shelf-and river-derived elements to the central Arctic Ocean • The TPD is rich in dissolved organic matter (DOM), which facilitates long-range transport of trace metals that form complexes with DOM • Margin trace element fluxes may increase with future Arctic warming due to DOM release from permafrost thaw and increasing river discharge

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This study reports increasing iron concentrations in rivers draining into the Baltic Sea. Given the decisive role of iron to the structure and biogeochemical function of aquatic ecosystems, this trend is likely one with far reaching consequences to the receiving system. What those consequences may be depends on the fate of the iron in estuarine mixing. We here assess the stability of riverine iron by mixing water from seven boreal rivers with artificial sea salts. The results show a gradual loss of iron from suspension with increasing salinity. However, the capacity of the different river waters to maintain iron in suspension varied greatly, i.e. between 1 and 54% of iron was in suspension at a salinity of 30. The variability was best explained by iron:organic carbon ratios in the riverine waters – the lower the ratio the more iron remained in suspension. Water with an initially low iron:organic carbon ratio could keep even higher than ambient concentrations of Fe in suspension across the salinity gradient, as shown in experiments with iron amendments. Moreover, there was a positive relationship between the molecular size of the riverine organic matter and the amount of iron in suspension. In all, the results point towards a remarkably high transport capacity of iron from boreal rivers, suggesting that increasing concentrations of iron in river mouths may result in higher concentrations of potentially bioavailable iron in the marine system.

Recent studies report trends of strongly increasing iron (Fe) concentrations in freshwaters. Since Fe is a key element with a decisive role in the biogeochemical cycling of major elements, it is important to understand the mechanisms behind these trends. We hypothesized that variations in Fe concentration are driven mainly by redox dynamics in hydraulically connected soils. Notably, Fe(III), which is the favored oxidation state except in environments where microbial activity provide strong reducing intensity, has several orders of magnitude lower water solubility than Fe(II). To test our hypothesis, seasonal variation in water chemistry, discharge, and air temperature was studied in three Swedish rivers. Methylmercury and sulfate were used as indicators of seasonal redox changes. Seasonal variability in water chemistry, discharge, and air temperature in the Emån and Lyckeby Rivers implied that the variation in Fe was primarily driven by the prevalence of reducing conditions in the catchment. In general, high Fe concentrations were observed when methylmercury was high and sulfate was low, indicative of reducing conditions. The Fe concentrations showed no or weak relationships with variations in dissolved organic matter concentration and aromaticity. The seasonal variation in Fe concentration of the Ume river was primarily dependent on timing of the snowmelt in high‐ versus low‐altitude areas of the catchment. There were long‐term trends of increasing temperature in all catchments and also trends of increasing discharge in the southern rivers, which should increase the probability for anaerobic conditions in space and time and thereby increase Fe transport to the aquatic systems.

Abstract. The photochemical oxidation of oceanic dissolved organic carbon (DOC) to carbon monoxide (CO) and carbon dioxide (CO 2 ) has been estimated to be a significant process with global photoproduction transforming petagrams of DOC to inorganic carbon annually. To further quantify the importance of these two photoproducts in coastal DOC cycling, 38 paired apparent quantum yield (AQY) spectra for CO and CO 2 were determined at three locations along the coast of Georgia, USA over the course of one year. The AQY spectra for CO 2 were considerably more varied than CO. CO AQY spectra exhibited a seasonal shift in spectrally integrated (260 nm-490 nm) AQY from higher efficiencies in the autumn to less efficient photoproduction in the summer. While full-spectrum photoproduction rates for both products showed positive correlation with pre-irradiation UV-B sample absorption (i.e. chromophoric dissolved organic matter, CDOM) as expected, we found no correlation between AQY and CDOM for either product at any site. Molecular size, approximated with pre-irradiation spectral slope coefficients, and aromatic content, approximated by the specific ultraviolet absorption of the pre-irradiated samples, were also not correlated with AQY in either data set. The ratios of CO 2 to CO photoproduction determined using both an AQY model and direct production comparisons were 23.2 ± 12.5 and 22.5 ± 9.0, respectively. Combined, both products represent a loss of 2.9 to 3.2 % of the DOC delivered to the estuaries and inner shelf of the South Atlantic Bight yearly, and 6.4 to 7.3 % of the total annual degassing of CO 2 to the atmosphere. This result suggests that direct photochemical production of CO and CO 2 is a small, yet significant contributor to both DOC cycling and CO 2 gas exchange in this coastal system.

