Since the Industrial Revolution, economic prosperity has increased the input of phosphorus from land to the coastal zone, leading to severe phosphorus pollution in some offshore waters. However, numerous studies have shown that the Bohai Sea is nutrient-imbalanced and phosphorus-limited. What role does the Yellow River, an important source of nutrient input to the Bohai Sea, play in phosphorus limitation in the Bohai Sea? Most studies have focused on the transport of phosphorus to the Bohai Sea from the lower reaches of the Yellow River and the Yellow River Delta. They ignore the fact that the Yellow River runs east to west across China, and that phosphorus from the upper and middle reaches may also be imported into the Bohai Sea with the Yellow River. In addition, the Yellow River is a seasonal river that varies greatly between the rainy and dry seasons in terms of precipitation, suspended particulate matter concentration, and flow rate. According to studies, the Yellow River receives almost half of its annual precipitation in the summer and only approximately 3% in the winter.

To solve the above outstanding questions, using the standards, measurements and testing (SMT) method and solution ³¹P nuclear magnetic resonance (³¹P-NMR) spectroscopy, this study investigated the cross-regional variations of suspended particulate matter and phosphorus concentrations and compositions from the Yellow River source to the estuary in the rainy season of 2020 and the dry season of 2021. According to the spatial and temporal distribution of phosphorus in the Yellow River, the distribution pattern of phosphorus in the suspended particulate matter and the water body in the rainy and dry seasons was opposite. It indicated that the majority of the phosphorus in the Yellow River was adsorbed or surrounded by suspended particulate matter. In addition, suspended particulate matter of the Yellow River was prone to carry phosphorus for deposition in reservoirs and rivers.

Subsequently, we analyzed the phosphorus fractionation of suspended particulate matter in the Yellow River. Since the suspended particulate matter in most sections of the Yellow River did not vary noticeably, we selected 16 representative sampling sites to measure and analyze the phosphorus fractionation of suspended particulate matter in the Yellow River. During the rainy season, it can be seen from the Figure 1(a) that NaOH-P in the suspended particulate matter of Yellow River accounted for only 2.9-5.2% of the total particulate phosphorus, while HCl-P accounted for 59.0-83.5% of the total particulate phosphorus. NaOH-P represents the Fe/Mn/Al bound phosphorus in the sediment, which is the main fraction of phosphorus exchange with the Yellow River water column. HCl-P stand for phosphorus bound to calcium carbonate, usually not bioavailable. It indicated that the suspended particulate matter of Yellow River have a low content of reactive Fe and a low ratio of exchangeable phosphorus to total particulate phosphorus. However, in dry season, the NaOH-P content in suspended particulate matter was 2.5-6.8 times higher than in rainy season, and the HCl-P content was comparable to that in rainy season (Figure 1). The results indicated that the bioavailable phosphorus content in dry season was higher than that in rainy season. However, the runoff volume of the Yellow River in the dry season decreases dramatically, the water velocity slows down, and the concentration of suspended particulate matter in the Yellow River is much lower than that in the rainy season. Meanwhile, we found that the NaOH-P contents of suspended particulate matter in the upper reaches of the Yellow River were generally higher than those in the middle and lower reaches.

Figure 1 The contents of different forms of P and TP in suspended particles in the Yellow River. (a) Rainy season; (b) Dry season. “OP”, “HCl-P” and “NaOH-P” mean organic phosphorus, phosphorus associated with calcium carbonate and iron-manganese-aluminum bound phosphorus, respectively.

In addition, in Figure 2, the results of ³¹P-NMR analysis of the suspended particulate matter of the Yellow River showed that only 3.6-20.3% of phosphorus in suspended particulate matter could be extracted as biogenic-P by NaOH-EDTA, which was different from other rivers sediments. The non-extractable phosphorus may be Ca-bound and refractory organic phosphorus, which may not be bioavailable. The content of biogenic-P in the upper reaches exhibited wide fluctuations during the rainy season, but that in middle and lower reaches remained constant, with only a slight difference between the two seasons. Meanwhile, Mono-P accounted for more than 78% of biogenic-P (except Ortho-P), and Mono-P was considered as phosphorus that was not directly bioavailable and immobile in suspended particulate matter. Moreover, a clear wave-like upward trend in the Mono-P: Pyro-P ratios indicated that only a small amount of phosphorus can be transported from the source of the Yellow River to the estuary.

Figure 2 ³¹P-NMR spectra of 0.25 M NaOH extracted from SPM in the Yellow River in different seasons. (a) Upper reaches in rainy season; (b) Middle reaches in rainy season; (c) Lower reaches in rainy season; (d) Lower reaches in dry season. The colored lines are used to distinguish different ³¹P-NMR spectra. “QH”, “GS”, “NX”, “NM”, “SX”, “HN” and “SD” stand for Qinghai, Gansu, Ningxia, Inner Mongolia, Shanxi, Henan and Shandong provinces of China, respectively. The Arabic numerals “12”, “3”, “6”, etc. represent the order of sampling points. For example, “QH-12” represents the 12th sampling site in Qinghai Province and “SD-4” represents the 4th sampling site in Shandong Province.

This research is of great significance for exploring the changes of phosphorus during long-distance migration in rivers with high suspended particulate matter concentration. It revealed a gradual decrease in the reactivity of phosphorus pools in the Yellow River from the source to the estuary, suggesting a low potential for biologically effective phosphorus export from the Yellow River to the Bohai Sea. This is scientifically important for further revealing the causes of long-term phosphorus limitation in the Bohai Sea. In the future, we will investigate the impact of the Yellow River nutrient input on the nutrient status of the Bohai Sea and its intrinsic mechanism, with the goal of providing a scientific foundation for a comprehensive understanding of the impact of other high sand-containing rivers on the nutrient status of the oceans around the world.