Abstract
Dissolved organic matter (DOM) is ubiquitous in natural waters and plays a central role in the biogeochemistry in riverine, estuarine and marine environments. This study quantifies and characterizes solid-phase extractable DOM and trace element complexation at different salinities in the Weser and Elbe River, northern Germany, and the North Sea. Dissolved organic carbon (DOC), total dissolved nitrogen (TDN), Co and Cu concentrations were analyzed in original water samples. Solid-phase extracted (SPE) water samples were analyzed for DOC (DOCSPE), dissolved organic nitrogen (DONSPE), sulfur (DOSSPE) and trace metal (51V, 52Cr, 59Co, 60Ni, 63Cu, 75As) concentrations. Additionally, different pre-treatment conditions (acidification vs. non-acidification prior to SPE) were tested. In agreement with previous studies, acidification led to generally higher recoveries for DOM and trace metals. Overall, higher DOM and trace metal concentrations and subsequently higher complexation of trace metals with carbon and sulfur-containing organic complexes were found in riverine compared to marine samples. With increasing salinity, the concentrations of DOM decreased due to estuarine mixing. However, the slightly lower relative decrease of both, DOCSPE and DONSPE (~77%) compared to DOSSPE (~86%) suggests slightly faster removal processes for DOSSPE. A similar distribution of trace metal and carbon and sulfur containing DOM concentrations with salinity indicates complexation of trace metals with organic ligands. This is further supported by an increase in Co and Cu concentration after oxidation of organic complexes by UV treatment. Additionally, the complexation of metals with organic ligands (analyzed by comparing metal/DOCSPE and metal/DOSSPE ratios) decreased in the order Cu > As > Ni > Cr > Co and thus followed the Irving-Williams order. Differences in riverine and marine trace metal containing DOMSPE are summarized by their average molar ratios of (C107N4P0.013S1)1000V0.05Cr0.33Co0.19Ni0.39Cu3.41As0.47 in the riverine endmember and (C163N7P0.055S1)1000V0.05Cr0.47Co0.16Ni0.07Cu4.05As0.58 in the marine endmember.
Highlights
Dissolved organic matter (DOM) is actively cycling in natural waters and participates in most biogeochemical processes
The original Dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) concentrations in the water samples decreased with increasing salinity from a maximum of 407 μmol L-1 and 190 μmol L-1, respectively, in riverine water (W1) to 293 μmol L-1 DOC and 101 μmol L-1 TDN in estuarine water (E3), and 97 μmol L-1 DOC and 14 μmol L-1 TDN in the marine sample (M1; Fig 3, Table 3)
Using reversed-phase chromatography, we found a good relationship of both fluorescence (260/430 nm) and absorption (210 nm) data with measured DOC and DOCSPE concentrations (R2 = 0.3 and p < 0.01 for DOC concentrations of 0–40 μmol L-1 versus UV peak areas and R2 = 0.6 and p < 0.001 for DOC concentrations > 100 μmol L-1 versus UV peak areas, S1 Fig), confirming that UV absorption in the extracts serves as a suitable predictor of DOC concentration [46]
Summary
Dissolved organic matter (DOM) is actively cycling in natural waters and participates in most biogeochemical processes. The biogeochemistry of marine dissolved organic carbon, nitrogen and phosphorus (DOC/N/P) was extensively studied in the past, e.g. The knowledge on quantity, distribution, and the biogeochemical role of dissolved organic sulfur (DOS) in aquatic environments is limited, though not less important. Several studies focused primarily on volatile organic sulfur compounds, such as dimethylsulfide (DMS) and carbonyl sulfide (COS), because they are actively involved in climate processes [15,16,17]. Those climate relevant organic sulfur compounds contribute less than 3% to the total marine DOS pool [3]
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