Chromophoric dissolved organic matter (CDOM) in natural waters: Distribution, dynamics and nature

Chromophoric dissolved organic matter (CDOM) plays a critical role in determining the aquatic light field. Because its absorption spectrum decreases exponentially from ultraviolet (UV) to visible wavelengths, CDOM limits the penetration of UV light, thus shielding aquatic organisms from potentially damaging radiation. Because its absorption spectrum extends into the visible, it can also affect primary productivity and interfere with satellite remote sensing retrieval of phytoplankton biomass. To better understand the factors controlling its distribution and dynamics, the spatial and seasonal distributions of CDOM optical properties (absorption, fluorescence), dissolved organic carbon content and particulate material (phytoplankton, detritus) absorption were acquired for coastal waters of the Middle Atlantic Bight (MAB) and compared to those for the Southwest Florida shelf. The absorption of CDOM is shown to dominate the total absorption in the UV and to contribute substantially in the blue region. The quasi-conservative dependence of CDOM on salinity in the Chesapeake and Delaware Bays indicates a primarily terrestrial source for CDOM, with no strong in situ sources or sinks within the Bays. In contrast, a strong surface loss of CDOM is observed on the shelf of the MAB over the summer months and attributed to a photochemical sink. To investigate this photochemical sink, the effects of monochromatic and polychromatic UV-visible radiation on the optical properties of CDOM were examined. A simple model was developed to predict the time and spectral dependence of the photobleaching of natural waters under polychromatic light fields. These predictions were consistent with the field observations in the MAB. An analysis of the monochromatic photobleaching kinetics argues that a model based on a simple superposition of multiple chromophores undergoing independent photobleaching cannot apply. Thus, to study the nature of the constituents giving rise to the absorption spectrum of humic substances, species were selectively destroyed at specific wavelengths in high viscosity solutions using a tunable-laser source. The results indicates that the absorption spectrum of humic substances cannot simply originate from the superposition of the absorption spectra of numerous-discrete chromophores, but instead requires electronic coupling among its constituents. Optical charge-transfer transitions represent a possible explanation for the long wavelength absorption of CDOM.