Sea Ice Melting Drives Substantial Change in Dissolved Organic Matter in Surface Water off Prydz Bay, East Antarctic

Polar ecosystems are vulnerable to climate warming and are characterized by a rapid decline in sea ice. In this study, we collected surface water samples from the Southern Ocean off Prydz Bay during and after sea ice melting season (SIM). After removing the particles using 0.7-mu m pore size filters, we measured the concentration of dissolved organic carbon (DOC) and the composition of dissolved organic matter (DOM) by using the total organic carbon analyzer, optical spectrometry, and high-resolution mass spectrometry. The mean DOC concentration during the SIM is 1.3-fold higher than that during the post-SIM, whereas the a(350) value, an indicator of chromophoric DOM, showed the opposite trend by a factor of 2.8. Five components, including three humic-like and two protein-like substances were identified by fluorescence excitation emission matrices-parallel factor analysis, in which protein-like biolabile components were enriched during the SIM and humic-like refractory components were enriched during the post-SIM. The DOM in the water samples from the SIM had higher proportions of polycyclic aromatics, polyphenols, and compounds within molecular lability boundary, whereas the DOM from the post-SIM had a higher proportion of highly unsaturated compounds. Statistical analysis revealed significant correlations between DOM and sea-ice melting/associated environmental factors (e.g., water temperature, salinity, and microbial activity). These collective results suggest that sea ice melting is an important factor driving a transition from more labile to more refractory DOM in the surface water off Prydz Bay, which may have a profound impact on the marine ecosystem and carbon cycle in the polar ocean. Plain Language Summary The polar ocean is sensitive to climate warming and characterized by rapid sea ice decline. It is important to understand how sea-ice melting influences the marine carbon cycle in the Southern Ocean. In this study, we collected 73 surface water samples from the Southern Ocean close to Prydz Bay, East Antarctica, during the two cruises that corresponded to the sea ice melting season and the post-sea ice melting season. While analyzing the total dissolved organic carbon (DOC) concentration, optical properties, and molecular formulas, we found substantial differences in the concentration and composition of dissolved organic matter (DOM) between the two cruises. Water samples from the sea ice melting season had higher bulk DOC concentrations, more protein-like components, and more compounds within molecular lability boundary. In contrast, water samples from the post-sea ice melting season had lower DOC concentrations, more humic-like components, and higher proportions of highly unsaturated compounds. There were significant correlations between the DOM parameters and sea ice melting/associated environmental changes (e.g., water temperature, salinity, and microbial activity). These results suggest that rapid sea-ice changes in the Southern Ocean can cause substantial changes in the concentration and composition of DOM in surface water.