Detrital spectral absorption: Laboratory studies of visible light effects on phytodetritus absorption, bacterial spectral signal, and comparison to field measurements
Abstract:Visible light-dependent changes in the visible and near-ultraviolet spectral absorption (250–750 nm) of phytodetritus (killed and disrupted phytoplankton and copepod fecal pellets) were determined in laboratory experiments, as were spectral changes due to the growth of bacteria. Bacteria carbon and nitrogen content were estimated. Results of the laboratory studies are compared to field measurements of particulate spectral absorption in waters from the South Atlantic Bight off the southeastern U.S. The correction for “pathlength amplification” of filtered particulate samples (β) is also examined.
Light-dependent changes in the spectral absorption of phytodetritus are principally due to the bleaching of chlorophylls, pheopigments, and carotenoids. With bleaching of these pigments the residual phytodetrital absorption is due to more light-stable chromophores which absorb in the violet to near-ultraviolet. In the visible region, detrital-type absorption appears to represent the longer wavelength “tails” of absorption spectra which reach maxima in the ultraviolet.
Extension of the spectral measurements to 250 nm revealed two general patterns. Biological macromolecules associated with phytoplankton, fresh phytodetritus and bacteria contribute to a broad maximum in absorption centered around 260 nm (likely due to nucleic acids and the aromatic amino acids of proteins). In the turbid near-shore waters of the South Atlantic Bight, detrital-type absorption showed a rather featureless, steady increase in absorption with decreasing wavelength, similar to typical DOM spectra, which may be due to relatively refractory particulate geomacromolecules (i.e., complex organics such as fulvic acids). Some near-surface field samples also indicated the presence of uv-photoprotective pigments associated with the phytoplankton community.
Detrital type spectral absorption in the visible may be contributed by a variety of chromophores, some portion of which may be contained in heterotrophic organisms. The nature of the chromophores and particle types which constitute detrital-type absorption may differ between coastal and open ocean waters.
Document Type: Research Article
Publication date: February 1, 1993
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