| Restoration of the eutrophicd lake has been one of the most important hotpots in environmental sciences. Generally, environmental effects produced by submerged plants and benthic mollusks were the foundation of water environmental quality improvement. In this paper an investigation had been conducted at eight typical sampling stations to learn the community structure of phytoplankton and assess the water quality in Baiyangdian lake during 2009-2010 and situ enclosure experiments were also undertaken in Baiyangdian lake to study effects submerged plants and mollusks on water quality and its stability . The major results were as follows:1、Phytoplankton were investigated at 8 sites in Baiyangdian lake in the early days of April, and mid-June, 2009. This investigation showed that Chlorophyta, Cyanophyta, and Bacillariophyta were dominant in the phytoplankton community among the total 133 species identified. In spring, Chlorophyta and Bacillariophyta were dominant phylum, and Chlorella sp., Chroomonas acuta and Microcystis incert. were dominant species; The cell density of phytoplankton ranged from 496×10~4 to 6 256×10~4 cells/L with an average of 2 384×10~4cells/L. However, in summer, Chlorophyta and Cyanophyta were dominant phylum, and Chlorella sp., Leptolyngbya valderiana and Nephrocytium agardhianum were dominant species; the cell density of phytoplankton ranged from 318×10~4 to 4 630×10~4 cells/L with an average of 1 785×10~4cells/L. According to the comparison between this investigation and previous two ecological investigations since 2005, the abundance of phytoplankton had increased significantly. The index of Carlson nutritional status (TSIM) and the dominant genus assessment revealed that most water columns of Baiyangdian lake were eutrophic.2、30 situ enclosures (9m2 each) were constructed in Baiyangdian lake, some of which were selected for Myriophyllum spicatum and Ceratophyllum demersum being planted with gradient density(0, 10, 20, 30 individual plant/m~3) . Due to results of six weeks observation, the biomass of phytoplankton decreased and the SD of Baiyangdian lake improved in enclosures. Especiallly the enclosures with 10 plants/m~3 of Myriophyllum spicatum and 20 plants/m~3 of Ceratophyllum demersum were improved well, of which TN and TP decreased significantly, SD and the biodiversity of phytoplankton increased distinctly. But submerged macrophytes with high density may bring chlorophyll a and nutrient concentration rebound. Hence, it could be seen that rational density of Myriophyllum spicatum and Ceratophyllum demersum could contribute to water quality improvement of Baiyangdian lake.3、The optimum planting density for Myriophyllum spicatum, Ceratophyllum demersum was filtered after situ enclosure experiments. Furthermore, control effects of Myriophyllum spicatum, Ceratophyllum demersum, Unio douglasiae and the combination of the last two species on the eutrophic water body were studied in six weeks situ observation, which showed that the enclosure with 195g/m~3 of U. douglasiae removed TN, TP and Chla with some degree, but had no effection on SD, TN, TP and nutrient conditions. Submerged macrophytes with the rational density in the exposure can inhibit the growth of phytoplankton effectively as well as can reduce chlorophyll a and nutrients concentration of water. Combined with U douglasiae and submerged macrophytes can maintain SD in an optimum condition combined, and the purification effect brought by the combination was better than only submerged macrophytes.It was suggested that submerged macrophytes and zoobentheos with rational density would benefit compound food chains formation and water quality improvement of Baiyangdian lake. In a word, this mix and match test for submerged macrophytes and benthic mollusks was carried out with both of them arranged rationally, bringing together optimal adjustment on food chains and trophic structure of Baiyangdian lake, decreasing the eutrophication degree, and thus, maintaining safe aquatic ecosystem and stable diversity patterns of Baiyangdian lake. |
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