379 / 2024-09-15 19:29:36

Atmospheric nitrogen deposition and its contribution to phytoplankton biomass in the South Yellow Sea

atmospheric deposition,nutrients,Yellow Sea,primary productivity

The South Yellow Sea (SYS) is a semi-enclosed marginal sea of the northwest Pacific. The SYS is characterized by two primary production processes: spring phytoplankton bloom (SPB), and subsurface chlorophyll maximum (SCM) in summer. Atmospheric deposition is one of the main nitrogen sources supporting the SPB and SCM in the SYS. However, its contribution to the primary production has not been clarified. In this study, to track the dissolved inorganic nitrogen (DIN) derived from atmospheric deposition (AD-N), and calculate its contribution to the SPB and SCM, a physical-biogeochemical coupled model with nutrient tracking module was applied.

The annual mean AD-N flux over the SYS is 0.26 mmol N/m²/day. The surface DIN concentration is highest in winter, and consumed up in spring by SPB, whose average concentration in the central SYS (CSYS) reaches 2.4 mg Chl-a/m³. The SCM appears at the depth of around 20 m with a concentration of 0.6 mg Chl-a/m³. Under the steady state after long-term accumulation, AD-N contributes approximately 52% of the DIN in the water column of CSYS. The Chlorophyll-a supported by AD-N accounts for 57% of the surface SPB, and 53% of the SCM at 20 m. The surface AD-N proportion to total DIN in the CSYS increases from February to July due to stratification, and begins to decrease in August due to the input of Changjiang diluted water, whose contribution increases from 18% in July to 22% in August. The bottom AD-N proportion decreases in winter due to appearance of Yellow Sea Warm Current. The annual cycle of surface Chl-a supported by AD-N follows that of the AD-N proportion in the total DIN.

Furthermore, the atmospheric deposition in a year contributes 12.7% of the total nitrogen (TN, sum of DIN, phytoplankton, and detritus) in the CSYS water column. For the SPB, the AD-N deposited in last summer, last autumn, last winter, and this spring contribute 1.9%, 3.5%, 4.4%, 5.3% of the total Chlorophyll-a, respectively. For the SCM between 10 and 22 m, the AD-N deposited in this summer contributes less chlorophyll-a than the background deposition accumulated until one year ago.