Environmental Drivers of Dissolved Organic Matter Molecular Composition in the Delaware Estuary

Abstract

Estuaries as connectors of freshwater and marine aquatic systems are hotspots of biogeochemical element cycling. In one of the best studied temperate estuaries, the Delaware Estuary (USA), we investigated the variability of dissolved organic matter (DOM) over five sampling cruises along the salinity gradient in August and November of 3 consecutive years. Dissolved organic carbon (DOC) concentrations were more variable in the upper reaches of the estuary (245 ± 49 µmol DOC L −1 ) than at the mouth of the estuary (129 ± 14 µmol L −1 ). Bulk DOC decreased conservatively along the transect in November but was non-conservative with increased DOC concentrations mid-estuary in August. Detailed analysis of the solid-phase extractable DOM pool via ultrahigh resolution mass spectrometry (Fourier-transform ion cyclotron resonance mass spectrometry, FT-ICR-MS) revealed compositional differences at the molecular level that were not reflected in changes in concentration. Besides the mixing of terrestrial and marine endmember signatures, river discharge levels and biological activity impacted DOM molecular composition. DOM composition changed less between August and November than along the salinity gradient. Relative contributions of presumed photolabile DOM compounds did not reveal non-conservative behavior indicative of photochemical processing, suggesting that on the timescales of estuarine mixing photochemical removal of molecules plays a minor role in the turbid Delaware Bay. Overall, a large portion of molecular formulae overlapped between sampling campaigns and persisted during estuarine passage. Extending the analysis to the structural level via the fragmentation of molecular masses in the FT-ICR-MS, we found that the relative abundance of isomers along the salinity gradient did not change, indicating a high structural similarity of aquatic DOM independent of the origin. These results point toward a recalcitrant character of the DOM supplied by the Delaware River. We demonstrate that in addition to bulk DOC quantification, detailed information on molecular composition is essential for constraining sources of DOM and to identify the processes that impact estuarine DOM, thereby controlling amount and composition of DOM eventually discharged to the ocean through estuaries.

Microbially-Mediated Transformations of Estuarine Dissolved Organic Matter

“…The correlation coefficient between PC1 and salinity for the pre-incubation samples was −0.87 (p < 0.01). The loading of PC1 ( Figure 1B) shows a pattern typical of river-to-ocean transects (Medeiros et al, 2015c), with high salinity samples enriched with compounds with high H/C ratios compared to lowsalinity samples, which is consistent with the higher abundance of aliphatic structures in marine DOM (Sleighter and Hatcher, 2008;Osterholz et al, 2016). This is also consistent with the strong gradient in DOM sources in the system associated with terrigenous vs. oceanic inputs (Medeiros et al, 2015a).…”

“…All PCs shown here are significantly different (95% confidence level) from results of PC analysis of spatially and temporally uncorrelated random processes (Overland and Preisendorfer, 1982). The Wilcoxon rank-sum test was used for comparisons between groups of compounds, as in Osterholz et al (2016).…”

“…In general, DOC concentrations and the DOM aromaticity decrease toward the ocean (Abdulla et al, 2013) at the same time that DOM lability increases (D'Andrilli et al, 2015). Despite intense cycling, a large portion of compounds persist during estuarine passage (Osterholz et al, 2016).…”

Composition and Transformation of Dissolved Organic Matter in the Baltic Sea

“…Several other studies have shown that the transformation of terrestrial and the new production of autochthonous (marine) DOM compounds leave characteristic molecular imprints (Sleighter and Hatcher, 2008;Kujawinski et al, 2009;Medeiros et al, 2015a,b;Seidel et al, 2015b). Aromatic terrestrial DOM compounds generally decrease along salinity gradients while the DOM heteroatom content increases due to autochthonous production (Sleighter and Hatcher, 2008;Osterholz et al, 2016). Likewise, the alteration of DOM under different redox-regimes can leave identifiable molecular imprints such as the relative enrichment of sulfurcontaining compounds (Schmidt et al, 2009;Seidel et al, 2014Seidel et al, , 2015aSleighter et al, 2014;Gomez-Saez et al, 2016).…”

“…Consequently, this technique was used to study the transformation of DOM in a wide range of aquatic systems, including terrestrial-to-marine and oxicto-anoxic transition zones (Koch et al, 2005;Kim et al, 2006;Sleighter and Hatcher, 2008;Schmidt et al, 2009;Longnecker and Kujawinski, 2011;Lechtenfeld et al, 2013). For example, in the Delaware Estuary (USA), compositional differences at the molecular level were not reflected in changes of DOC concentrations (Osterholz et al, 2016). Several other studies have shown that the transformation of terrestrial and the new production of autochthonous (marine) DOM compounds leave characteristic molecular imprints (Sleighter and Hatcher, 2008;Kujawinski et al, 2009;Medeiros et al, 2015a,b;Seidel et al, 2015b).…”