Rates and Environmental Regulating Factors of Nitrification in the Yellow Sea and Bohai Sea in Spring

Nitrification is a key process that controls the transformation of diverse valence of nitrogen within the water column. It plays a crucial role in controlling nitrogen cycling within the oceans. For marginal seas, which are profoundly influenced by anthropogenic activities, the study of exogenous nitrogen input is relatively mature. However, the study of the nitrification process within the water column and its impact on nitrogen cycling transformation remains poorly understood. In this study, we conducted in situ observations in the Bohai Sea and the North Yellow Sea between May and June 2021. The objective was to quantify the rate of the nitrification in the Yellow Sea and Bohai Seas and to identify the key environmental control factors. This is achieved through ¹⁵N enrichment incubation experiments, and combined with light and nutrient. The results demonstrated that the nitrification rates in the Yellow Sea and the Bohai Sea during the spring ranged from 0.06 to 2.56 nmol·L^-1·d^-1 and from 0.11 to 3.15 nmol·L^-1·d^-1, respectively. Furthermore, the nitrification rates in the surface waters of the observation area are generally lower than those in the bottom layer. In the Yellow Sea and Bohai Sea in spring, the nitrification rate and the concentration of ammonia, the substrate of the nitrification reaction, conformed to the relationship of Michaelis-Menten equation. Light significantly suppressed the nitrification process, and the relationship between photosynthetically active radiation (PAR) and nitrification rate was consistent with the kinetic characteristics of inhibition. This study provides new insights into the internal nitrogen transformation process of seawater at the continental marginal seas and its environmental control factors.