| Popis: |
Many China's lakes are facing with serious eutrophication problem. Although some measures for controlling lake eutrophication have been employed in practice, the deterioration tedndency of water quality has not been controlled efficiently. Lake sediuments are rich in nutrients, which usually have a higher concentration than that of in the water. Therefore, as one of the possible measures, dredging is applied for controlling eutrophication in some lakes around the world. This measure needs a great deal of funds. In fact, there are no obvious samples, which indicate the efficiency of dredging, as the key measure for controlling the lake eutrophication. Can we control eutrophication in (usually shallow) lakes by dredging? Theoretical analysis shows that there are some basic problems limiting the effciency of dredging for lake eutrophication control: (1)The lake bottom is a reservoir of nutrients, not a source of that in the long run, for example, a couple of months or years. The self purification capacity of lakes usually functions. In the water sediment interface, the total nutrient fluxes are shown downwards, not upwards, even when the liquid nutrient flux is upward sometimes, but the solid (including inorganic and organic substance) flux is downward. (2)The high concentration of nutrients in the water sediment interface is formed mainly by decomposition of recent detritus of organisms. The liquid and diluted mud are movable and easy for re suspension, forming fractional “pollutant cloud”, as main internal source of pollution in lakes. (3)Experiments in laboratory show that nutrients are released from sediment just in the first few days. The release may become less and negative for longer periods of 20-40 days. Self purification dominates as time goes on. Laboratory experiments also show that the nutrient release decreases at the beginning after dredging, and after several months it differs little with those in the period before dredging. (4)The nutrient concentration in the sediment is of 10 4 orders more than that in the water body, and 10 6 orders more than that for algae bloom (when all the nutrients would be released from the phytoplankton). Dredging can decrease the nutrient in sediment only 10 times, and could not be the key factor for controlling eutrophication and the algae bloom. To cut off the pollutant loading into lake and to improve the environment of aquatic ecosystem should the key approaches instead. (5)The efficiency of dredging is related with the dredging method. Attentions should be paid to the negative influences, which may occur by dredging on ecological restoration. (6)The water quality problem, such as the algae bloom, could be solved alternatively through ecological endeavors. To improve the environment of aquatic ecosystem, approaches such as Physico Ecological Engineering (PEEN) and Bio Environmental Enterprise (BEEN) are favored in lakes step by step from local to all the open water system. |
