Effects Of Phosphorus Concentration On The Shifts Between The Grass-stable State And The Algae-stable State Of Gehu Lake

Eutrophication of shallow lake is one of the most important problems in China. Recently, with the development of industrial, agricultural and artificial fence breeding of Gehu lake, the diversity of aquatic organisms and submerged macrophytes decreased sharply. During its process of eutrophication, it had shifted from the grass-stable state to the algae-stable state. The article mainly demonstrated the investigation of the lake ecological environment and evaluation of historical survey through the collection of water quality, biological community compositon, social activities and other related information. The degradation reasons of submerged macrophytes were comprehensively analyzed. The effects of phosphorus on shifts between the grass-stable state to the algae-stable state was also studied through field investigation and mesocom experiment. The main results are as following:The water body of Gehu Lake had reached the early stage of hyper eutropher and the water quality had been to inferiorⅤ. Furthermore, the nutrient concentration played an important role on the degradation of submerged macrophytes. Based on the long-term investigation, we came to the conclusion that the TP played a determinant role during the regime shifts of Gehu Lake. It experienced mainly three stages. Firstly, the ecosystem had been in the grass-stable state when the TP was 30～94μg·L^-1; Secondly,the grass-stable state shifted to the grass-algae intermediate state with the TP increase up to 94μg·L^-1, it had been in this stage with the TP of 99～201μg·L^-1; Finally the ecosystem shifted to the algae-stable state with the TP increase up to 201μg·L^-1. In Da Hong Gang—the submerged macrophytes remaining area, the result indicated that it was in grass-stable state when the TP was 1.62～61μg·L^-1and the grass-stable state shifted to the grass-algae intermediate stable state with the TP increase up to 61μg·L^-1, it had been in this stage with the TP of 61～115μg·L^-1. The thresholds of TP were reasonable and would probably provide theoretical guidance for the ecological restoration and regime shifts of Gehu Lake.The indoor mesocom experiment results showed that Ceratophyllum demersum was suitable with the TP of 0.1～0.3 mg·L^-1while Vallisneria natans with the TP of 0.2～0.4 mg·L^-1. The system maintained stable state in the phosphorus thresholds. The two submerged macrophytes are more preferable to eutrophic condition and will become the dominant species with the growing eutrophication. However, the high level of nutrient concentration is a sress to them. It deserves further studies on the nutrient concentration thresholds. Although Gehu lake had shifted to the algae-stable state, the submerged macrophytes remaining area still showed the hope of restoration. In order to make the ecosystem shift from the algae-stable state to the grass-stable state, the nutrient concentration must be cut down to a lower level. The TP should drop to below 61μg·L-1 in Da Hong Gang while the TP should drop to below 94μg·L-1 in Gehu lake. Several solutions were suggested in order to realize the regime shift. Firstly, we should cut and control the lake's nutrient sources including the exogenous pollution caused by the river into the lake and the endogenous pollution caused by the release of the sediment; Secondly, we must reduce fence breeding area, remove rich sediment full of organic matter, improve intense reductive environment; Finally, overall management should be taken such as dredging the sediment, constructing wind wave belt, strengthening basins management and so on.