Production, degradation, and flux of dissolved organic matter in the subterranean estuary of a large tidal flat

doi:10.1016/j.marchem.2012.08.002

Marine Chemistry

Volumes 142–144, 10 October 2012, Pages 1-10

https://doi.org/10.1016/j.marchem.2012.08.002 Get rights and content

Abstract

The distributions of dissolved organic carbon (DOC), total dissolved hydrolysable amino acids (THAA), and colored dissolved organic matter (CDOM) were studied in pore-water/groundwater samples (including seeping water) from a subterranean estuary (STE) of a large tidal flat in Hampyeong Bay, Korea, in July 2011. The relatively low alanine D/L ratios and high THAA concentrations in the pore-water closest to the sediment surface (0–10 cm) indicate the active production of dissolved organic matter (DOM) from benthic algae, and the relatively low THAA concentrations and high D/L ratios in the subsurface pore-water (10–35 cm) indicate a relatively large presence of bio-degraded DOM. In the deep pore-water (35–75 cm), relatively low D/L ratios, high DOC concentrations, and intense humic-like fluorescence were observed, suggesting a net accumulation of less-reactive DOC in this layer. Overall, this STE appears to have net DOM sources because the concentrations of DOC (60–1700 μM) in the pore-water decreased toward the land, the surface, and the low-salinity waters. The concentrations of DOC in the seeping water (185 ± 52 μM) were higher than those in the overlying seawater (144 ± 9 μM), resulting in net DOC fluxes of 2–5 × 10⁹ g·C·yr^− 1 through submarine groundwater discharge (SGD) into Hampyeong Bay. The organic matter compositions in the seeping water indicated that SGD introduced DOM from both the surface and subsurface layers. Our results highlight that tidal flats are important sources for DOM, implying that SGD-driven DOM plays an important role in coastal carbon cycles and biogeochemistry.

Highlights

► DOM in the subterranean estuary is produced mainly in the sandy parts of the aquifer. ► Fresh DOM was effectively produced in the surface layer of this subterranean estuary. ► A higher proportion of bio-degraded DOM was observed in the subsurface layer. ► A large amount of less-reactive DOM was produced and accumulated in the deep layer.

Introduction

Submarine groundwater discharge (SGD) has been emphasized as an important source of nutrients and trace metals in coastal zones during the last few decades (Burnett et al., 2003, Kim et al., 2005, Moore, 2006). The fluxes of the chemical constituents introduced through SGD are influenced by their biogeochemical behavior in subterranean estuaries (STEs), which are a dynamic part of coastal aquifers where fresh groundwater mixes with seawater intruding into the aquifer (Moore, 1999). Thus, it is necessary to explore the processes occurring in STEs to assess the impact of SGD on coastal areas. However, the biogeochemical behaviors of chemical constituents in STEs are poorly understood because they involve complex interactions among hydrogeologic, oceanic, and geochemical processes. For example, the biogeochemical cycling of nutrients can be affected by the redox conditions of fresh groundwater and seawater and by microbial activity (Slomp and van Cappellen, 2004, Spiteri et al., 2008, Santos et al., 2009).

As the term SGD covers both fresh groundwater (terrestrial origin) and re-circulated seawater (marine origin) (Burnett et al., 2003), the sources of dissolved organic matter (DOM) transported via SGD could be diverse (Gao et al., 2010, Roy et al., 2010). Only a few studies have investigated the distributions of DOM in STEs. Beck et al. (2007) showed that the concentration of dissolved organic carbon (DOC) in West Neck Bay, USA, was lower in the groundwater than in the seawater of the adjacent ocean and that it was conservatively mixed throughout the flow path. In contrast, Santos et al. (2009) reported that DOC is produced in a STE in the Gulf of Mexico and that land-derived DOM makes up 16–34% of the total SGD flux. Similarly, Goñi and Gardner (2003) showed that the DOC flux from saline groundwater discharge contributes a significant fraction of the annual DOC budget in the North Inlet tidal estuary, South Carolina. However, there are still large unknowns concerning the sources and behavior of DOM in STEs under various hydrogeologic, oceanic, and geochemical conditions.

