Based on an extensive literature survey containing more than 12,000 paired measurements of dissolved organic carbon (DOC) concentrations and absorption of chromophoric dissolved organic matter (CDOM) distributed over four continents and seven oceans, we described the global distribution and transformation of dissolved organic matter (DOM) along the aquatic continuum across rivers and lakes to oceans. A strong log-linear relationship (R=0.92) between DOC concentration and CDOM absorption at 350nm was observed at a global scale, but was found to be ecosystem-dependent at local and regional scales. Our results reveal that as DOM is transported towards the oceans, the robustness of the observed relation decreases rapidly (R from 0.94 to 0.44) indicating a gradual decoupling between DOC and CDOM. This likely reflects the decreased connectivity between the landscape and DOM along the aquatic continuum. To support this hypothesis, we used the DOC-specific UV absorbance (SUVA) to characterize the reactivity of the DOM pool which decreased from 4.9 to 1.7m × gC along the aquatic continuum. Across the continuum, a piecewise linear regression showed that the observed decrease of SUVA occurred more rapidly in freshwater ecosystems compared to marine water ecosystems, suggesting that the different degradation processes act preferentially on CDOM rather than carbon content. The observed change in the DOM characteristics along the aquatic continuum also suggests that the terrestrial DOM pool is gradually becoming less reactive, which has profound consequences on cycling of organic carbon in aquatic ecosystems.

An important function of coastal ecosystems is the reduction of the nutrient flux from land to the open sea, the coastal filter. In this study, we focused on the two most important coastal biogeochemical processes that remove nitrogen and phosphorus permanently: denitrification and phosphorus burial. We compiled removal rates from coastal systems around the Baltic Sea and analyzed their spatial variation and regulating environmental factors. These analyses were used to scale up denitrification and phosphorus burial rates for the entire Baltic Sea coastal zone. Denitrification rates ranged from non‐detectable to 12 mmol N m−2 d−1, and correlated positively with both bottom water nitrate concentration and sediment organic carbon content. The rates exhibited a strong decreasing gradient from land to the open coast, which was likely driven by the availability of nitrate and labile organic carbon, but a high proportion of non‐cohesive sediments in the coastal zone decreased the denitrification efficiency relative to the open sea. Phosphorus burial rates varied from 0.21 g P m−2 yr−1 in open coastal systems to 2.28 g P m−2 yr−1 in estuaries. Our analysis suggests that archipelagos are important phosphorus traps and account for 45% of the coastal P removal, while covering only 17% of the coastal areas. High burial rates could partly be sustained by phosphorus import from the open Baltic Sea. We estimate that the coastal filter in the Baltic Sea removes 16% of nitrogen and 53% of phosphorus inputs from land.

Abstract. The microbial degradation of dissolved organic carbon and nitrogen (DOC, DON) was studied in three Finnish boreal estuaries with contrasting land use patterns (Kiiminkijoki -natural forest and peatland; Kyrönjoki -agricultural; Karjaanjoki -mixed/urban). Bioassays of 12-18 d long durations were used in 3 seasons at in situ temperatures. Besides the bulk parameters, a suite of dissolved organic matter (DOM) quality parameters were also investigated, including colored DOM (CDOM), fluorescent DOM and the molecular weight of DOM. Bioavailable DOC and DON pools varied significantly between the estuaries, from 7.9 to 10.6 % and from 5.5 to 21.9 %, respectively. DOM originating from the catchment dominated by natural forests and peatlands (Kiiminkijoki) had the lowest DOC and DON degradation rates, as well as the lowest proportions of biodegradable DOC and DON. A greater proportion of agricultural land in the catchment increased the bioavailability of DON, but not the bioavailability of DOC (Kyrönjoki). Additionally, DOM quality varied significantly between the estuaries, and DOM originating from the agricultural Kyrönjoki catchment sustained higher DOC and DON degradation rates and higher bacterial growth efficiency (BGE) compared to those of the natural forest and peat dominated Kiiminkijoki catchment. The quality of DOM, indicated by differences in CDOM, fluorescent DOM and molecular weight, varied between estuaries with differing land use and was concluded to be major driver of BGE of these systems and thereafter to the microbial CO 2 fluxes from the estuaries. The differences in BGE resulted in a 5-fold difference in the calculated daily bacterial CO 2 emissions between the study's estuaries due to bacterial activity, ranging from 40 kg C d −1 in the Karjaanjoki estuary to 200 kg C d −1 in the Kyrönjoki estuary. Lower DOC : DON ratios, smaller molecular weight and higher CDOM absorption spectral slope values of DOM resulted in higher proportion of the initial DOC and DON being transferred to microbial growth and therefore to the pelagic food web. The pristine, peatland and forest-dominated Kiiminkijoki catchment had the lowest BGE, and therefore proportionally highest CO 2 fluxes.

The flocculation of dissolved organic matter (DOM) was studied along transects through three boreal estuaries. Besides the bulk concentration parameters, a suite of DOM quality parameters were investigated, including colored DOM (CDOM), fluorescent DOM, and the molecular weight of DOM as well as associated dissolved iron concentrations. We observed significant deviations from conservative mixing at low salinities (<2) in the estuarine samples of dissolved organic carbon (DOC), UV absorption (a (CDOM254) ), and humic-like fluorescence. The maximum deviation from conservative mixing for DOC concentration was À16%, at salinities between 1 and 2. An associated laboratory experiment was conducted where an artificial salinity gradient between 0 and 6 was created. The experiment confirmed the findings from the estuarine transects, since part of the DOC and dissolved iron pools were transformed to particulate fraction (>0.2 μm) and thereby removing them from the dissolved phase. We also measured flocculation of CDOM, especially in the UV region of the absorption spectrum. Protein-like fluorescence of DOM decreased, while humic-like fluorescence increased because of salt-induced flocculation. Additionally, there was a decrease in molecular weight of DOM. Consequently, the quantity and quality of the remaining DOM pool was significantly changed after influenced to flocculation. Based on these results, we constructed a mechanistic, two-component flocculation model. Our findings underline the importance of the coastal filter, where riverine organic matter is flocculated and exported to the sediments.