Abstract
AbstractDissolved organic matter (DOM) in the transition zone from freshwater to marine systems was analyzed with a new approach for parameterizing the size distribution of organic compounds. We used size‐exclusion chromatography for molecular size analysis and quantified colored DOM (CDOM) on samples from two coastal environments in the Baltic Sea (Roskilde Fjord, Denmark and Gulf of Gdansk, Poland). We applied a Gaussian decomposition method to identify peaks from the chromatograms, providing information beyond bulk size properties. This approach complements methods where DOM is separated into size classes with pre‐defined filtering cutoffs, or methods where chromatograms are used only to infer average molecular weight. With this decomposition method, we extracted between three and five peaks from each chromatogram and clustered these into three size groups. To test the applicability of our method, we linked our decomposed peaks with salinity, a major environmental driver in the freshwater‐marine continuum. Our results show that when moving from freshwater to low‐salinity coastal waters, the observed steep decrease of apparent molecular weight is mostly due to loss of the high‐molecular‐weight fraction (HMW; >2 kDa) of CDOM. Furthermore, most of the CDOM absorbance in freshwater originates from HMW DOM, whereas the absorbing moieties are more equally distributed along the smaller size range (< 2 kDa) in marine samples.
Highlights
Despite that mathematical approaches for deconvoluting chromatograms have been used for decades in other disciplines, here we used the approach for the first time to assess the size-dependent changes in the optical properties of natural Dissolved organic matter (DOM) in the aquatic continuum from land to sea
Our data show that different chromophores have different size ranges, and using different wavelengths for measurements will result in different colored fraction of DOM (CDOM) molecules being detected
It is apparent that CDOM in freshwater results from relatively large molecules, whereas in marine systems small molecules are responsible for most CDOM absorbance
Summary
It has been shown that apparent molecular weight (i.e., the size of the DOM molecules) does not conform with conservative mixing, but decreases rapidly with increasing salinity, and faster than the colored fraction of DOM (CDOM) (Sholkovitz et al 1978; Zhou et al 2016; Asmala et al 2018). The most important effect of the choice of the eluent is the resolution in the chromatograms of natural organic matter, as some eluents are able to produce more detailed chromatograms than others This is due to the non-size-exclusion interactions of humic substances in DOM with the gel matrix in the column, which can be effectively suppressed by adding electrolytes and hydrophobic solvents to the eluent, leading to higher accuracy of the analysis (Swift 1999). Each size-exclusion chromatography analysis yields a detailed chromatogram with a wealth of information, which is usually not utilized to its full potential with traditional metrics of, e.g., average molecular weight
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