Influence of river discharge on phytoplankton absorption properties: a case study in the East China Sea and Tsushima Strait.

To investigate the influence of fresh water on phytoplankton absorption properties, the phytoplankton absorption coefficient and pigments identified by high-performance liquid chromatography (HPLC) were measured at the surface and subsurface chlorophyll a maximum (SCM) in the East China Sea (ECS), which is highly influenced by river discharge from the Changjiang during summer. For comparison, data were also collected in the Tsushima Strait (TS) at the surface and SCM. The majority of ECS surface samples taken from the low-salinity Changjiang diluted water (CDW), and even most of SCM samples taken from waters beneath the CDW, displayed significant freshwater influences. The specific absorption coefficient normalized by total chlorophyll a concentration (Tchl a), a_ph*(λ), of these samples was substantially higher than values derived from global regressions between a_ph*(λ) and the Tchl a. Using the pigment data derived from HPLC, the increase of a_ph*(λ) was found to be mainly caused by the phytoplankton size structure, which indicated that both surface and SCM samples in the ECS still incorporated considerable portions of picophytoplankton (cyanobacteria), even though the Tchl a was high. When water from the surface and the SCM were merged, variations in the phytoplankton size-fractions and a_ph*λ) vs. Tchl a that were consistent with values for the global ocean were found in the TS but not in the ECS. Data for the ECS indicated that there was no correlation between Tchl a and the size-fraction or total pigment absorption. As a consequence, a_ph*(λ) was poorly correlated with Tchl a and displayed large variability within a small Tchl a range. These findings suggest the need for care when considering the changing patterns of size-fractions vs. Tchl a and the relationship between a_ph*(λ) and Tchl a in coastal regions that are significantly influenced by fresh water. [ABSTRACT FROM AUTHOR]