The Current Status and 20 Years of Evolution of Nutrient Structure in the Yellow River Estuary

Abstract

The distribution and evolution of nutrients in the Yellow River estuary and its adjacent waters have always been a hot research topic on the geochemical cycle of biogenic elements in the northwest continental margin of the west Pacific Ocean as well as the focus of research on the eutrophication, environmental quality supervision, and comprehensive pollution control in the Bohai Sea. We conducted a survey on the current status of nutrients and related environmental factors in the Yellow River estuary and its adjacent waters in May and August 2023, during which period the primary production was high. Historical data from the past 20 years were collected and summarized to analyze and reveal the potential trends of nutrient composition and structure in the Yellow River estuary and to elaborate the current status and main influencing factors of nutrients in the Yellow River estuary. The main results are presented as follows.The concentrations of dissolved inorganic nitrogen (DIN) varied wider and higher in August than that in May, 2023. The higher concentration was located outside the Yellow River mouth and west Laizhou Bay, and the DIN concentration gradually decreased toward the northwest and southeast from the Yellow River mouth. The lower value appeared in the northern waters of the Yellow River estuary. Although the average DIN concentrations in August were about half that in May, the variation of DIN concentrations in August was larger than that in May, with the highest value in August being about twice that of May and the lowest value being only about 1/10 that of May. Vertically, the surface layer showed the highest DIN concentrations, followed by the bottom layer, and the middle layer showed the lowest concentration. The concentration of DIP located at wider and higher ranges in August than that in May, which was similar to DIN. The DIP distribution was generally similar to that of DIN in tendency, whereas the average DIP concentration in May was comparable to that in August. Meanwhile, the difference in the surface, middle, and bottom concentrations in August was smaller than that in May. The concentration of DIN and DIP in the lower reaches of the Yellow River in May was about 4 and 2 times higher than that outside the mouth of Yellow River, whereas that in August was about 10 and 4 times higher, respectively.Nitrate was the main component of DIN, accounting for 50.7%–96.5% (average 85.2%) in May and 3.9%–99.1% (average 57.1%) in August, respectively. Although the DIN composition was consistent between May and August, the percentage variation in the DIN composition was significant, and the percentage variation between DIN compositions in August was much larger than that in May. The DIN to phosphate ratio (N/P) waved at a wide range in both May and August, with a high average N/P of far from Redfield Ratio, among which the lowest N/P of 43.7 was observed in May. The average N/P of the lower reaches of the Yellow River was 910.7 in May and 410.9 in August. In comparison, the surface N/P was relatively high, approximately 1.5 and 3 times that of the bottom and middle layers.The DIN concentration in the Yellow River estuary increased to 60 μmol/L before 2006, decreased to 30 μmol/L in 2009, and then remained under 30 μmol/L until 2023. The DIP concentration showed peak vales in 2005 and 2013 after increasing from those in 2006 to 2008 and were maintained at lower levels in the other years. The DIN was dominated mainly by nitrate in the past 20 years, followed by ammonia, and then nitrite. In episodic cases, ammonia and nitrite contributed to more than 80% of the total DIN, which may be related to strong terrestrial inputs and weak oxidizing conditions in the bottom water. The nitrogen/phosphorus ratio in the Yellow River estuary continued to increase in the past 20 years. Based on the developing trend of nutrient composition and structure in the Yellow River estuary and its adjacent waters, the imbalance of "more nitrogen and less phosphorus" may be further intensified. DIN and silicate both showed a significant linear correlation with salinity, indicating that surface runoff was the major input pathway for nutrients in the Yellow River estuary. The discrete point plots of DIN components and DO indicated a clustered distribution, indicating that DIN components and DO were influenced by some common factors, and the direct interaction between the two was weak or rapidly reached an equilibrium state. There was no obvious evidence of groundwater transport in study area, and the contribution and mechanism of dissolved organic matter conversion and primary production to nutrients must be understood through further investigation.The results of principal component analysis indicated that the composition and structure of nutrients may significantly characterize the water properties of the Yellow River estuary. The extreme variations in the composition and distribution of nutrients, as well as the high nitrogen/phosphorus ratio, could push the water environment quality situation into a more severe status in the Yellow River estuary.

Keywords

• yellow river estuary
• nutrients
• composition and structure
• historic evolution