Spatial distribution of chlorophyll concentration controlled by limiting factors and physical environment in the Yellow Sea

Abstract

To understand the spatial changes in chlorophyll concentration in the Yellow Sea, the ocean circulation and lower tropic ecosystem were simulated from 2008 to 2010 and analyzed using a three-dimensional coupled physical-biogeochemical model, the Proudman Oceanographic Laboratory Coastal Ocean Modeling System (POLCOM)–European Regional Seas Ecosystem Model (ERSEM). The model included three types of nutrients (nitrates, phosphates, and silicates), four types of phytoplankton (one for diatoms and three for non-diatoms with size fraction), and three size fractional groups of zooplankton. The analysis was focused on the ocean circulation and chlorophyll oncentration distribution. In the spatial distribution of chlorophyll concentration averaged from 2008 to 2010, relatively high chlorophyll concentrations (1 mg/m3 or more) were found over the entire water column in the shallow coastal region of the Yellow Sea. However, in the deep central ocean, chlorophyll concentration was lower in the upper and lower layers except approximately 20 m depth. Based on the distribution of 1 mg/m3 isoconcentration line, which lies with the largest spatial gradient of chlorophyll concentration, the three-dimensional ecological zones were divided into surface offshore (SO), subsurface chlorophyll maximum layer (SCML), deep layer (DL), eastern Yellow Sea (EYS), and western Yellow Sea (WYS). Seasonal variation of the spatially averaged chlorophyll concentration for each zone was examined using a nitrate-phosphate chart. In SO, the chlorophyll concentration peaked in April with the increase of light and the chlorophyll concentration decreased from May due to phosphate depletion as vertical stratification increased. In contrast, in SCML, the concentration of chlorophyll slightly increases-decreases-suddenly increases during the spring bloom in the SO. The maximum chlorophyll concentration in SCML appeared in June, and the chlorophyll concentration decreased from July due to phosphate depletion. In DL, seasonal change of chlorophyll concentration occurred similar to that in the SCML.