Characteristics of nitrogen and phosphorus pollution in the middle and small creeks, suburban Shanghai

(1) The middle and small creeks in suburban Shanghai were under heavy eutropic condition, with high loadings of nitrogen, phosphorus and organic pollutants. KN, TP and COD_Cr of the surface water of them were several times greater than the critical values of Types V, the worst level of water. (2) Spatial variations of N and P pollution were observed, as the creeks in different places accepted different types and amounts of pollutants. Affected by domestic sewage, the creeks nearby residential areas were often heavily polluted by NH₄⁺ and P. The creeks in agricultural fields were more easily polluted by NO₃^- than by NH₄⁺ and phosphorus. The creeks nearby livestock farms often witnessed sudden soaring of NH₄⁺ and P. (3) N and P loadings of the creeks were also affected by the seasonal evolvement of ecological environment. With the increase of temperature and biomass, NH₄⁺ and NO₃^- in the surface water were largely consumed and decreased as a result; while organic N and particle P increased as the enhancement of suspending biological particles. NO₂^- in the surface water increased with temperature as nitrifying bacteria became active. Total P and water soluble P also showed the trend of enhancement with the intensification of biological activities, however, they were also controlled by the condition of pH and DO in the water. (4) At the height of the summer, the surface and bottom water of the creeks differed in N and P loadings. NH₄⁺, soluble P and total reactive P in the bottom water were higher than those in the surface; while NO₃^- and NO₂^- in the surface were higher than those in the bottom. Such water-quality stratification inevitably resulted from the enhancement of phytoplankton in the surface water and intensified release of NH₄⁺ and P from the sediments. In the autumn, as the temperature decrease, such phenomenon declined gradually.