I

Mar-TFF Est-TFF Fresh-TFF ER-TFF ER-C18 ER-Nexus CDOM Source and Type of Isolation

Figure 3. Comparison of CDOM recovery from tangential flow filtration (TFF) and solid phase extraction using two commercially-available resins (AbsElut and Nexus). Percent removal of initial CDOM absorbance at 320 nm.

SPE provides the best retention of marine CDOM based on recovery of chromo-phoric properties resembling the original marine DOM. There exists some debate on the extraction efficiencies of either technique and we refer the reader to current discussions on the subject [82,91-93].

Naturally, these parameters vary with the source of the CDOM. For example, freshwater (terrestrial or allochthonous) CDOM is higher in molecular weight and aromaticity than marine DOM, and freshwater CDOM typically has a lower C/N ratio as marine DOM. Carbon stable isotope evidence shows that marine DOM is on average — 23%, reflecting a marine plankton source, whereas freshwater DOM is lighter, around — 28%. The lighter isotopic signature reflects the contribution of terrestrial land plants. Additionally, Opsahl and Benner [31] have demonstrated the photoreactivity of lignin-derived phenols, suggesting their use as a proxy for terrestrial CDOM. Lignin originates primarily in terrestrial land plants and thus provides a good indicator of CDOM source. These analyses may be difficult in mixing environments, such as estuaries. Thus, one must consider the source of CDOM when interpreting the effects of solar radiation on CDOM chemical properties (see section 6.7).

6.5 Photochemical changes in DOM

Chemical changes in CDOM have been measured on several scales. Most common is measurement of bulk DOC by high temperature combustion or wet oxidation [94]. Other bulk measurements include carbon gases in the form of CO and C02. However, in recent years, compound-specific and molecular analyses have provided more detailed information on the products of CDOM photodegradation.

6.5.1 Measurement of organic photoproducts

LMW organic compounds produced by irradiation of DOM cover a wide range of compound classes, but are generally carbonyl compounds such as aldehydes and carboxylic acids, compounds readily available to aquatic microbes [26-28,30,33,35,38,95-111]. These compounds appear to arise from the degradation, or fragmentation, of larger humic structures into its component molecules by either direct photolysis or indirect secondary reactions discussed in Section 6.3. Measurement of these photoproducts usually involves gas chromatography/mass spectrometry of derivatized compounds or capillary ion electrophoresis (e.g., [37,38,52,112-114]).

Indirect evidence of organic photoproduct generation comes from structural analyses of DOM. Several reports have used 13C NMR in the solid and solution state to show that the abundance of aromatic (ring containing) groups in the bulk DOM and humic and fulvic isolates is reduced after exposure to UVR [115,116]. Unpublished results by Thorn and Younger show reductions in both aromatic and carboxyl C groups of the Nordic fulvic acid after UV irradiation, corresponding to a 35% loss of dissolved absorbance at 465 nm. Osburn et al. [115] were able to suggested that these results are observable on a seasonal basis in a humic lake (Figure 4).

6.5.2 Measurement of inorganic photoproducts

The production of inorganic carbon compounds (DIC, primarily C02 and CO) has been widely reported from various CDOM sources [29,56,104,117-123]. Allard [124] has observed the photoproduction of DIC as carbonate ion from capillary electrophoresis. Production of DIC in natural waters was shown to have a strong dependence on wavelength band [120,123], pH [29,125], and cumulative dose [118]. C02 concentrations in fresh waters are easily measured by acidifying the sample with concentrated phosphoric acid and then measuring the evolved C02 either by gas chromatography or with a nondispersive infrared detector. However, Kieber (personal communication) notes the difficulty with measuring photoproduced C02 in marine samples or samples with high DIC background concentrations, in which case the DIC must be stripped out of the

0.90

Figure 4. Change in the ratio of aromatic-to-aliphatic C in freshwater DOM exposed to solar radiation. Dark bars are 0.2 ¡im filtered and reverse osmosis concentrated samples collected from Lake Lacawac, Pennsylvania, USA during 1999. Stippled bars are results from sunlight exposure of bog DOM during 1998. From Osburn [158].

sample prior to irradiation. The reason is that the amount of DIC photo-produced is too small to measure above background DIC concentrations. CO concentrations are measured by gas chromatography fitted with either a meth-anizing flame ionization detector [29,117] or HgO/UV detector [126], Photoproduction of CO2 is at least an order of magnitude greater than CO [29,104].

