono- and dinitrile complexes trans-[FeBr(NCCH2C6H4OMe-4)(depe)2]+ and trans-[Fe(NCCH2C6H4OMe-4)2(depe)2]2+, have been investigated by stopped-flow spectrophotometry. The substitution reaction occurs by a mechanism involving rate-limiting dissociation of bromo ligands to form the unsaturated intermediates [FeBr(depe)2]+ (k1 = 1.52 ± 0.02 s−1) and [Fe(NCR)(depe)2]2+ (k3 = 0.063 ± 0.008 s−1) which add the nitrile ligand to form those nitrile complexes. The competition between the nitrile and Br− for such metal centres has also been investigated and a stronger inhibiting effect of added Br− is observed for the substitution of the second bromo ligand relative to the first one. The kinetic data are rationalized in terms of π-electronic effects of these unsaturated metal centres and of the bromide and nitrile ligands. 相似文献
746.
747.
We tested the hypothesis that changed microclimate at induced forest edges causes reduced growth of epiphytic lichens. Two
foliose, green algal lichens were transplanted to the lower canopy of a mature Picea abies forest at six distances (2, 6.25, 12.5, 25, 50 and 100 m) from a clearcut. The biomass growth in Platismatia glauca (6.2% in 16 months) was 41% higher than in Lobaria pulmonaria (4.4%). We found no growth reduction near the forest edge. In contrast, the highest growth in both species occurred within
12 m from the edge. Further, fluorescence and chlorophyll measurements showed that lichen vitality was unaffected by distance
from edge. The light intensity was 4.3 times higher at the edge than in the interior during the growing season, but there
were only minor differences in air temperature and relative humidity. Monitoring of thallus water content revealed clear differences
in both number and length of wetting and drying cycles. However, the total time with water content sufficient for photosynthetic
activity was only slightly higher at the edge. The data thus indicate that our gradient in microclimate was too small to significantly
affect lichen growth, and that lichens are largely metabolically inactive when large edge-interior contrasts in microclimate
occur. Lichen response to forest edge microclimate results from intricate interactions among several biotic and abiotic factors.
Linking data on lichen growth, microclimate and thallus water content with physiological measurements provides a framework
for future studies of the mechanisms behind abiotic edge effects.
Received: 15 April 1996 / Accepted: 21 June 1996 相似文献
748.
Gerlinde Linne von Berg Alexander Samoylov Manfred Klaas Peter Hanelt 《Plant Systematics and Evolution》1996,200(3-4):253-261
The utility of chloroplast DNA variation for checking a recently proposed infrageneric classification of the genusAllium was tested. cpDNA restriction patterns of 49 species representing the main subgenera, sections, and subsections of the existing classification were compared. 363 different fragments generated by 4 restriction enzymes were identified and analysed by UPGMA clustering. The resulting phenogram largely confirms the subgeneric classification based on an integration of morphological and other methods. 相似文献
749.
Fungal spore populations in the outdoor and indoor atmosphere of Ismailia have been studied during the period from March 1992
to May 1993. A total of 23 350 cfu and 73 species were recorded,Cladosporium cladosporioides, Aureobasidium pullulans andAspergillus flavus were the most abundant. The indoor and outdoor mycoflora showed marked quantitative and qualitative differences. In view
of count, recorded species could be categorized into three groups as follows: (a) species showing higher counts in out- than
indoor, (b) species showing the opposite trend i.e. lower counts in out-door than indoor, (c) species showing approximately
equal counts in out- and indoor. Regarding seasonal periodicity, March and either September or October showed the highest
count for both normal fungal flora (NFF) and opportunistic fungal flora (OFF). While January and July showed the lowest count
of them both, May but not July was the lowest as for outdoor NFF. 相似文献
750.
An in vitro procedure for large scale multiplication of Sterculia urens Roxb. (Gum Kadaya Tree) has been developed using cotyledonary node segments. An average of 4.0 shoots per node were obtained on Murashige and Skoog's (MS) medium containing 2.0 mgl–1 6-benzyl amino-purine (BAP) within 21 days of initial culture. Upon subsequent subculture 16 shoots/node could be harvested every three weeks and upto three times. Sixty per cent of the shoots were successfully rooted. Rooted plantlets were transferred to plastic pots containing soil under mist house conditions before they were finally exposed to an external environment. Fifty seven per cent of the plantlets survived in nursery sheds. 相似文献