The finescale distribution, origin, and photobleaching of the spectral absorption of dissolved organic matter in coastal waters

Michael Stewart Twardowski, University of Rhode Island


Accurate, finescale absorption profiles of colored dissolved organic matter (CDOM) were measured in coastal waters. CDOM has also been called Gelbstoff, gilvin, yellow substances, humic substances, and "unknown chromophores". The processes responsible for the observed temporal and spatial variability were investigated. The sources of CDOM were diluted inputs from the terrestrial biosphere and in situ biological processes. The primary sink was photodegradation.^ The finescale distribution of inherent optical properties was measured with a WET Labs ac-9. The ac-9 is an innovative in situ absorption and attenuation meter with a precision about $\pm$0.001 m$\sp{-1}$ in the raw signal. A pressurized flow calibration procedure is described in detail. Vertical profiles of spectral total absorption, $a\sb{t}(\lambda)$, CDOM absorption, $a\sb{g}(\lambda)$, and particulate absorption, $a\sb{p}(\lambda)$, were measured by multiple meters, corrected for drift, temperature, salinity, and scattering effects, and compared. The accuracy of the absolute absorption values and their vertical position with respect to the hydrography was evaluated.^ These new techniques were used to investigate the relationship between autochthonous CDOM formation and primary production in a phytoplankton thin layer (2-4 m thick) in East Sound, WA. The estimated allochthonous component, associated with salinity, was subtracted from $a\sb{g}(\lambda)$ profiles. The remaining residual was linearly correlated with primary production parameters. The bio-optical and remote sensing implications are discussed.^ At the surface, the $a\sb{g}(\lambda)$ residual was negative, indicating photolytic removal. To better characterize the photobleaching of $a\sb{g}(\lambda)$ by UV and visible radiation, rates were measured in incubation experiments and then used to predict absorption loss in surface waters. Differing rates of absorption loss appeared to be a function of cumulative past levels of exposure. Daily absorption loss in stratified surface waters was estimated at 20%.^ The spectral slope in the visible range increased as a function of irradiation dosage in several closed incubations. Increasing spectral slope was also observed in surface waters. Additionally, the absorption spectrum in the visible was more accurately modeled using a double exponential versus a single exponential. Changes in spectral absorption were used to describe the underlying modifications to the CDOM pool. ^

Subject Area

Biology, Oceanography|Biogeochemistry|Chemistry, Organic

Recommended Citation

Michael Stewart Twardowski, "The finescale distribution, origin, and photobleaching of the spectral absorption of dissolved organic matter in coastal waters" (1998). Dissertations and Master's Theses (Campus Access). Paper AAI9902583.