Abstract
Endogenous nutrients released from sediments are a potential hazardous source in aquatic ecosystems, especially reservoirs. Here, we investigated seasonal and spatial variations of different species of nitrogen and phosphorus and evaluated the pollution levels of nutrients in sediments from one of the world's largest tidal reservoirs. The results indicate that most of the total nitrogen and phosphorus were accumulated in sediments from the reservoir downstream, which were enhanced by runoff and precipitation during seasonal alteration. Total nitrogen was increased to 2471.17 mg/kg during the saltwater intrusion period. Nitrate and ammonium were the major nitrogen fractions in flood and dry seasons, respectively, while the highest level of phosphorus, especially inorganic phosphorus, was found in summer. The sediment was slightly to moderately contaminated by nitrogen but not phosphorus, especially downstream in winter, according to the applied indices. Multivariate statistical analyses reveal that nutrient input in flood seasons was more complex than that in dry seasons, which possibly originated from upstream agricultural and domestic sewage.