Seasonal Variation Of Bioavailablity Of Nitrogen And Phosphorus In Surface Sediments Of Taihu Lake

Eutrophication in Chinese lakes is one of the most serious water-pollution problems present and will still be in the near future. Since 1990s cyanobacteria bloom become increasingly serious in Taihu Lake which is a typical large shallow lake. Normal water supply of citys around it and its lake ecosystem ecology environment have been seriously affected. Sediments play an important part in eutrophication as inner nutrient resource.It is important to forecast the water quality in lakes and then find a way to reduce its impact on the environment Therefore it is necessary to get knowledge about the function mechanism between sediments and cyanobacteria bloom.Sediments supply nutrient for water plant (phytoplankton and large hydrophyte ),especially nitrogen and phosphorus .Nutrient actions in sediments correlate nearly with water plant and the growth of water plant is a typical seasonal process.It is highly necessary to study the seasonal variation of bioavailability in Sediments.In order to discuss the impact of sediments on water plant in different seasons, nitrogen and phosphorus in the surface sediments(0-10 cm) from three different trophic regions of northeast of Taihu Lake were investigated which is sampled respectively in 3,6,9 and 12 in 2004.Thereinto Meiliang Bay and Gonghu Bay are of phytoplankton-dominated turbid-water phase, Xukou Bay is of macrophyte-dominated clear-water state.The results show that the concentrations of total phosphorus (TP) and bioavailable phosphorus (BAP) in the sediments are coincided with the corresponding trophic level of the overlying water namely Meiliang Bay> Gonghu Bay> Xukou Bay.And there is a significant correlation between BAP and TP in surface sediments.The concentrations of TP and BAP in the sediments varied widely not onlybetween sampling sites but also between seasons. Percentage of bioavailable phosphorus forms of TP in lake sediments is in same sequence algal available-P (AAP) > Olsen-P> water soluble P(WSP)>readily desorbable P(RDP).Especially AAP, occupies great proportions.Seasonal variations of BAP show different characters in different sediments. Contents of TP and BAP in sediments of phytoplankton-dominated turbid-water phase decrease in summer and markedly increase in autumn which is coincided with cyanobacteria bloom. Contents of TP and BAP in sediments of macrophyte-dominated clear-water state decrease in summer and increase in autumn along with death and catabolism of plant. Seasonal variations of contents of TP and BAP in sediments of hardpan vary are not visible.There is a significant correlation between TP and dissolved total phosphorus (DTP) in spring and summer, and dissolved organic phosphorus (DOP) is the main form of DTP. There is also a significant correlation between dissolved inorganic phosphorus (DIP) and DOP in spring and summer.The transformation of DOP and dissolved inorganic phosphorus DIP by the biological processes is more active in spring and summer than in autumn and winter.Contents of total nitrogen(TN)decrease in sediments of phytoplankton-dominated turbid-water phase and increase in sediments of macrophyte-dominated clear-water state in summer.Contents of NH4+-N released from sediments are higher in summer and winter and highest in summer.Contents of N03~-N released from sediments increase in summer and the contents are low in winter. Contents of TN,NH4+-N and N03"-N in sediments of macrophyte-dominated clear-water state are markedly higher than thatin sediments of phytoplankton-dominated turbid-water phase.Study bioavailablity of nitrogen and phosphorus in Sediments with Scendesmus quadricauda, Microcystis aeruginosa and Nitzschia palea according to algal growth potential (AGP)method by simulating respectively algal growth condition in spring, summer and autumn in Taihu Lake. The results show that in spring biomass density is low while just Scendesmus quadricauda have a little growth.In summer algae grows rapidly and dies out rapidly and growth statu of Microcystis aeruginosa is better than Scendesmus quadricauda. Nitzschia palea have a certain extent biomass density. In autumn algae grows better in filtrated water of Meiliang Bay sediment release than in filtrated water of Xukou Bay sediment release.During the early experimental period, Scendesmus quadricauda grow better and subsequently Microcystis aeruginosa grow better and reach highest biomass density.The results accord with the fact that Scendesmus quadricauda is familiar without bloom and Microcystis aeruginosa becomes bloom in summer in Taihu Lake.The results also show that contents of nitrogen and phosphorus are not always the control factor in algal growth.In summer and autumn algae grows better in filtrated water of high trophic sediments release under fitting conditions while algae grows better in filtrated water of low trophic sediment release under un-fitting conditions.This paper indicates bioavailablity of nitrogen and phosphorus in sediments is different in different seasons.lt is more exact to evaluate nutrient supply and potential release risk of sediments aiming at corresponding season.lt is also necessary to take measures aiming at corresponding seasons.