Specific UV absorbance (SUVA) is defined as the UV absorbance of a water sample at a given wavelength normalized for dissolved organic carbon (DOC) concentration. Our data indicate that SUVA, determined at 254 nm, is strongly correlated with percent aromaticity as determined by 13C NMR for 13 organic matter isolates obtained from a variety of aquatic environments. SUVA, therefore, is shown to be a useful parameter for estimating the dissolved aromatic carbon content in aquatic systems. Experiments involving the reactivity of DOC with chlorine and tetramethylammonium hydroxide (TMAH), however, show a wide range of reactivity for samples with similar SUVA values. These results indicate that, while SUVA measurements are good predictors of general chemical characteristics of DOC, they do not provide information about reactivity of DOC derived from different types of source materials. Sample pH, nitrate, and iron were found to influence SUVA measurements.

A new approach for parameterizing dissolved organic matter (DOM) ultraviolet-visible absorption spectra is presented. Two distinct spectral slope regions (275-295 nm and 350-400 nm) within log-transformed absorption spectra were used to compare DOM from contrasting water types, ranging from wetlands (Great Dismal Swamp and Suwannee River) to photobleached oceanic water (Atlantic Ocean). On the basis of DOM size-fractionation studies (ultrafiltration and gel filtration chromatography), the slope of the 275-295-nm region and the ratio of these slopes (S R ; 275-295-nm slope : 350-400-nm slope) were related to DOM molecular weight (MW) and to photochemically induced shifts in MW. Dark aerobic microbial alteration of chromophoric DOM (CDOM) resulted in spectral slope changes opposite of those caused by photochemistry. Along an axial transect in the Delaware Estuary, large variations in S R were measured, probably due to mixing, photodegradation, and microbial alteration of CDOM as terrestrially derived DOM transited through the estuary. Further, S R varied by over a factor of 13 between DOM-rich wetland waters and Sargasso Sea surface waters. Currently, there is no consensus on a wavelength range for log-transformed absorption spectra. We propose that the 275-295-nm slope be routinely reported in future DOM studies, as it can be measured with high precision, it facilitates comparison among dissimilar water types including CDOM-rich wetland and CDOM-poor marine waters, and it appears to be a good proxy for DOM MW.

Using reversed-phase high performance liquid chromatography, 25 amino acids as o-phthaldialdehyde derivatives were determined with a 30-min sample turnover. The derivatization procedure is rapid, performed in aqueous medium, and has few transfer steps. The precision is ±0.5% at the 80-pmol level. Recoveries of standards added to urine samples were >98%. Intercallbration with an amino acid analyzer showed an agreement of >95 %. The detection limit Is about 50 fmol.The overall retention is controlled by the organic modifier, but second-order effects are ascribed to Ion repulsion between anions In the eluent and the ionized carboxylate on the derivatives. Clinical, biochemical, and environmental applications are given. Amino acids and ammonia were determined In 25

Congo River water was filtered and then irradiated for 57 d in a solar simulator, resulting in extensive photodegradation of dissolved organic matter (DOM). Whole‐water (i.e., unfractionated) DOM was analyzed pre‐ and post‐irradiation using ultrahigh resolution Fourier transform ion cyclotron mass spectrometry (FT‐ICR MS), revealing the following three pools of DOM classified based upon their photoreactivity: (1) photo‐resistant, (2) photo‐labile, and (3) photo‐produced. Photo‐resistant DOM was heterogeneous, with most molecular classes represented, although only a small number of aromatics and no condensed aromatics were identified. The photoproduced pool was dominated by aliphatic compounds, although it included a small number of aromatics, including condensed aromatics. Aromatic compounds were the most photoreactive, with > 90% being lost upon irradiation. Photochemistry also resulted in a significant drop in the number of molecules identified and a decrease in their structural diversity. The FT‐ICR MS signatures of two classes of refractory organic matter, black carbon and carboxylic‐rich alicyclic molecules (CRAM), were present in the sample prior to irradiation, indicating that the Congo River could be a significant exporter of recalcitrant DOM to the ocean. All black carbon‐like molecules identified in the initial sample were lost during irradiation. Molecular signatures consistent with CRAM were also highly photo‐labile, demonstrating that environmental solar irradiation levels are capable of removing these refractory compounds from aquatic systems. Irradiation also shifted the molecular signature of terrestrial DOM toward that of marine DOM, thereby complicating the task of tracking terrestrial DOM in the ocean.

