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

[摘要] Phytoplankton light absorption properties were investigated at the surfaceand subsurface chlorophyll a maximum (SCM) layer in the East China Sea(ECS), a marginal sea which is strongly influenced by the Changjiangdischarge in summer. Results from ECS were compared with those from theTsushima Strait (TS) where the influence of Changjiang discharge is less. Theprobable controlling factors, packaging effect (cell size) and pigmentcomposition of total chlorophyll a (Tchl a)-specific absorptioncoefficient (a_ph^*(λ)) were examined by thecorresponding measurements of pigments identified by high-performance liquidchromatography. We observed distinct phytoplankton size structure and therebyabsorption properties between ECS and TS. At the surface, mixed populationsof micro-, nano- and pico-phytoplankton were recorded in ECS whilepico-phytoplankton dominated in TS, generating a lower averagea_ph^*(λ) in ECS than in TS. Within SCM, averagea_ph^*(λ) was higher in ECS than in TS because of thedominance of nano- and micro-phytoplankton in ECS and TS, respectively. Bypooling surface and SCM samples, we found regular trends in phytoplanktonsize-fraction versus Tchl a; and correlations between a_ph^*(λ) and Tchl a consistent with previous observations for theglobal ocean in TS but not in ECS. In ECS phytoplankton size-fraction was notcorrelated with Tchl a, which consequently caused poor relationshipsbetween a_ph^*(λ) and Tchl a. The abnormal valuesmainly originated from the surface low-salinity waters and SCM waters beneaththem. At high Tchl a, a_ph^*(λ) of these samples wassubstantially higher compared to the values in TS and from the globalregressions, which was attributable to the lower micro-phytoplanktonfraction, and higher nano- and/or pico-phytoplankton fractions in ECS. Theseobservations indicated that the distinct light absorption properties ofphytoplankton in ECS were possibly influenced by the Changjiang discharge.Our findings imply that general bio-optical algorithms proposed based on thecorrelations between a_ph^*(λ) and Tchl a or thepatterns in size-fraction versus Tchl a are not applicable in ECS, and needto be carefully considered when using these general algorithms inriver-influenced regions.

[学科分类] 地球化学与岩石