Application of an in situ fluorometer to determine the distribution of fluorescent organic matter in the open ocean

The application of in situ fluorometers to determine the high resolution vertical profiles of oceanic fluorescent organic matter (FOM) enhances the study of FOM distributions in the open ocean and will lead to better understandings of the environmental dynamics of FOM, including the distribution/fate of terrestrial FOM in offshore waters. Here, we applied a commercially available in situ FOM fluorometer that was connected to a conductivity, temperature and depth (CTD) sensors with a carousel multi-sampling system to determine full vertical profiles of the FOM in oceanic environments. It was shown that temperature standardization was necessary to produce robust depth profiles and was performed preceding calibration with a benchtop fluorometer. The calibrated output of the in situ fluorometer was linearly related to apparent oxygen utilization when corrected for differences in fluorometer response over the length of the cruise. The results of this study indicated that (1) in situ FOM fluorometers that are mounted on CTD systems can be used to study the distribution of FOM in the open ocean, and (2) temperature standardization and appropriate calibration are crucial to evaluate the distribution of FOM in the open ocean. Although we used a window filter (weighted average of the output) to overcome the low S/N ratio of the output of the in situ fluorometer, better S/N ratios for in situ fluorometers are expected for open ocean studies.