Abstract
Abstract. Phytoplankton light absorption properties were investigated at the surface and subsurface chlorophyll a maximum (SCM) layer in the East China Sea (ECS), a marginal sea which is strongly influenced by the Changjiang discharge 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 factors, packaging effect (cell size) and pigment composition of total chlorophyll a (Tchl a)-specific absorption coefficient (aph*(λ)) were examined by the corresponding measurements 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 average aph*(λ) in ECS than in TS. Within SCM, average aph*(λ) was higher in ECS than in TS because of the dominance of nano- and micro-phytoplankton in ECS and TS, respectively. By pooling surface and SCM samples, we found regular trends in phytoplankton size-fraction versus Tchl a; and correlations between aph*(λ) and Tchl a consistent with previous 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 between aph*(λ) and Tchl a. The abnormal values mainly originated from the surface low-salinity waters and SCM waters beneath them. At high Tchl a, aph*(λ) 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 indicated that the distinct light absorption properties of phytoplankton in ECS were possibly influenced by the Changjiang discharge. Our findings imply that general bio-optical algorithms proposed based on the correlations between aph*(λ) 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 regions.
Highlights
Phytoplankton light absorption is a major determinant of the optical properties of oceanic and coastal waters
The difference in average aph(λ) between the surface and subsurface chlorophyll a maximum (SCM) was larger in East China Sea (ECS) than in Tsushima Strait (TS)
By comparing the packaging effect and ap∗igm(440), we found that the packaging effect was the main cause for the differences in ap∗h(λ) between East China Sea SCM (ECS_SCM) and Tsushima Strait SCM (TS_SCM) (Figs. 4a and 8)
Summary
Phytoplankton light absorption is a major determinant of the optical properties of oceanic and coastal waters. The packaging effect, together with phytoplankton pigment composition, have been suggested to be the major factors causing the variability in ap∗h(λ) (Morel and Bricaud, 1981; Stuart et al, 1998; Lohrenz et al, 2003). These two factors are different among and within phytoplankton populations growing under various environmental conditions such as nutrient levels and irradiance (Bricaud, 2004; Stæhr et al, 2004)
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