Terrestrially derived dissolved organic matter (DOM) impacts the optical properties of coastal seawater and affects carbon cycling on a global scale. We studied sequential long-term photochemical and biological degradation of estuarine dissolved organic matter from the Satilla River, an estuary in the southeastern United States that is dominated by vascular plant-derived organic matter. During photodegradation, dissolved organic carbon (DOC) loss (amounting to 31% of the initial DOC) was much less extensive than colored dissolved organic matter (CDOM) or fluorescent dissolved organic matter (FDOM) loss (50% and 56% of the initial CDOM and FDOM), and analysis of kinetics suggested a reservoir of DOC that was resistant to photodegradation. In contrast, CDOM photodegradation closely followed first-order kinetics over two half-lives with no indication of a nondegradable component. FDOM loss was slightly biased toward fluorophores considered representative of terrestrial humic substances. Additional changes in optical properties included increases in spectral slope and shifts in fluorescence excitation/ emission maxima that were generally consistent with previous observations from field studies of photobleached DOM. Biological degradation of photobleached DOM was more rapid than that of unbleached material, and this net positive effect was evident even for extensively photodegraded material. Bacterial degradation caused shifts in the opposite direction from photochemical degradation for both spectral slope and excitation/emission maxima and thus dampened but did not eliminate changes in optical properties caused by photobleaching.Photochemical processes that induce changes in natural dissolved organic matter (DOM) can influence many aspects of carbon cycling in marine environments. The sunlight-mediated conversion of DOM into inorganic forms (dissolved inorganic carbon, carbon monoxide) and into bacterial substrates (low-molecular-weight carbonyl compounds and others) increases turnover rates of DOM in surface waters (Bushaw et al. 1996;Miller and Moran 1997;Moran and Zepp 1997). The loss of color (i.e., photobleaching) that accompanies photochemical modification of DOM affects the optical properties of seawater and influences penetration of ultraviolet and photosynthetically active wavelengths (Vodacek et al. 1997). Changes in light penetration depth subsequently affect exposure of marine microorganisms to sunlight and can modify the activities of autotrophs and heterotrophs in both positive and negative ways (Herndl et al. 1997;Jeffrey et al. 1999;Moran and Zepp 2000). Photoreactions of DOM therefore play key roles in oceanic carbon cycling, potentially controlling the degradation of terrestrially derived DOM in estuaries and shallow continental shelf systems (Miller and Moran 1997;Vodacek et al. 1997) and driving the turnover of marine-derived DOM in the surface ocean (Opsahl and Benner 1998;Cherrier et al. 1999).Several models have been used to predict the effects of Acknowledgments We thank L. R. Pomeroy...

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In August 1993, we measured photosynthesis, chlorophyll a, bacterial secondary production, microb~al community respiratory rate, bacterial abundance, dissolved free armno acids, nitrate, phosphate, silicate, and dissolved oxygen in the eastern Chukchi Sea. Our crulse track was mostly in loose pack ice e x c e e d~n g 50% ice cover, with heavier ice cover near 7 5" N. We sclmpled over the continental shelf and slope, in deep water in the Canadian Basin, and over the Chukchi Cap. Primary production was highest over the upper continental slope, averaging 748 mg C m-' d -' In deep water and heavier ice cover in the Canadian Basin, primary productivity averaged 123 mg C m-' d-l. However. microbial community respiratory rates averaged 840 mg C m ' d-l over the upper slope and 860 mg C m-2 d.' in the Canadian Basin. Nitrate was virtually depleted In the .upper m~x e d layer, suggesting some nutrient limitation and dependence on regenerated ammonium in late summer. This is supported by f-ratios ranging from 0.05 to 0.38. Estimates of annual prlmary production of organlc carbon, both from our 'v and 13<1 assimilation measurements and from the supersaturation of dissolved oxygen in the upper mixed layer at all stations, suggest that significant primary production occurs well beyond the continental shelves out into the so-called perennial pack ice. Respiratory activity in the upper mixed layer exceeded primary productivity at the deep-water stations, a s it often does in summer oligotrophic conditions at lower latitudes. These observations suggest that rates of both autotrophic and heterotrophic biological activity in the upper mixed layer of the deep waters of the Arctic Ocean may be considerably higher than suspected and should b e incorporated lnto models of polar proccsscs.

Quantum yield spectra for the photochemical formation of biologically labile photoproducts from dissolved organic matter (DOM) have not been available previously, although they would greatly facilitate attempts to model photoproduct formation rates across latitudinal, seasonal, and depth-related changes in spectral irradiance. Apparent quantum yield spectra were calculated for two coastal environments from the southeastern United States using postirradiation bacterial respiration as a measure of total labile photoproduct formation and a cutoff filter method to model spectral dependence. As has been the case for previously studied classes of DOM photoproducts (i.e., dissolved inorganic carbon, CO, and H 2 O 2 ), ultraviolet (UV)-B irradiance was significantly more efficient at forming labile photoproducts (i.e., compounds readily assimilated by marine bacterioplankton) than UV-A and visible irradiance. Calculations of DOM photoproduct formation in southeastern U.S. coastal surface waters indicate a formation ratio for biologically labile photoproducts : CO of 13 : 1. The slope of a natural log plot of the apparent quantum yield spectrum obtained for biologically labile photoproducts was similar to that for CO (0.028 nm Ϫ1 vs. 0.034 nm Ϫ1 ). Modeled kinetic rates therefore indicate that the production ratio of these photoproduct classes is approximately maintained despite variations in the solar spectrum that occur with depth in a water column or distance from shore. Application of the apparent quantum yield to coastal regions worldwide predicts an annual formation rate of biologically labile photoproducts in coastal waters of 206 ϫ 10 12 g C.Exposure of marine dissolved organic matter (DOM) to natural sunlight modifies the chemical structure of DOM and produces a suite of organic and inorganic photoproducts that can affect biological productivity (Kieber et al. 1989), oceanic carbon cycling (Mopper et al. 1991;Miller and Zepp 1995), atmospheric flux of CO and CO 2 (Miller and Zepp 1995;Gao and Zepp 1998;Erickson et al. 2000), and many other processes. Although significant progress has been made in identifying DOM photoproducts and documenting