[1] Photochemical degradation of Congo River dissolved organic matter (DOM) was investigated to examine the fate of terrigenous DOM derived from tropical ecosystems. Tropical riverine DOM receives greater exposure to solar radiation, particularly in large river plumes discharging directly into the open ocean. Initial Congo River DOM exhibited dissolved organic carbon (DOC) concentration and compositional characteristics typical of organic rich blackwater systems. During a 57 day irradiation experiment, Congo River DOM was shown to be highly photoreactive with a decrease in DOC, chromophoric DOM (CDOM), lignin phenol concentrations (S 8 ) and carbon-normalized yields (L 8 ), equivalent to losses of $45, 85-95, >95 and >95% of initial values, respectively, and a +3.1 % enrichment of the d C; r = 0.97, p < 0.01), highlighting the potential of CDOM absorbance measurements for delineating the photochemical degradation of lignin and thus terrigenous DOM. It is apparent that these commonly used measurements for examination of terrigenous DOM in the oceans have a higher rate of photochemical decay than the bulk DOC pool. Further process-based studies are required to determine the selective removal rates of these biomarkers for advancement of our understanding of the fate of this material in the ocean.

Photochemical production rates and steady-state concentrations of hydroxyl radicals (.OH) were measured in sunlight-irradiated seawater. Values ranged from 110 nanomolar per hour and 12 x 10(-18) molar in coastal surface water to 10 nanomolar per hour and 1.1 x 10(-18) molar in open ocean surface water. The wavelengths responsible for this production are in the ultraviolet B region (280 to 320 nanometers) of the solar spectrum. Dissolved organic matter (DOM) appears to be the main source for .OH over most of the oceans, but in upwelling areas nitrite and nitrate photolysis may also be important. DOM in the deep sea is degraded more readily by .OH (and its daughter radicals), by a factor of 6 to 15, than is DOM in open-ocean surface water. This finding may in part bear on major discrepancies among current methods for measuring dissolved organic carbon in seawater.

Low-molecular-weight (LMW) carbonyl compounds, e.g. formaldehyde, acetaldehyde, and the a-keto acid glyoxylate, were produced in a wide variety of natural waters upon irradiation with sunlight. Production rates were linearly related (r2 > 0.98) to initial absorbance at 300 nm and initial fluorescence (360/460 nm) in all waters tested. Photochemical production was also linearly related to loss (photobleaching) of absorbance and fluorescence during irradiation, irrespective of irradiation time and prior photobleaching history of the sample. These results were attributed to absorption of light by humic substances in the waters, as determined by experiments where purified humic and fulvic extracts were added to open-ocean water. The wavelengths in the solar spectrum responsible for photoproduction of LMW carbonyl compounds and bleaching of dissolved organic matter light absorbance are in the UV-B region (280-320 nm). These results, plus results from time-course irradiations, suggest that photoproduction of carbonyl compounds from humic substances is closely related to photobleaching of absorbance. Based on our photoproduction rates of LMW carbonyl compounds, we estimate that the half-life of humic-rich riverine dissolved organic C in the oceanic mixed layer is 5-l 5 yr.Photochemical reactions have been shown to play important roles in the chemistry and biology of the upper water column of the sea and other water bodies (Blough and Zepp 1990;Zafiriou et al. 1984). Light-initiated processes affect the chemical makeup of aqueous systems because of their involvement in redox processes (Moffett and Zika 19 8 7) and in transformation of naturally occurring and anthropogenic organic compounds (Cooper et al. 1989;Zepp et al.

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