Can the watershed non-point source phosphorus flux amount be reflected by lake sediment?

Abstract

An understanding of the quantitative relationship between watershed non-point source (NPS) pollution and lake nutrient enrichment is essential for the environmental management of water bodies. However, a lack of data availability and integration limit our understanding of this relationship. The most critical bottlenecks are the lack of adequate support for watershed-lake integrated data, a lack of data of the nutrient loading from watersheds into a lake as NPS, and the long-term time series data of lake nutrient concentrations. Since lake sediment is the ultimate destination of watershed nutrients, this study aimed to reveal the relationship between watershed phosphorus (P) concentrations that are transported into lakes and lake P concentrations by using inlet sediments as an indicator. Three typical sub-basins within Hongze Lake, China were selected as the study site, and 30-year dataset was calculated using the technology integration of remote sensing, sediment chemical analysis and SWAT model assessment. It can be concluded that, first, the Mineral-P (Min-P) accounted for nearly 65% of the watershed Total P (TP) loss, which varied over different basins and temporal periods. Second, the relationship between watershed P loss into lakes and sediment P concentrations represented obvious variation among different basins, sediment depths and P states. With respect to different basins, the highest correlation appeared in Basin 1, which had a R2 value of 0.65 for Min-P. With respect to different P states, the Inorganic-P (IP) showed a closer relationship between watershed and lake P than Organic-P (OP), and the correlation coefficient (r) was higher than 0.6 in Basin 1 and Basin 3 for IP. In addition, the calcium-phosphorus ratio (Ca-P), which is largely sourced from watershed rock detritus by eroded runoff, played the most critical role among different Min-P states in the relatively ideal relationship of IP. With respect to different sediment depths, a stratification phenomenon existed in Basin 2 and Basin 3, which differed at specific depths. In particular, the ideal relationship only existed within the 7 cm depth from the bottom in basin 2, and this value expands to 10 cm for basin 3. This diversity was influenced by the integration roles from external and endogenous sources. All the findings indicated that lake surface sediment can be used as a proxy to represent NPS P loading under specific conditions. The relationship between watershed P loss and lake sediment P concentration was largely decided by land use/cover change (LUCC), meteorological conditions, and lake resuspension.