Cross-shelf variation in carbon-to-chlorophyll a ratios in the East China Sea, summer 1998

Spatial variations of the phytoplankton carbon-to-chlorophyll a ratio (C:chl a) in the East China Sea were investigated during a June 1998 cruise. Based on a regression analysis between particulate organic carbon and chlorophyll a concentrations measured at 2-m depths, estimated values of C:chl a were 13.0 and 92.8 g g−1 for coastal and offshore waters, respectively. In addition, water samples were collected from 5-m depths at three stations with different hydrographic characteristics, and phytoplankton carbon biomass was estimated from microscope-measured cell volumes. At the coastal zone station, chlorophyll a concentration reached 7.9 mg m−3 with Skeletonema costatum as the dominant species. The total phytoplankton carbon was 142.8 mg m−3, and the estimated C:chl a was 18.0 g g−1. At the midshelf station, Synechococcus spp. and Pseudosolenia calcar-avis were the major contributors to phytoplankton carbon. The chlorophyll a concentration was 1.3 mg m−3, and C:chl a was 67.4 g g−1. In contrast, chlorophyll a concentration decreased to 0.1 mg m−3 at the Kuroshio station, where the filamentous cyanobacteria Trichodesmium spp., contributed to most of the phytoplankton carbon, and C:chl a was estimated to be 94.4 g g−1. The C:chl a ratios estimated by the two methods were in close agreement, and a linear relationship was established between the logarithm of chlorophyll a concentration and phytoplankton carbon. The estimated carbon biomass was used to calculate intrinsic growth rates of phytoplankton in the East China Sea. The results indicate that phytoplankton grow actively in the coastal zone, with growth rates often higher than 1.4 day−1, but much lower rates were observed near the margin of the continental shelf.