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 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 were found to impact 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 cell, 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 towards 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.
Highlights
Large amounts of terrigenous organic matter are channeled through rivers and estuaries into the oceans—approximately 0.25 × 1015 g are transported in the form of dissolved organic carbon (DOC) annually (Hedges et al, 1997)
Environmental Characterization of the Dissolved organic carbon (DOC), total dissolved nitrogen (TDN) and nitrate concentrations as well as turbidity generally decreased along the salinity gradient (Figure 1, Figure S1)
The analytical window provided by the combination of PPL-solid-phase extractable (SPE) and FT-ICRMS allowed us to examine a large, otherwise inaccessible portion of the dissolved organic matter (DOM) pool; in this study on average 67 ± 6% of the DOC was captured with our solid-phase extraction method
Summary
Large amounts of terrigenous organic matter are channeled through rivers and estuaries into the oceans—approximately 0.25 × 1015 g are transported in the form of dissolved organic carbon (DOC) annually (Hedges et al, 1997). The size and composition of the riverine dissolved organic matter (DOM) pool is transformed through a multitude of biotic and abiotic processes. Bulk DOC concentrations decrease along estuaries from the river to the ocean while at the same time, the composition of the DOM pool changes. Estuaries are complex systems where, in addition to mixing of fresh- and marine waters, diffuse sources such as subterranean groundwater discharge (Taniguchi et al, 2002), aeolian and anthropogenic input (Liu et al, 2005; Tzortziou et al, 2015) add to the intricacy of biogeochemical cycling. Biological processes, in temperate regions influenced by seasonality, such as selective uptake and transformation by heterotrophic microbes (Azam et al, 1994; Bourgoin and Tremblay, 2010), as well as addition of compounds via autochthonous production (Pennock and Sharp, 1986) likewise shape the estuarine DOM pool. Biological processes, in temperate regions influenced by seasonality, such as selective uptake and transformation by heterotrophic microbes (Azam et al, 1994; Bourgoin and Tremblay, 2010), as well as addition of compounds via autochthonous production (Pennock and Sharp, 1986) likewise shape the estuarine DOM pool. Medeiros et al (2015b) for example showed that phytoplankton-derived
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