Distribution Of Nitrogen And Phosphorus In Sediment Core And Overlying Water Of Donghu Lake

Donghu Lake is a typical urban shallow lake. Like many other lakes, it has become hypertrophic due to decades of excessive increases in the input of nutrients, through domestic sewage and agricultural run-off. In order to provide scientific guide for prediction and control of the eutrophic level in Donghu Lake, it is necessary to study the distribution of key nutrients, such as nitrogen and phosphorus, and their biogeochemical processes. The research is of important theoretical and practical values.Characteristics of sediment cores and their overlying water were analyzed and assessed with field investigation and laboratory chemical analysis from 2004 to 2005. Two sites were selected for sampling in Donghu Lake to observe water quality, vertical profiles of nitrogen and phosphorus chemical species, their seasonal changes, and the correlations between different factors. Main results and conclusions were shown as follows:(1) Characteristics of nutrients in the overlying water: the concentrations of total nitrogen (TN) were relatively low in both summer and winter, while much high in both spring and autumn. The maximum concentration of nitrate was recorded in autumn and that of nitrite was recorded in summer. The concentration of phosphorus reached its minimum in autumn, and primarily contained its particle form. The portion of dissolvable phosphorus in total phosphorus reached its maximum in spring.(2) Vertical profiles of nitrogen species showed that concentration of TN decreased with the increase of sediment depth. The concentration of TN reached its minimum in spring, then increased till autumn and reached its maximum. The levels of two spring seasons were about the same. Ammonium was controlled by both the content of TN and the sediment environment, so the vertical profiles and seasonal changes of NH₄⁺-N presented the same trend as TN.(3) Vertical profiles of phosphorus species showed that total phosphorus(TN), labile phosphorus(LP), Al- and Fe- bound phosphorus(Al-P, Fe-P) were characterized by a similar distribution pattern and enrichment in the upper 0-10cm sediment layers. Theconcentration decreased steeply within the top 10 cm sediment layers, then it became steady. The changes of Ca- bound phosphorus, organic phosphorus and occluded phosphorus were quite complex.(4) The portions of P fractions were Fe-P>OrgP>OcP>Ca-P>Al-P>LP.(5) The seasonal changes of P fractions in sediments: the maximum value of TP and Fe-P were recorded in autumn, while organic P and occluded P reached their minimum value at that time. Considered that the concentration of TP in the overlying water reached its minimum, sediment might be the phosphorus trap in autumn. During winter and spring, the contents of TP, Fe-P and Al-P decreased. In winter organic P reached its maximum, while the concentration of TP in the overlying water increased, so organic phosphorus in sediment might be a possible phosphorus source.(6) From the analyses of correlations between different factors in sediment, TP, Fe-P and Al-P revealed strong correlations with other P fractions. The inertia of occluded phosphorus made it no obvious correlation with any other P species. TN and TP in sediment were found positive intercorrelations. Fe-P correlated with TN and NHi+-N.(7) Large proportion of Fe- bound phosphorus in sediments showed a severe pollution level in Miaohu Lake. Strong positive intercorrelation between Fe-P and other nutrient species revealed its high capacity for internal loading.