Seasonal variation of primary productivity in the East China Sea: A numerical study based on coupled physical-biogeochemical model

The seasonal variation of phytoplankton growth in the East China Sea (ECS) is simulated with a three-dimensional coupled physical-biogeochemical model, which includes discharges from Changjiang (Yangtze River). The purpose is to determine the main control on the seasonality of primary productivity in the ECS shelf, which nurtures rich biological resources. The model has a horizontal resolution of 1/6° in the domain from 23°N to 41°N and from 116°E to 134°E, excluding the Japan/East Sea, and 33 layers in the vertical. The nitrogen-based biogeochemical model has four compartments: dissolved inorganic nitrogen (DIN), phytoplankton, zooplankton and detritus. The chlorophyll to phytoplankton ratio depends on light and DIN availability. The model is driven by monthly climatological winds with the sea-surface temperature, salinity and DIN relaxed towards the climatological mean values. It successfully reproduces the observed seasonal variation of primary productivity over the ECS shelf with a strong peak in later spring and summer. The modeled annual mean primary production (PP) over the entire ECS shelf is 441 mg C m−2 d−1, which falls within the reported range of 390–529 mg C m−2 d−1. It also reproduces the marked seaward gradient of DIN that decreases away from the Changjiang plume. Strong dependency on the amount of photosynthetically active radiation (PAR) is demonstrated for primary production. For 1% change in PAR 0.7% change in PP is effected. Numerical experiments show that the strong summer peak of DIN load from Changjiang cannot generate the seasonality of PP without the seasonal cycle of PAR. On the other hand, the model can still produces the seasonal pattern with Changjiang nutrient load set to zero, indicating light availability as the major control. Yet the Changjiang DIN load induces a PP increase of 77 mg C m−2 d−1, which represents 13–20% of the observation based estimates of PP. It is noted that the increase in nitrogen uptake associated with PP is 2.7 times the DIN provided by the Changjiang discharge, implying efficient recycling of the riverine nutrients in the shelf water. The model needs improvement on the sluggish dispersion of the Changjiang plume and insufficient vertical mixing. It also needs a more complicated biogeochemical model with more size classes of organisms, multiple-nutrient schemes and additional geochemical processes.