Historical accumulation and environmental risk of nitrogen and phosphorus in sediments of Erhai Lake, Southwest China

The release of endogenous nutrients from sediments significantly affects water quality and accelerates eutrophication. In general, lakes from the Yungui Plateau of China are characterized by high sediment nutrient contents and relatively good water quality. Thus, the risk of nutrient release from sediments may be enhanced under certain environmental conditions, and this enhanced risk would become a general concern during eutrophication. Knowledge about historical nutrient accumulation and effect of environmental parameters on nutrient dynamics at the sediment-water interface in the lakes from this region is important to understand the eutrophication processes for these lakes. Hence, this study reconstructs the historical accumulation of nutrient and the effects of environmental parameters on possible release risk of nutrient at the sediment interface for Erhai Lake in the Yungui Plateau region. This study also analyzes historical changes in environmental conditions to predict the future release of nutrient from sediment. In the past decades, the burial fluxes (BFs) of nitrogen (N) in the sediments have continuously increased, whereas those of phosphorus (P) have not or only slightly increased, except in highly polluted areas. These situations may be attributed to the long-term fertilization practices of farmers in the watershed, where the application of N fertilizers is much higher than that of P fertilizers. The mentioned phenomena can also be ascribed to the different biogeochemical behaviors of N and P. Nutrient pollution started in the 1970s and worsened from the 1990s. Before the 1970s, nutrients were relatively low and stable. Thereafter, N in the entire area and P in seriously polluted areas (northern area) dramatically increased because of natural and anthropogenic processes, such as excessive artificial N and P fertilization. After the early 1990s, the BFs of nutrients were steady but high because of the degradation of aquatic vegetation and the implementation of pollution control policies. The burial efficiencies (BEs) of TN range between 44% and 85%, with a mean value of 71%. Combined with the high increase rate of BFs during the past decade, this relative low BE indicates that the release of sediment N into overlying water has increased yearly. The BEs of TP range between 98% and 102%, with a mean value of 99%, and the increase rate of historical BFs is small. This result suggests that the sediment generally serves as a sink for P. In addition, the diffusion fluxes of nutrients is an important factor because their forms and environmental conditions (dissolved oxygen (DO), pH, etc.) influence water quality. However, the risk of nutrient release from sediment source might increase if these environmental conditions change (i.e., decreased DO and increased pH). (C) 2015 Elsevier B.V. All rights reserved.