Analyzing the phosphorus flow characteristics in the largest freshwater lake (Poyang Lake) watershed of China from 1950 to 2020 through a bottom-up approach of watershed-scale phosphorus substance flow model

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.

[Transport Characteristics of Phosphorus Sources at the Multi-scale Watershed and the Associated Ecological Effects on Poyang Lake]

In this study, a cascaded watershed system in the Poyang Lake area was selected as the study site, which ranged from the primary tributaries to the lake area (Xiangxi River→Jiazhu River→Ganjiang River→Poyang Lake). The aims of the study were to monitor the P wet deposition and runoff process in the Poyang Lake area and discuss the P transport characteristics at the multi-scale watershed and its impact on phytoplankton community structure in the Poyang Lake. The results showed that the P concentration in the Poyang Lake area exhibited significant seasonal changes. Apart from the Xiangxi River watershed, the concentrations of total phosphorus (TP), dissolved total phosphorus (DTP), and orthophosphate(PO₄^3-) were higher in the low flow period than in the high flow period at other watersheds. There was a significant correlation between TP concentration and diatom density during the high flow period, and between TP concentration and cryptophyta during the low flow period. The order of the amounts of TP and PO₄^3- transport by runoff under different rain intensities is as follows:light rain > moderate rain > heavy rain. There is a significant difference in the deposition flux between the wet season and the dry season with seasonal change, in which the flux during the wet years was about 2.8 times higher than in the dry years. The export flux of P was also shown to be higher in the high flow than in the low flow period. As watershed size increased, the contribution rate of P export did not increase, indicating that P transported at different scales was not the main source of P in each watershed.

Evaluation of the Human Interference on the Microbial Diversity of Poyang Lake Using High-Throughput Sequencing Analyses

The Poyang Lake Watershed (PLW) is regarded as an air temperature moderator, as well as a wind energy, food resources and good habitat in the Jiangxi Province, People’s Republic of China. However, with the increasing of anthropogenic disturbance on PLW, there are few studies focused on the effects of human activities on microbial composition in Poyang Lake. In the present study, a high-throughput sequencing method was used to identify the microbial composition in water and sludge in Dahuchi (DHC, sub-lake of Poyang Lake National Nature Reserve), Shahu (SH, sub-lake of Poyang Lake National Nature Reserve), Nanhu (NH, sub-lake out of Poyang Lake National Nature Reserve), Zhelinhu (ZLH, artificial reservoir), Sixiahu (SXH, sub-lake artificially isolated from Poyang Lake) and Qianhu (QH, urban lake). Results of the present study illustrated the various bacterial diversity between different lakes, for example, at the phylum level, Actinobacteria and Cyanobacteria showed low abundance in water samples of ZLH and QH, and high abundance in DHC. In addition, anthropogenic disturbance and human activities decreased the abundance of probiotic bacteria (Actinobacteria, Cyanobacteria, Chloroflexi and Acidobacteria) and increased the abundance of pathogenic bacteria (Acinetobacter, Aeromonas and Noviherbaspirillum). The enrichment of pathogenic bacteria in polluted lakes, in turn, may cause potential threats to human health.

[C and N Transport Flux and Associated Changes of Water Quality Parameters from a Multiscale Subtropical Watershed in the Poyang Lake Area]

In this study, a connected waterflow watershed system in the Poyang Lake area was selected as the study site, which ranged from the primary tributary to the lake area (Xiangxi River Jiazhu River Ganjiang River Poyang Lake). The aims of the study were to monitor different forms of C and N and evaluate the transport flux of C and N, and then, the transport mechanisms of C and N and the variation characteristics of water quality parameters in Poyang Lake were discussed, with the intent of providing a scientific basis for the comprehensive management of watershed health within the Poyang Lake Basin ecosystem. The main results were as follows. ① The concentrations of C and N in the Poyang Lake watershed exhibited significant seasonal changes, wherein the TIC, TOC, and TC concentrations in the Poyang Lake Basin were higher in the wet season than those in the dry season, and the NO₃^--N and DTN concentrations were higher in the dry season than those in the wet season. The main reason for the increase of TC in the wet season was the increase of TIC. Most of the TN in the wet season was transported by non-dissolved forms of N, while the TN in the dry period mostly was transported by DTN, and the DTN was mostly in the form of NO₃^--N. ② The C and N transport fluxes in the Poyang Lake watershed also showed significant seasonal variation. The C transport flux of Xiangxi River was lower during the wet season than that during the dry season, and the C transport flux of Jiazhu River and Ganjiang River was higher during the wet season than that during the dry season. The various forms of N transport flux in Xiangxi River, Jiazhu River, and Ganjiang River watershed were higher in the wet season than those in the dry season. There was a very significant positive correlation between the flux and runoff at the 99% confidence level. ③ The COND, TDS, and pH in the Poyang Lake watershed were lower during the wet season than those during the dry season, while the ORP in the wet season was higher than that in the dry season.