Hydro-biogeochemical alterations to optical properties of particulate organic matter in the Changjiang Estuary and adjacent shelf area

Abstract

Base-extracted particulate organic matter (BEPOM) collected in low (spring) and high (summer) runoff seasons in the Changjiang Estuary and adjacent nearshore/offshore shelf areas was examined using absorption and fluorescence spectroscopy. Parallel factor analysis (PARAFAC) identified two humic-like (C1p and C2p) biologically-refractory components and two protein-like (C3p and C4p) bioreactive components. Absorption coefficient a350p, C1p and C2p of BEPOM showed strong positive correlations with particulate organic carbon (POC), and were generally higher in spring than in summer. C4p correlated positively with Chl a, and was higher in summer than in spring. In the inner estuary, an anthropogenic POM signal from the highly polluted Huangpu River was effectively identified by the fluorescent component ratio (IT:IC)P. In the nearshore area during spring, resuspension within the well-developed turbidity maximum zone (TMZ) supplied large amounts of refractory POM into the water column. POM fractions from an autochthonous source (i.e., algal bloom) were dominant in the offshore area during the summer. A multivariate regression model inferred that POM export fluxes from the Changjiang River during spring had a higher percentage of labile POM fractions, which were subsequently retained by the TMZ. However, these labile fractions were discharged into the offshore shelf area contributing to hypoxia in the summer. The summer high runoff season also transported large amounts of refractory POM that was subsequently buried in the sediment. This study highlights the utilization of optical analyses to trace pollution sources and reveal POM fate and transport dynamics allowing better assessment of its ecological consequences in estuaries.