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In order to elucidate factors controlled the water column stratification in canyon-reservoirs, and describe the relative response of water quality, we present a 13-month survey and study of meteorological, physical and hydro chemical patterns during 2001 in a canyon-reservoir, southwestern China, especially including a intensive frequency study during the period from August to November of this year in which an event of water quality abruptly degeneration occurred. The study showed that: an obscure thermal stratification has been formed on vertical profile of water column during summer (about from April to October) in Baihua Lake reservoir, however, it could cause the very strong hydro chemical stratification (i. e. dissolved oxygen stratification) in same period in water column of the reservoir. During summer of 2001, the oxide-reduce interface could move up to 8 meter-depth from bottom of Baihua Lake reservoir. The survey results also confirmed that gravitationally unstable of the thermal stratification caused by surface cooling of water when the air temperature decreased abruptly in early fall, leaded to a vertical convection between the upper layer water and deep layer water. The reductants (e.g. Mn2+ , Fe2+ and H2 S ) formed in the anoxic hypolimnion companying with processes of organic maters degeneration would be brought to the epilimnion through the vertical eddy diffusion. These reductive matters would consume up the dissolved oxygen in the utter water rapidly as if such oxidization reactions were strong enough, and than a large number of aquatic organism dead due to oxygen deficient. This study showed that the seasonal thermal stratification, although not strong like typical alpine lakes, could play a key role in change of water quality in canyon-reservoirs.

Phosphorus is one of the important limited elements to lake eutrophication.The regenera-tion of phosphorus from sediment may be an important phosphorus resource for lakes.Several undisturbed sediment cores have been sampled at the deepest area of a canyon-reservoir,Lake Hongfeng,on Guizhou Plateau,southwestern China.The concentration and distributions of various phosphorus forms in sediment cores have been studied.The chronology data have been obtained by means of 210 Pb, 137 Cs for the same sediment cores.The geochemical process-es of various forms of sedimental phosphorus were mainly controlled by early diagenesis and anthrogenic disturbances on the phosphorus burial in sediment.In our study,the SEDEX method from Ruttenberg has been used to identify the various forms of phosphorus in sedi-ment samples.The species of phosphorus include:loosely sorbed P,Fe-bound P,Ca-bound P,detrial-P and Organic-P.The results showed that the organic-P and Fe-bound P could been seen as two main sorts phosphorus in sediment of Lake Hongfeng,about60%and28%in total sediment phosphorus respectively.The forms and vertical distrubition profiles of sedi-ment phosphorus implied that the sediment phosphorus could have strong ability of regenera-tion during the terminal burial in the bottom of Lake Hongfeng.Under certain conditions,the sediment of Lake Hongfeng could be regarded as the important resource of phosphorus for lake system rather than a sink.

Nine sediment-water columns were taken from three stations of East Taihu Lake (about lm in depth) in June, 1997. The overlying water of the cores was filtered (0. 45m), and then the cores were placed in a dark room for 50 days. Air-bubbling was used to keep oxidized conditions in the overlying water and upper sediment, without resuspending the sediment. The room temperature was between 23. 5 and 31. 2, and the pH value of the overlying water and upper sediment was 7. 2 0. 2. Concentration of PO_4~3-P in the overlying water was measured every 2 or 3 days to follow the P-release process. In the first 15 days, PO_4~3 -P level in the overlying water increased 2 orders of magnitude. But in the following 33 days, the PO_4~3-P level decreased 1 order of magnitude and then stabilized. Such a process was also found in the West Lake, Hangzhou and Gehu Lake, Changzhou. We think it not a normal sediment-P-release process, but more complex as follows. In natural conditions of East Taihu Lake, the sediment is covered by a layer of detritus. Biological degradation is so intensive in this layer that PO_3~4 accumulates to a high level and diffuses to both the overlying water and the sediment beneath it. In the sediment a steep PO3~4 -P concentration gradient forms because of the high diffusive resistance. But in the overlying water, PO_4~3-P is used by aquatic plants and is kept at a low level. We are not able to measure PO_4~3-P concentration in the detritus layer, but an indirect evidence was found in the same lake: a PO_4~3- P concentration gradient from 200mg m_~3 in the upper sediment layer to 80mg m_~3- in the sediment of 18cm depth existed when PO_4~3-P concentration in the overlying water was only 22 mg m_~3. In the core experiment, phosphorous biological cycling was broken when phytoplankton was removed from the overlying water by filtration, and thus, PO_4~3- accumulated in the water quickly; After 15 days, the p-release reaction in the detritus layer was about to end, and p-diffusion from overlying water to the sediment triggered. In East Taihu Lake and the other similar lakes with stable bottom conditions, detritus layer may exist and affect p-release core experiment seriously. In such a situation, it is better to measure internal-P-loading by other means, such as whole lake p-budget and in-situ enclosure experiment.

Systematic investigations into the distribution of aquatic species and variation of community structure in Baiyangdian Lake, the largest natural freshwater body in the Northern China Plain, were conducted during 1993–1994. Water quality of this shallow and eutrophic lake was assessed by qualitative and quantitative analyses of aquatic communities. Chemical and biological data in 1990s were compared with data from 1950s, 1970s and 1980s. Results from this research showed that Baiyangdian Lake was highly enriched by ammonia (NH3), nitrate (NO3) and phosphate (PO4) at the present time. The negative responses to increase degradation included: (1) a clear decrease in biodiversity; (2) a dramatic increase in individual abundance of tolerant species; and (3) a great decline in aquatic products in the recent years. A demonstration project for water pollution control and ecological restoration in Baiyangdian Lake is proposed.

Shallow and eutrophic Baiyangdian Lake is a decaying lake and has broad ecotone wetlands, composed of reed communities and ditches. Experiments were carried out to study the fate of nutrients and organic pollutants during the runoff, flowing from a contaminated river through the ecotone to the lake. Under moderate hydrological conditions, water flows through the ditches and through the root zone soil of the reed community. The retention of total nitrogen (TN) and total phosphorus (TP) by surface flow through a 300-m ditch was measured to be 42% and 65% respective-ly. The retention of TN and TP by an 8-m stretch of reed community soil was 59% and 88%. The retention by the reed community mainly happened at the rhizosphere below the soil surface. The removal of organic matter was also observed. The retention of nutrients by the subsurface flow was more than 10 times greater than that by surface flow. The harvest of reed biomass recycles 1120 metric tons of nitrogen and 53 tons of phosphorus from lake-side ecotones back to the terrestrial ecosystems each year. There is a potential use of ecotone wetlands as the advanced treatment facility for nutrient-rich water, and a good design can enhance the treatment efficiency. Ecotone engineering for advanced water treatment is an economical method with low environmental risks and high efficiency.