Distribution of Nitrification and Its Regulating Factors in Coastal Bays with Distinct Trophic Gradients

Nitrification is the key process linking the oxidized and reduced forms of reactive nitrogen, playing an important role in the nitrogen biogeochemical cycle. Quantifying the nitrification rate and evaluating its environmental regulators in different aquatic environments at both regional and global scales has received increasing attention. However, the spatiotemporal variations in nitrification rates in coastal waters, particularly with different trophic states, remain unclear. By using the 15N-labeling technique, here, we quantified the nitrification rates under dark and light conditions in the surface waters of Shenzhen Bay and Dapeng Bay, representing eutrophic and oligotrophic environments, respectively. The nitrification rates were 2–3 orders of magnitude higher in Shenzhen Bay (eutrophic) than those in Dapeng Bay (oligotrophic). The concentrations of ammonium and total suspended matter play key roles in regulating the spatiotemporal distribution and difference in nitrification in these two coastal bays. The nitrification rate under the dark condition (0.13–49.37 nmol N L−1 h−1) was greater than that under light incubation (0–10.15 nmol N L−1 h−1), indicating light inhibition of 33–100% in the surface water. Such results imply that daily integrated nitrification based on the rates under dark incubation may have been overestimated. An environment with high turbidity is preferable for nitrification, as it reduces the damage caused by light to ammonia-oxidizing microbes. Collectively, the differences in nitrification rates further result in a distinct composition of dissolved inorganic nitrogen, with Shenzhen Bay dominated by nitrate and Dapeng Bay dominated by ammonium. Our results provide scientific references for the mitigation of nitrogen pollution in different trophic coastal bays.