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• Internal P loading in the diversion input shallow Lake Hengshui was studied. • High water P concentration was mainly regulated by the high internal P loading. • Littoral wetland showed higher sedimentation and P release than open water areas. • Detritus burial, coprecipitation, and deposition accelerate P retention in wetlands. • Fe-P dominated the internal P release in the lake, accompanied by Org-P. Diversion input lakes usually have a low catchment area/lake area ratio and pulsing pollution input. Various pollutants might accumulate in the lake continuously owing to the concentration effect under high evaporation but low precipitation over the entire area, typically for sedimentary cyclic elements such as phosphorus (P). However, the detailed transportation, sedimentation, and internal release mechanisms of P in the diversion input lakes remain unclear. This study conducted a year-long investigation of the littoral wetlands and open water areas of the shallow Lake Hengshui in the semi-humid region of North China. Results revealed that the average total P concentrations in the water and surficial sediment reached as high as 0.202 mg L−1 and 878.21 mg kg−1 in summer. The high water P levels in the lake were mainly regulated by the high internal P loading during summer and autumn, with the internal P loading being approximately nine times the external P loading. The littoral wetland area serves as a higher sedimentation sink and release source of P than the open water area. The concentrated P was continuously transported to the littoral wetland area through detritus burial, coprecipitation, and deposition of suspended particles. The release of P was mainly controlled by the dissolution of redox-sensitive Fe-P and Org-P at high temperatures and organic matter mineralization in the sediment, accompanied by the potential release capacity of apatite P (Ca-P). Future management of eutrophication and P levels in similar diversion input lakes should pay more attention to the high internal P loading in the sediment and the differentiated sedimentation and release processes in the littoral wetland and open water areas. [Display omitted] [ABSTRACT FROM AUTHOR]

• A watershed P-SFA model with comprehensive subsystems and detailed P flows was developed. • The P utilization patterns of cultivation and livestock farming influence the P cycling. • The average annual growth rate of P emissions is 2.41%, from 1.67 × 104 t to 8.73 × 104 t. Understanding the historical patterns of phosphorus (P) cycling is essential for sustainable P management and eutrophication mitigation in watersheds. Currently, there is a lack of long-term watershed-scale models that analyze the flow of P substances and quantify the socioeconomic patterns of P flow. This study adopted a watershed perspective and incorporated crucial economic and social subsystems related to P production, consumption, and emissions throughout the entire life cycle. Based on this approach, a bottom-up watershed P flow analysis model was developed to quantify the P cycle for the first time in the Poyang Lake watershed from 1950 to 2020 and to explore the driving factors that influence its strength by analyzing multi-year P flow results. In general, the P cycle in the Poyang Lake watershed was no longer a naturally dominated cycle but significantly influenced by human activities during the flow dynamics between 1950 and 2015. Agricultural intensification and expansion of large-scale livestock farming continue to enhance the P flow in the study area. Fertilizer P inputs from cultivation account for approximately 60% of the total inputs to farming systems, but phosphate fertilizer utilization continues to decline. Feed P inputs have continued to increase since 2007. The expansion of large-scale farming and the demand for urbanization are the main factors leading to changes in feed P input patterns. The P utilization rate for livestock farming (PUEa) is progressively higher than international levels, with PUEa increasing from 0.64% (1950) to 9.7% (2020). Additionally, per capita food P consumption in the watershed increased from 0.67 kg to 0.80 kg between 1950 and 2020. The anthropogenic P emissions have increased from 1.67 × 104 t (1950) to 8.73 × 104 t (2020), with an average annual growth rate of 2.41%. Watershed-wide P pollution emissions have increased by more than five-fold. Population growth and agricultural development are important drivers of structural changes in P flows in the study area, and they induce changes in social conditions, including agricultural production, dietary structure, and consumption levels, further dominating the cyclic patterns of P use, discharge, and recycling. This study provides a broader and applicable P flow model to measure the characteristics of the P cycle throughout the watershed social system as well as provides methodological support and policy insights for large lakes in rapidly developing areas or countries to easily present P flow structures and sustainably manage P resources. [Display omitted] [ABSTRACT FROM AUTHOR]

Understanding various biogeochemical processes, especially in eutrophic sediments, necessitates fine-scale phosphorus (P) measurements in pore waters. To the best of our knowledge, the fine-scale distributions of P across the sediment profiles of Lake Nansi have rarely been investigated. Herein we evaluated the dynamic distributions of labile P and Fe across the sediment-water interface (SWI) of Lake Nansi at two-dimensional (2D) and sub-millimeter resolution, using well-established colorimetric diffusive gradients in thin films (DGT) methodology. The concentrations of labile P in all investigated sediment profiles exhibited strong spatial variations, ranging from 0 to 1.50 mg/L with a considerable number of hotspots. Lake Nanyang (0.55 ± 0.21 mg/L) had the highest mean concentration of labile P, followed by Lake Dushan (0.38 ± 0.19 mg/L), Lake Weishan (0.28 ± 0.21 mg/L), and Lake Zhaoyang (0.18 ± 0.09 mg/L). The highest concentrations of labile P were always detected in Lake Dushan, which had been subjected to excessive exogenous P pollution. The co-distributions of labile P and Fe in the majority of the sediment of Lake Nansi confirmed highly positive correlations (P < 0.01), suggesting that the mobility of labile P throughout the SWI was likely governed by iron redox processes. The apparent diffusion fluxes of P across the SWI ranged from −7.7 to 33.6 μg/m2·d, with a mean value of 5.26 ± 7.80 μg/m2·d. Positive apparent fluxes for labile P were recorded in most sediment cores, demonstrating the strong upward mobility of P from the sediment to the overlying water. Our results provided accurate and extensive information regarding the micro-distribution and dynamic exchange of labile P across the SWI. This allows for a better understanding of eutrophication processes and the implementation of P management strategies in Lake Nansi. [Display omitted] • The spatial variation and mobility of P in Lake Nansi were characterized using DGT. • 2D, heterogeneously micro-distribution of P in the sediment profile was observed. • Labile P had a northward and downward spatial gradient, related to human activity. • The mobility of P across the SWI was likely governed by iron redox processes. • Fine-scale measurement improves the accuracy P-cycle depiction for eutrophic lake. [ABSTRACT FROM AUTHOR]