Assessing depth-integrated phytoplankton biomass in the East China Sea using a unique empirical protocol to estimate euphotic depth

• A unique empirical protocol for the estimation of euphotic depth (Ze) is developed.
• Depth-integrated Chl-a from the surface to the Ze(∑ChlZe)(∑ChlZe) is evaluated spatially.
• The ∑ChlZe∑ChlZe is higher within the river plume than outside the plume.
• The impact of the plume extension on phytoplankton biomass is discussed.

The Changjiang (Yangtze) River plume has a direct impact on phytoplankton biomass in the East China Sea (ECS). The present study aimed to analyze the spatial distribution of depth-integrated chlorophyll a (Chl-a) concentrations from the surface to the euphotic depth (Ze; ∑ChlZe∑ChlZe) using samples collected at 50 stations in the ECS during a cruise in June 2007. However, spatial coverage was limited because the Ze, obtained from radiometric measurement of the vertical diffuse attenuation of solar irradiance, was only available at the 20 daytime stations. To address this limitation, it was determined that Ze could be expressed empirically using the vertical means of Chl-a concentration, turbidity, and salinity in the euphotic zone. Using this relationship, the potential value of Ze at night-time or low-light stations was calculated, and a dataset of (∑ChlZe)(∑ChlZe) for the entire research area was obtained. A low salinity surface water mass (LSSW) was identified on the eastern continental shelf (125.0°–126.5°E, 30.0°–31.0°N), probably part of the Yellow Sea Mixed Water, but clearly influenced by Changjiang Diluted Water (CDW) extending from the west. The Taiwan Current Warm Water mass (TCWW) was located to the south of the LSSW. Other oceanic water masses, including Kuroshio Surface Water, were located to the east of the LSSW. The means of the Ze and ∑ChlZe∑ChlZe in the LSSW were significantly shallower and higher, respectively, compared with the TCWW and other oceanic water masses (p < 0.01). The present study suggests that the extension of the Changjiang River plume beyond the CDW affects the phytoplankton biomass on the eastern continental shelf of the ECS, more than 300 km from the river mouth.

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