In addition, photoproduction of inorganic nutrients such as phosphate [127] and ammonium [107,128] have been reported. Phosphate was shown to be bound to an iron-humic complex in a Midwestern US bog lake, and released upon irradiation with UVR. Ammonium was similarly released from irradiation of humic substances isolated from Skidaway and Satilla River estuaries. This result has opened up a new dimension to the biogeochemical cycling of nitrogen that may be mediated by UVR. Kieber et al. [129] have shown that humic substances isolated from various substances may produce nitrite upon photodegradation, though the rates of production were much less than those reported for ammonium production from ref. [16] (4 nM h~1 vs. 50 nM h~1). Thus, UVR effects on CDOM may liberate inorganic nutrients that become active in the biogeochemical cycling of natural waters.

6.5.3 Recent approaches to measuring photochemical changes to CDOM

Other chemical methods have recently been employed to examine the effects of UVR on CDOM. Opsahl and Benner [31] have studied lignin-derived phenols

L = Lake Lacawac

mix = mixed water column

epi = epilimnion

hypo = hypolimnion

init = initial

exp = exposed (7 d) ■ .1

L-mix L-epi L-hypo Bog-init Bog-exp

L-mix L-epi L-hypo Bog-init Bog-exp

of both HM W and LMW DOM in the Mississippi River plume. They showed that 75% of total dissolved lignin in riverine HMW DOM was lost during 28 d exposure to solar radiation; 80% of the remaining fraction of dissolved lignin was present as LMW material and the remaining result was less susceptible to further photochemical degradation. HMW lignin from the equatorial Pacific Ocean was found to be resistant to photodegradation. Interestingly, the photo-degraded riverine dissolved lignin was similar to the marine dissolved lignin, suggesting that DOM photochemistry is an important factor in the composition of marine DOM. Furthermore, they found that the ratio of vanillic acid to vanillin reflects photochemical alteration and this may be a useful tracer for chemical changes in CDOM at the molecular level. Osburn et al. [115] have reported a correlation between the loss of DOC concentration, the loss of dissolved absorbance, and an enrichment in the carbon stable isotope value of DOM after sunlight exposure of 0.2 /mi-filtered DOM from a Sphagnum bog. They were also able to show experimentally that the aromaticity of decreased by 47% after the exposure to sunlight. The aromatic loss corresponded to an increase in the stable carbon isotopic value of the DOM from —28% to —27% and a decrease in DOC concentration of 16% (Figure 5). These chemical changes suggest a removal of C as C02 and a change in the composition of the DOM to smaller molecular weight compounds. Similarly, Opsahl and Zepp [130] have also reported an increase in stable carbon isotopic values of riverine water exposed to sunlight. They also show loss of DOC and dissolved absorbance concurrent with isotopic enrichment; furthermore, they observed substantial reductions in lignin phenol concentration after sunlight exposure (>65%).

Vahatalo and coworkers [121] have provided direct evidence that lignin is photochemically reactive. Using synthetic lignin, radiolabeled with 14C on the aromatic C ring only, they determined that approximately 20% of the ringlabeled C was mineralized to C02. Simultaneous exposure of DOC from lake water produced slightly higher (2-3%) results. These results provide convincing evidence that the aromatic fraction of CDOM is largely responsible for its photoreactivity.

These examples show the utility of using 13C NMR spectroscopy, dissolved lignin, and stable isotopes as molecular tracers of photodegradation of CDOM. Other tools of mass spectroscopy and compound separation and identification should provide additional information on changes to DOM as it is photodeg-raded. Recently, several groups have presented spectroscopic and spec-trophotometric methods for identification of chromophores [47-49,131].

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