Investigation of seasonal variability of CDOM fluorescence in the southern changjiang river estuary by EEM-PARAFAC

The southern Changjiang River Estuary has attracted considerable attention from marine scientists because it is a highly biologically active area and is biogeochemically significant. Moreover, land-ocean interactions strongly impact the estuary, and harmful algal blooms (HABs) frequently occur in the area. In October 2010 and May 2011, water samples of chromophoric dissolved organic matter (CDOM) were collected from the southern Changjiang River Estuary. Parallel factor analysis (PARAFAC) was used to assess the samples’ CDOM composition using excitation-emission matrix (EEM) spectroscopy. Four components were identified: three were humic-like (C1, C2 and C3) and one was protein-like (C4). Analysis based on spatial and seasonal distributions, as well as relationships with salinity, Chl a and apparent oxygen utilization (AOU), revealed that terrestrial inputs had the most significant effect on the three humic-like Components C1, C2 and C3 in autumn. In spring, microbial processes and phytoplankton blooms were also important factors that impacted the three components. The protein-like Component C4 had autochthonous and allochthonous origins and likely represented a biologically labile component. CDOM in the southern Changjiang River Estuary was mostly affected by terrestrial inputs. Microbial processes and phytoplankton blooms were also important sources of CDOM, especially in spring. The fluorescence intensities of the four components were significantly higher in spring than in autumn. On average, C1, C2, C3, C4 and the total fluorescence intensity (TFI) in the surface, middle and bottom layers increased by 123%–242%, 105%–195%, 167%–665%, 483%–567% and 184%–245% in spring than in autumn, respectively. This finding corresponded with a Chl a concentration that was 16–20 times higher in spring than in autumn and an AOU that was two to four times lower in spring than in autumn. The humification index (HIX) was lower in spring that in autumn, and the fluorescence index (FI) was higher in spring than in autumn. This result indicated that the CDOM was labile and the biological activity was intense in spring.