Determination and uncertainty of spring net community production estimated from O2/Ar measurements in the northern East China Sea and southern Yellow Sea

Abstract

The East China Sea (ECS) and Yellow Sea (YS) are East Asian marginal seas and mainly cover shallow continental shelves with depths of <200 m. These seas have high biological productivities with large spatial and temporal variabilities. Nevertheless, there are a limited number of studies on the biological pump in the ECS and YS. We observed net community production (NCP) for the first time in this region based on high resolution O2/Ar measurements, which act as a measure of biological pump activity. Observations were conducted underway using an equilibrator inlet mass spectrometer in April 2018. Three distinct water masses were identified in the study area: a cold and less saline water mass in the southern YS (Yellow Sea Cold Water; YSCW); a warm and saline water mass along a branch of the Kuroshio current (Eastern Kuroshio Branch Water; EKBW); and a water mass near the southern coast of the Korean Peninsula (Southern Coast Water; SCW). The average NCP of 49 ± 19 mmol O2 m−2 d−1 in the YSCW was almost twice as high as that of 26 ± 13 and 23 ± 12 mmol O2 m−2 d−1 in the EKBW and SCW, respectively. Vertical mixing between the surface mixed layer and subsurface layers was assessed using vertical measurements of microscale turbulence and discrete O2/Ar values and appeared to bias the NCP estimates by ∼2%. In contrast, horizontal advection, which was assessed using the horizontal O2/Ar gradient and current speeds, could be a major source of uncertainty up to 38% in the estimates. The spatial variability of the NCP, shown in the standard deviations (39–52% of the three averages), may largely be ascribed to the horizontal advection in the region, which is affected by strong surface currents.