CHROMOPHORIC DISSOLVED ORGANIC MATTER (CDOM) IN THE OCEAN: OPTICAL PROPERTIES AND RELATION TO CDOM STRUCTURE AND SOURCE

Abstract

CDOM is a ubiquitous component of the dissolved organic matter pool which absorbs UV and visible light. Owing to its optical properties it plays a critical role in photochemistry and biogeochemistry in aquatic systems. Extensive data exist on the optical properties of CDOM from both terrestrial and coastal environments, yet the open oceans have been under-sampled. Consequently, the source and structural basis of marine CDOM optical properties are still debated. To address this, CDOM optical properties were acquired for both untreated and sodium borohydride (NaBH4) reduced natural waters and C18 extracts for waters of the Equatorial Atlantic Ocean. The similarities and differences were examined relative to optical data obtained for CDOM from terrestrial sources. Ocean CDOM was found to comprise of a marine CDOM component (absorbs and emits in the UV), as well as a terrestrial CDOM component (absorbs in the UV-Vis, and emits in the visible). CDOM concentrations in oceans are low and seawater contains high salt concentrations, impeding the use of some analytical techniques. These salts can be removed by solid phase extraction which results higher concentrations of organic matter. The extent to which the C18 extracts represent the whole water samples was examined. Our data indicate that the optical properties of the extracted material is largely representative of the original CDOM. It is generally suggested that optical measurements for CDOM be acquired as close to the time of collection as possible. However, CDOM is commonly stored at 4°C in the dark and there is no consensus regarding the effect of storage on CDOM. To address this, the effect of storage on the optical properties of CDOM samples stored for 6 years was examined. Taking into account standard deviation in optical measurements, it was concluded that the storage protocol did not significantly affect CDOM optical properties. In-situ production has been suggested as one of the main sources of CDOM in oceans. The optical properties of DOM produced by one of the most abundant bacteria in oceans (P. ubique) was examined. The results indicate that under our incubation conditions CDOM is not produced.