Sunlight-induced changes in fluorophores of dissolved organic matter in the eastern Indian Ocean

The fluorescence of seawater provides a bulk characterization of seawater organics. Two main components of fluorescent dissolved organic matter (FDOM) are protein- and humic-like fluorophores. The environmental dynamics of FDOM in the open ocean has been reported to include photodegradation at surface layers and production through microbial decomposition of organic matters in deeper layers. During a KH-18-6 cruise in November 2018, observations were conducted along a transect at 88ºE in the eastern Indian Ocean, where changes in biological activities of organic matter production and decomposition are remarkable in the north-south direction. Filtered seawater samples were placed in 500 ml quartz bottles and these bottles were exposed to sunlight in an on-deck running seawater tank. Changes in fluorescent intensity of FDOM were monitored every 24 hours for 5 days. The vertical distributions of FDOM fluorescence intensity showed nutrient-type profile. A distinct peak of fluorescence intensity was also observed at the oxygen minimum layer just below the subsurface chlorophyll maximum. The FDOM fluorescence intensity generally correlated with apparent oxygen utilization in the intermediate and deep layers. In the solar irradiation experiments, a significant photobleaching was observed within 24 hours followed by gradual decrease in FDOM fluorescence intensity during the remaining period of the experiments. It indicate the existence of fluorophores having different photo-lability in the FDOM. One of the experiments also showed control of fluorescence degradation kinetics by the irradiation level. Some of the deep water samples exposed to sunlight showed an increase in protein-like fluorescent intensity along with a decrease in humic-like fluorescent components, suggesting photochemical transformation between them. Therefore, the light field and vertical mixing process could have a strong influences on dynamics of FDOM in the eastern Indian Ocean.