Bioavailability of dissolved organic carbon linked with the regional carbon cycle in the East China Sea

The regional carbon cycle on continental shelves has created great interest recently due to the enigma of whether these areas are a carbon sink or a source. It is vital for a precise carbon cycle model to take the bioavailability of dissolved organic carbon (DOC) into account, as it impacts the sink and source capacity, especially on dynamic shelves such as the East China Sea. Nine bio-decomposition experiments were carried out to assess differences in the bioavailability of DOC. Samples were collected from different water masses in the East China Sea, such as the Coastal Current, the Taiwan Current, and the Kuroshio Current, as well as from the Changjiang (Yangtze River), the main contributor of terrestrial DOC in the East China Sea. This study aimed to quantify and qualify bioavailable DOC (BDOC) in the East China Sea. Both the degradation constant of BDOC and the carbon output from microorganisms have been quantitatively evaluated. Qualitatively, excitation-emission matrix fluorescence spectra (EEMs) were used to evaluate the intrinsic reasons for BDOC variation. By using EEMs in conjunction with parallel factor analysis (PARAFAC), five individual fluorescent components were identified in this study: three humic-like and two protein-like components (P1, P2). The highest P1 and P2 fluorescence intensities were recorded in the coastal water during a phytoplankton algal bloom, while the lowest intensities were recorded in the Changjiang estuary. Quantitatively, BDOC observed during the incubation ranged from 0 to 26.1 µM. The DOC degradation rate constant varied from 0 to 0.027 (d−1), and was lowest in the Changjiang and highest in algal bloom water and warm shelf water (the Taiwan current). The Taiwan Current and mixed shelf water were the major contributors of BDOC flux to the open ocean, and the East China Sea was a net source of BDOC to the ocean. The results verified the importance of BDOC in regional carbon cycle modeling. Combining the data of BDOC and EEMs, there is significant positive linear correlation between BDOC and protein-like peaks (P<0.01). The transformation and degradation of BDOC, which plays an important role in the microbial carbon pump, could be viewed by EEMs. This suggests the potential of EEMs in the quantitative prediction of BDOC in large-scale carbon cycle modeling.