A modelling approach to assess the effects of atmospheric nitrogen deposition on the marine ecosystem in the Bohai Sea, China

Atmospheric deposited nitrogen (AD-N) approximates or exceeds riverine input in many coastal ecosystems, such as the Bohai Sea (BHS) which is one of the most eutrophic coastal waters in China. We construct a three-dimensional (3D) physical-biogeochemical model to understand the influence of atmospheric dissolved inorganic nitrogen (DIN) on the intra- and inter-annual variations of phytoplankton blooms and nutrient dynamics in the BHS. The biological component, which is coupled to the Regional Ocean Modelling System (ROMS), is a simple but widely used Nutrient-Phytoplankton-Zooplankton-Detritus (NPZD) model with eight state variables. The model simulation successfully reproduces the spatial and temporal variations of observed DIN and dissolved inorganic phosphorus (DIP) and the climatological features of phytoplankton biomasses (chlorophyll a), which confirms the major role of air-transported nutrients in controlling standing stocks and nutrient limitations. The modelling results show that the relative contributions of deposited nitrogen to the total DIN in the seawater for the Bohai, Laizhou and Liaodong Bays and Central Bohai Basin reach 84.8%, 49.3%, 37.4% and 44.4% on average, respectively. The relative contribution ratio is approximately 54% for the entire Bohai region, causing a 56.5% increase in the phytoplankton biomass on average. These results also indicate that the effects of AD-N on nutrient and phytoplankton dynamics vary widely in regional areas because of the uneven spatial distribution of nitrogen deposition fluxes and also partially because of the hydrodynamic conditions, shortwave radiation and water temperatures.