Distribution and structure of dissolved inorganic nitrogen and phosphorus in Laizhou Bay, 2018

Based on field data of nutrients observed in Laizhou Bay during May and August, 2018, the distribution characteristics of nitrogen and phosphorus in Laizhou Bay were analyzed and discussed. The results showed that the concentrations of dissolved inorganic nitrogen ranged from 0.0203 mg/L to 1.49 mg/L with an average content of 0.339 mg/L and which was significantly higher in May. The dissolved inorganic nitrogen concentrations were between 0.1 mg/L and 0.4 mg/L in 64.5% of survey stations. Approximate 17.8% of investigating stations with dissolved inorganic nitrogen levels exceeded the fourth class of seawater quality standard. The proportions of NO₃-N, NO₂-N and NH₄-N in dissolved inorganic nitrogen were 66.6%, 9.1 % and 24.3%, respectively, and NO₃-N was the main form of dissolved inorganic nitrogen. The concentrations of PO₄-P ranged from not detected to 62.3×10⁻³ mg/L with an average content of 0.00563 mg/L and which was significantly higher in August. Approximate 95.6% of investigating stations with PO₄-P levels were lower than the first class of seawater quality standard. Approximate 96.3% of investigating stations with N/P ratio were larger than 16, while 13.1% of investigating stations with E value were larger than 1. The high value areas of dissolved inorganic nitrogen, PO₄-P, N/P ratio and E were mainly located in the sea area near Xiaoqing estuary and Yellow River estuary in the west of Laizhou Bay. The proportion of NH₄-N in dissolved inorganic nitrogen was the highest in summer. The main factor leading to the increase of N/P ratio was low PO₄-P content.