Seasonal variations of dissolved organic matter chemistry in a semi-enclosed and eutrophic coastal bay in southeastern China: Implications for carbon cycling

As the widely distributed transition zones connecting the terrestrial and marine ecosystems, coastal bays have long water retention times to reconstruct the DOM pool affecting global carbon cycling. However, we still have a limited understanding of mechanisms shaping the carbon cycling of coastal bays at regional scales. To address this issue, we combined stable carbon isotopes, ultraviolet-visible absorption, fluorescence spectroscopy, and ultrahigh-resolution mass spectrometry to investigate the spatiotemporal chemistry of dissolved organic matter (DOM) in Xiangshan Bay, a semi-enclosed, eutrophic bay in southeastern China. The DOM in Xiangshan Bay was characterized by a higher degree of humification and aromaticity during winter, whereas the inverse signature during summer. Results showed that the significant seasonal variation of DOM chemistry was mainly modulated by intense primary productivity stimulated by nutrients during summer, and other factors including sediment pore water DOM release and photochemical reactions. Additionally, based on the microbial carbon pump (MCP) hypothesis, marine microorganisms may synthesize biologically recalcitrant DOM with millennia turnover time. The efficient MCP associated with reduced nutrients probably acted as a critical factor in Xiangshan Bay, leading to the formation of additional refractory molecular formulae (carboxyl-rich alicyclic molecules) and a more recalcitrant signature of DOM during winter. Incorporated with seven other coastal bays across China, it is revealed that anthropogenic inputs substantially contributed to Chinese coastal bays. Further studies need to better constrain the biogeochemical implications of these anthropogenic inputs. More importantly, decreasing the anthropogenic input of nutrients to coastal bays may enhance their functions in long-term carbon sequestration.