Stoichiometry, polarity, and organometallics in solid-phase extracted dissolved organic matter of the elbe-weser estuary

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 (DOC _SPE ), dissolved organic nitrogen (DON _SPE ), sulfur (DOS _SPE ) and trace metal ( ⁵¹ V, ⁵² Cr, ⁵⁹ Co, ⁶⁰ Ni, ⁶³ Cu, ⁷⁵ As) 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, DOC _SPE and DON _SPE (~77%) compared to DOS _SPE (~86%) suggests slightly faster removal processes for DOS _SPE . 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/DOC _SPE and metal/DOS _SPE 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 DOM _SPE are summarized by their average molar ratios of (C ₁₀₇ N ₄ P _0.013 S ₁ ) ₁₀₀₀ V _0.05 Cr _0.33 Co _0.19 Ni _0.39 Cu _3.41 As _0.47 in the riverine endmember and (C ₁₆₃ N ₇ P _0.055 S ₁ ) ₁₀₀₀ V _0.05 Cr _0.47 Co _0.16 Ni _0.07 Cu _4.05 As _0.58 in the marine endmember.