Influence of the Changjiang River on the light absorption properties of phytoplankton from the East China Sea

Phytoplankton light absorption properties were in- vestigated at the surface and subsurface chlorophyll a max- imum (SCM) layer in the East China Sea (ECS), a marginal sea which is strongly influenced by the Changjiang dis- charge in summer. Results from ECS were compared with those from the Tsushima Strait (TS) where the influence of Changjiang discharge is less. The probable controlling fac- tors, packaging effect (cell size) and pigment composition of total chlorophyll a (Tchl a)-specific absorption coeffi- cient (a ph ()) were examined by the corresponding mea- surements of pigments identified by high-performance liquid chromatography. We observed distinct phytoplankton size structure and thereby absorption properties between ECS and TS. At the surface, mixed populations of micro-, nano- and pico-phytoplankton were recorded in ECS while pico- phytoplankton dominated in TS, generating a lower aver- age a ph () in ECS than in TS. Within SCM, average a ph () was higher in ECS than in TS because of the dominance of nano- and micro-phytoplankton in ECS and TS, respec- tively. By pooling surface and SCM samples, we found reg- ular trends in phytoplankton size-fraction versus Tchl a; and correlations between a ph () and Tchl a consistent with pre- vious observations for the global ocean in TS but not in ECS. In ECS phytoplankton size-fraction was not correlated with Tchl a, which consequently caused poor relationships be- tween a ph () and Tchl a. The abnormal values mainly orig- inated from the surface low-salinity waters and SCM wa- ters beneath them. At high Tchl a, a ph () of these samples was substantially higher compared to the values in TS and from the global regressions, which was attributable to the lower micro-phytoplankton fraction, and higher nano- and/or pico-phytoplankton fractions in ECS. These observations in- dicated that the distinct light absorption properties of phyto- plankton in ECS were possibly influenced by the Changjiang discharge. Our findings imply that general bio-optical algo- rithms proposed based on the correlations between a ph () and Tchl a or the patterns in size-fraction versus Tchl a are not applicable in ECS, and need to be carefully considered when using these general algorithms in river-influenced re- gions.