Date Published: September 5, 2018
Publisher: Public Library of Science
Author(s): Kerstin B. Ksionzek, Jing Zhang, Kai-Uwe Ludwichowski, Dorothee Wilhelms-Dick, Scarlett Trimborn, Thomas Jendrossek, Gerhard Kattner, Boris P. Koch, Chon-Lin Lee.
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.
Dissolved organic matter (DOM) is actively cycling in natural waters and participates in most biogeochemical processes. Assessment of the DOM stoichiometry supports to unravel its origin and fate and to understand its role in different aquatic environments. The biogeochemistry of marine dissolved organic carbon, nitrogen and phosphorus (DOC/N/P) was extensively studied in the past, e.g. [1, 2]. In contrast, the knowledge on quantity, distribution, and the biogeochemical role of dissolved organic sulfur (DOS) in aquatic environments is limited, though not less important. We have previously estimated the global marine DOS inventory to range between 6.7 and 18.6 Pg S . In particular, rivers are known to be important sources of reduced carbon, nitrogen and phosphorus to coastal environments . Riverine DOM can be influenced by different transformation and removal processes along its way into estuarine and marine water: salt-induced flocculation [5, 6], adsorption to particulate matter [7, 8], photo-oxidative remineralization , and uptake by heterotrophs [10, 11]. Thus, typical concentrations of DOC and DON decrease over a salinity gradient from riverine to marine water [12, 13]. DOC/DON ratios usually also decrease from land to sea along the salinity gradient in estuaries  indicating differences in the stoichiometry of the organic matter precursors. In our previous study, we used existing literature and roughly estimated that the riverine transport of organic sulfur in particulate (POS) and dissolved form combined is about 0.25 Tmol S a-1 (8 Tg S a-1) . In estuarine and marine environments, the concentration of sulfate (up to 29 mmol S L-1) exceeds the concentration of DOS by up to five orders of magnitude. As the analysis of DOS has been analytically hampered, the composition and biogeochemistry of DOS remains widely unknown. 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 [15–17]. However, those climate relevant organic sulfur compounds contribute less than 3% to the total marine DOS pool .
Concentrations of DOC and total dissolved nitrogen (TDN) in filtered water were determined by high temperature catalytic oxidation (HTCO) and subsequent nondispersive infrared spectroscopy and chemiluminescence detection (TOC-VCPN analyzer, Shimadzu). For the determination of solid-phase extractable DOC (DOCSPE, pH 2 and pH 8) and DON (DONSPE, pH 2), 50 μL (250 μL for DONSPE, pH 8) of each methanol extract was evaporated under N2 and subsequently redissolved in 6.5 mL ultrapure water. All samples were acidified in the auto sampler (0.1 M HCl suprapur, Merck) and purged with O2 for > 5 min to remove inorganic carbon. Performance of the instrument was recorded by the analysis of potassium hydrogen phthalate standard solutions and the deep-sea reference samples (DSR, Hansell research lab). Final DOC and TDN concentrations are average values of triplicate measurements. If the standard variation or the coefficient of variation of DOC values exceeded 0.1 μM or 1%, respectively, up to two additional analyses were performed and outliers were eliminated. For DON, outliers of triplicate measurements were eliminated manually. The accuracy was ± 5% for DOC and ± 7% for DON.
In this study we presented the concentration and distribution of DOCSPE, DONSPE, DOSSPE, and dissolved metal-DOMSPE with changing salinity in two rivers draining to the North Sea. With regard to the research question/hypothesis stated in the introduction, we can conclude: