The Biogeochemical Cycles Of Nutrients Of Shallow Lakes In The Middle And Lower Reaches Of Yangtse River, China

Lake pollution, induced by human activities with the development of society and economy, has been a serious environmental problem, of which the eutrophication is one of the most universal and harmful environmental problem. Although the internal release or source of the nutrients in sediments is an important factor, we still have not understood adequately the mechanisms behind internal loading in shallow lakes. To control the eutrophication effectively, the cycling mechanisms of the nutrients in shallow lakes and the evolution process of the nutrients with lake eutrophication should be understood comprehensively.The middle and lower reaches of the Yangtze River are the central areas not only of shallow lakes, but also of eutrophication lakes in China: Taihu, Chaohu and Longgan Lakes are the typical shallow lakes in the middle and lower reach of the Yangtze River. The spatial and temporal distribution of the chemical forms of the nitrogen, phosphorus and silicon was studied in Taihu, Chaohu and Longganhu Lakes for a better understanding of the cycling mechanisms of the nutrients in shallow lakes, of the release mechanisms of internal loading in sediments and the action of algae. Meanwhile, the sedimentary records of carbon isotope, biogenic silica and polyphosphate were investigated for recognizing the evolution of the eutrophication, the effect of polyphosphate, and eco-environment change of Taihu Lake. The main conclusions have been reached as follows:1. The concentrations of TP (total phosphorus) in lake water of Taihu, Chaohu and Longganhu Lakes are high, among which the highest concentration was found in the Taihu Lake, then the Chaohu and the Longganhu Lakes. The phosphorus forms of lake water can be transferred with the season change: the average concentration of DOP (dissolved organic phosphorus) is higher in winter, but the SRP (solute reactive phosphorus) is higher in summer, as compared with other seasons. The intensive metabolism of algae can alter the phosphorus forms of water and cause the internal loading in sediments. Atomic ratios of N: P, Si: P and Si: P in lake water show that the nutrient limitation for phytoplankton is phosphorus during winter in all of the lakes, but is silicon during summer in Taihu and Chaohu Lakes according the Justuc's rules, which is consistent with the evolution of the nutrient structure and phytoplanktoncommunity in Taihu Lake.2. The density of phytoplankton is very high during summer in north part of Taihu Lake, with a highest value of 8.53 * 108 individuals L*1. There are six main phytoplankton groups including Cyanophyta, Cryptophyta, Bacillariophyta, Chlorophyta, Euglenophyta, Pyrrophyta, etc, among which the predominant species is Microcystis spp. At the same time, there are many algae species that can be used to indicate the extent of pollution and eutrophication. The water pollution is very serious according to algae indexes, and the trophic state relating to phytoplankton species varies from mesotrophic to eutrophic.3. Due to high hydraulic state of shallow lakes, the suspended matter is not easy to settle down to the bottom, so more suspended organic matter is degraded than that in other lakes, up to 98% of which can be degraded. The degradation of algae has an important influence on the cycle of the nutrients leaded by the great algae biomass in shallow lakes.4. The sedimentary organic matter was mainly originated from the lacustrine authigenesis in all of the lakes, and contribution of terrigenous organic matter was small, according to the low organic C/N atomic ratios. TOC (total organic carbon) shows evidently positive correlation with TN (total nitrogen), TP, Pin (inorganic phosphorus) and Porg (organic phosphorus) in sediments, suggesting that the N and P could be originated from the biological sedimentation accompanied by the lacustrine sedimentation in all of the lakes.5. The TP concentrations in sediments from all of the lakes are high, among which the sediments from Taihu Lake show the highest, while those from Chaohu Lake the lowest concentrations. TP shows evidently positive correlation with Pin and Porg in all of the lakes, suggesting that the all of the lake sediments have similar TP composition, although all of the lakes have different trophic status and ecological structure.6. Using the SEDEX method, a sequential extraction method for separating different forms of sedimentary phosphorus, to analyze different forms of phosphorous in the sediments of Taihu Lake, we found that the concentrations of PFe (iron bond phosphorus) are high, which account for not only the main composition of Pin (75 -88%), but also TP (53 - 75%). The variational trend of PFe is consistent with Pin andPtot, and PFe has an excellent positive correlation with TP and Pin (R=0.998, 0.993 respectively, n = 40, P < 0.01).7. The diffusive process at sediment-water interface has low impact on the nutrient cycle of overlain water in Taihu Lake, and even shows the scavenging effect in some region in the given season. Despite the atomic ratios of Fe/P > 2 in sediments and porewater of Taihu Lake, the release of phosphorus has still occurred. Fe oxides and oxyhydroxides are only a small part of sedimentary Fe (10 - 28%), and more Fe existed in the sediments is the Fe sulfide, such as FeS and FeS2. The absorption between Fe (III) and phosphorus has probably reached to equilibrium according to the low Fe/P atomic ratios (2.0 - 5.3). The main mechanisms of internal phosphorus release may be the sulfate reduction and the sulfide formation.8. The nutrient accumulation and eutrophication process induced by human activities in Wuli Bay of Taihu Lake is reconstructed by using TOC, TN and TP concentrations of the sediments. 81 C of bulk sedimentary organic matter is in response to the concentrations of TP and the organic C/N atomic ratios which suggests the evolvement of eutrophication, primary productivity and producer.9. The BSi (biogenic silica) concentrations in sediments of Taihu Lake is very low, with a variation between 0.4 to and 2.8 mg/g. Combining the changes of nutrients limitation factors and the evolution of algae species and biomass in lake water, we can discover the trends of Si limitation for algae is very obvious in Taihu Lake through the concentrations of BSi and the values of atomic ratios of BSi/TPN BSi/NAlP (non apatite inorganic phosphorus) and TOC/BSi in sediments.10. The concentrations of Poly-P in sediments of Taihu Lake are low, with a variation between 0.004 and 0.065 mg/g. The 22% of sedimentary TP is NAIP, so the NAIP is a main composition of sedimentary TP as well. In P-limited lakes, if algae biomass is very high and if NAIP is a large fraction of TP in sediments, Poly-P sedimentation is not only an important P sink stored by algae without increasing algae biomass, but also a sensitive indicator which can reflect the accelerated eutrophication caused by the increased input of anthropogenic P in the past.