Identifying the critical watershed regions creating lake nutrient enrichment (SLRs) based on a watershed-lake integrated perspective – A case study of Chaohu Lake Basin, China

Abstract

Identifying critical watershed source regions is essential for lake environments protection. However, related studies are mainly focused on assessing watershed Total Nitrogen (TN) and Total Phosphorus (TP) loads, while the impact of watershed on lake nutrients is rarely considered. To fill this gap, the amounts of watershed nutrient loss over the last ten years were assessed, and the corresponding lake TN and TP concentrations were acquired. Finally, the critical watershed regions creating lake nutrient enrichment (SLRs) were identified by analyzing the relationships from the integrated watershed-lake perspective. The results indicated that: i) The watershed TN and TP output amounts showed a rising trend in the last 10 years (mostly between 2012 and 2016); ii) The lake TP and TN in the western lake were much higher than those in the eastern lake; and iii) SLRs were located in the western Chaohu watershed. Specific findings were obtained from the statistical analysis in sub-watershed and intra-annual scales. First, the SLRs for TN were concentrated in the Nanfei (NF), which covered only 32 sub-watersheds and were mainly located in urban construction areas. In contrast, the SLRs for TP were more widely distributed and located contiguously across the major influent streams belonging to the Hangbu (HB) and NF. Second, the intra-annual variance is obvious. For TP, the spring season can be considered the most ideal period for SLRs identification, while the summer season is ideal for TN. These differences are driven by different loss processes, discharge capacities, and occurrence forms for TN and TP. In general, the integrated perspective of lake-watershed is helpful for identifying SLRs, and the temporal analysis at the seasonal scale and spatial analysis at the sub-watershed scale facilitate a deeper understanding of SLRs.