The photochemical oxidation of oceanic dissolved organic carbon (DOC) to carbon monoxide (CO) and carbon dioxide (CO₂) has been estimated to be a significant process with global photoproduction transforming petagrams of DOC to inorganic carbon annually. To further quantify the importance of these two photoproducts in coastal DOC cycling, 38 paired apparent quantum yield (AQY) spectra for CO and CO₂ were determined at three locations along the coast of Georgia, USA over the course of one year. The AQY spectra for CO₂ were considerably more varied than CO. CO AQY spectra exhibited a seasonal shift in spectrally integrated (260 nm–490 nm) AQY from higher efficiencies in the fall to less efficient photoproduction in the summer. While full-spectrum photoproduction rates for both products showed positive correlation with pre-irradiation UV-B sample absorption (i.e. chromophoric dissolved organic matter, CDOM) as expected, we found no correlation between AQY and CDOM for either product at any site. Molecular size, approximated with pre-irradiation spectral slope coefficients, and aromatic content, approximated by the specific ultraviolet absorption of the pre-irradiated samples, were also not correlated with AQY in either data set. The ratios of CO₂ to CO photoproduction determined using both an AQY model and direct production comparisons were 23.2 ± 12.5 and 22.5 ± 9.0, respectively. Combined, both products represent a loss of 2.2 to 2.6 % of the DOC delivered to the estuaries and inner shelf of the South Atlantic Bight yearly, and 5 to 6 % of the total annual degassing of CO₂ to the atmosphere. This result suggests that direct photochemical production of CO and CO₂ is a small, yet significant contributor to both DOC cycling and CO₂ gas exchange in this coastal system

To quantify the effects of photochemistry on the biological lability of dissolved organic carbon in a terrestrially influenced system, a quarterly sampling effort was undertaken at three estuarine locations along the coast of Georgia, producing a total of 15 apparent quantum yield (AQY) spectra for biologically labile products (BLPs). Prefiltered samples were irradiated and then inoculated with natural microbial communities isolated from the same sample. Oxygen consumption was used as a proxy for community carbon utilization over 10-12 d dark incubations. Seasonal microbial response to irradiated samples ranged from 221% to +155% relative to dark controls. AQY spectral shapes were not consistent over the data set, nor were there any apparent patterns in season or location. Progressive photon dose experiments showed that increasing irradiation length strongly decreased the total biolabile product as assayed with microbial community oxygen consumption, as well as altering the spectral shape of the AQY spectra. A conceptual model is presented to explain this dependence on photon dose, by illustrating the competition between the photochemical production and photochemical destruction of BLPs. The varying dependence on photon dose is fundamentally different from published results for other photochemical products and further complicates attempts to quantify the total effect of photochemistry on organic carbon cycles in natural environments.

Abstract. To examine the potential influence of terrestrially derived DOM on the Baltic Sea, a year-long study of dissolved organic matter (DOM) was performed in three river catchments in Sweden. One catchment drains into the Bothnian Sea, while two southern catchments drain into the Baltic proper. Dissolved organic carbon (DOC) concentrations were positively correlated with discharge from forested catchments over the year. While the overall concentrations of DOC were several times higher in the southern two catchments, higher discharge in the northern catchment resulted in the annual loadings of DOC being on the same order of magnitude for all three catchments. Biological oxygen demand (BOD) was used as a proxy for the lability of carbon in the system. The range of BOD values was similar for all three catchments, however, the ratio of BOD to DOC (an indication of the labile fraction) in Ume river was four times higher than in the southern two catchments. Total annual BOD loading to the Baltic Sea was twice as high in the northern catchment than in the two southern catchments. Lower winter temperatures and preservation of organic matter in the northern catchment combined with an intense spring flood help to explain the higher concentrations of labile carbon in the northern catchment. Lower lability of DOM as well as higher colour in the southern catchments suggest that wetlands (i.e. peat bogs) may be the dominant source of DOM in these catchments, particularly in periods of low flow. With climate change expected to increase precipitation events and temperatures across the region, the supply and quality of DOM delivered to the Baltic Sea can also be expected to change. Our results indicate that DOM supply to the Baltic Sea from boreal rivers will be more stable throughout the year, and potentially have a lower bioavailability.