DOM in seawater

Coble, P.G. 1996. Characterization of marine and terrestrial DOM in seawater using excitation-emission matrix spectroscopy. Marine Chemistry, 51, 325-346. 

Unfortunately, most of the DOM in seawater is LMW (75 to 80%) and its chemical composition has not been as well studied as that of the HMW fraction. LMW DOM is thought to be composed primarily of biopolymers containing 10 or fewer monomers. Radiocarbon measurements indicate LMW is older than HMW DOM, suggesting that LMW is fer less reactive than HMW DOM. 

After several decades of research, fundamental aspects of the chemical composition and structure of marine organic matter remain elusive. Advances in the chemical characterization of marine organic matter are, in large part, dependent on the development of quantitative methods for its concentration and isolation from seawater. Each of the major methods currently used for the isolation of marine DOM recovers around one-third of the DOM in seawater (solid-phase extractions, using XAD resins or C18 adsorbents, and ultrafiltration). A coupled reverse osmosis-electrodi-alysis method has recently been used to recover an average of 75% 12% of marine DOM from 16 seawater samples however, the method has emerged too recently to have been well tested at this time. 

Aluwihare, L. I. (1999). High molecular weight (HMW) dissolved organic matter (DOM) in seawater Chemical stmcture, sources and cycling. Massachusetts Institute of Technology/Woods Hole Oceanographic Institution, Doctoral Dissertations MIT/WHOI 99-08. 

Thus the surface chemistry of the ocean consists essentially of the chemistry of that part of the uncharacterised and complex part of the DOM in seawater which is surface active. Apart from other effects, this can lead to the entrainment of trace elements in the surface layer by complex formation with the surface active polymers and their enrichment in the microlayer (Barker and Zeitlin, 1972 Duce et al., 1972 Piotrowicz et al., 1972, Hunter, 1977) and possible enrichment of the atmospheric aerosol (Duce et al., 1972,1976), at least near the ocean surface (Chesselet et al., 1976). 

Time resolved fluorescence spectroscopy, i. e., lifetime measurement, has several advantages over the steady-state measurements described above. These include insensitivity to variables that may affect fluorescence measurements such as turbidity or scattering in the samples, photobleaching, changes in fiuo-rophore concentration, and optical misalignment. The presence of different lifetimes from samples with similar featureless spectra can provide unambiguous identification of the species. Lifetime measurements are also more sensitive to the fluorophore micro-environment. In spite of these advantages, there have been limited lifetime studies on DOM in seawater [50,51,58] and natural freshwater samples of DOM [58]. 

In one study, the decay of fluorescence for DOM in seawater could be reasonably fitted to a single exponential, with a fifetime of 2 ns [51]. Some recent experiments on DOM from the Baltic Sea have suggested that the fluorescence behavior of humic substances at ambient temperatures may best be described by a mixture of chromophores with only one fluorophore, i. e., one type of fluorescence molecule with only one chemical structure [58]. However, other stud-... 

More recent studies on hydrogen peroxide levels in natural waters attributed the observed correlation of H2O2 concentrations with DOM levels to the triplet mechanism for lake waters [96] and e q in seawater [97]. Fujiwara and coworkers [97] generated e by irradiating seawater samples at 355 nm and monitoring absorption at 750 nm. A positive correlation was found between the amount of e generated and colored DOM in seawater. 

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