To determine whether the DOM in the STE is derived from marine or terrestrial sources, colored dissolved organic matter (CDOM), the optically active portion of DOM that absorbs light over a broad range of visible and ultraviolet wavelengths, may be useful. CDOM has been used as a powerful tracer to identify the origins (marine versus terrestrial) of DOM in seawater and to study the bacterial degradation of DOM (Vodacek et al., 1997, Del Castillo et al., 1999, Murphy et al., 2008). EEMs (Excitation–Emission Matrix Spectroscopy) peaks and PARAFAC (parallel factor) analysis allow the discrimination of marine versus terrestrial DOM sources (Coble, 1996, Stedmon et al., 2003, Stedmon and Bro, 2008, Chen et al., 2010). In addition, amino acids, a major component of dissolved organic nitrogen (DON), can also be used to determine the degradation state of DOM (McCarthy et al., 1998, Dauwe et al., 1999, Amon et al., 2001, Dittmar et al., 2001). D-amino acids have been used as peptidoglycan biomarkers because relative increases in d- over l-amino acids have been interpreted as indicative of the preferential accumulation of bacterial peptidoglycans over proteinaceous material during organic matter degradation (Veuger et al., 2006). Recent studies showed that, in addition to peptidoglycans, other D-amino acid containing bacterial remains can contribute to the bacterial fraction of detrital organic matter (Benner and Kaiser, 2003, Niggemann and Schubert, 2006, Kaiser and Benner, 2008, Lomstein et al., 2009). Therefore, increases in the D/L ratio of amino acids are an indicator for bacterial degradation of fresh organic matter. In addition, the composition of the amino acids pool is commonly used to calculate a degradation index (DI, Dauwe et al., 1999) since the different amino acids are removed selectively during microbial remineralization.

In this study, we analyzed DOC, DON, CDOM, and total dissolved hydrolysable amino acids (THAA) in a large tidal flat STE located at Hampyeong Bay beach, Korea, which is known to have high seepage rates owing to rapid tidal recirculation (Waska and Kim, 2011). We aimed to (1) determine the sources (marine versus terrestrial) of DOM using CDOM; (2) evaluate the degradation state of DOM using THAA characteristics, such as the D/L ratio and the DI; and (3) determine the fluxes of DOC, DON, and DIN from the STE to the coastal waters.

Section snippets

Study area and sampling

Located along the west coast of South Korea (Fig. 1), Hampyeong Bay is a shallow (average depth: 4 m) and semi-enclosed bay with a large intertidal zone (average tidal range: 3.5 m) that covers more than 50% of the bay area (Waska and Kim, 2011). The subtidal area consists mainly of mud and muddy sand that is transported into the bay from the Yellow Sea. Along most of the shoreline, coarse sandy beaches sit atop the subtidal mud and muddy sands. In the intertidal zone, i.e., at the interface of

Results

For the depth profiles obtained from the multi-level pore-water samples taken on July 5 and 21, 2011, the salinities ranged from 25.3 to 32.2 (Fig. 3). The salinities ranged from 18.7 to 30.6 in shallow-well water samples, from 20.1 to 29.9 in the seeping water, and from 30.8 to 31.4 in the overlying seawater (Table 1). The concentrations of DIN ranged from 16 to 142 μM in the multi-level pore-water samples taken on July 5 and 21, 2011 (Fig. 3). In the shallow-well water and seeping water, the

Characteristics of CDOM peaks

According to Coble (1996), the T peak (tryptophan-like) is a “protein-like” component, the C and A peaks are “humic-like” components, and the M peak is a “marine humic-like” component (Table 2). We focus on the C and T peaks in this discussion because the fluorescence intensities of the M and A peaks were relatively weak and it was difficult to distinguish them from the background. The intensities of both the C and T peaks increased in the pore-water profiles from the surface down to the deep

Conclusions

On the basis of excitation–emission matrix spectroscopy analyses for CDOM and the analysis of THAA, different characteristics of DOM were found for the surface, subsurface, and deep groundwater layers of this subterranean estuary. DOM was effectively produced in the surface groundwater layer in this subterranean estuary, most likely by benthic algae. In the subsurface groundwater layer, a higher proportion of bio-degraded DOM was observed, whereas in the deep groundwater layer, a large amount

Acknowledgments

We would like to thank all EMBL members who helped with field sampling and three reviewers for providing valuable comments. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (no. 2011–0027523 and 2012–0006256).

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Production, degradation, and flux of dissolved organic matter in the subterranean estuary of a large tidal flat

Abstract

Highlights

Introduction

Section snippets

Study area and sampling

Results

Characteristics of CDOM peaks

Conclusions

Acknowledgments

Earth Planet. Sci. Lett.

Appl. Geochem.

Geochim. Cosmochim. Acta

Mar. Chem.

Appl. Geochem.

Mar. Chem.

Geochim. Cosmochim. Acta

Mar. Chem.

Geochim. Cosmochim. Acta

J. Chromatogr. A

Mar. Environ. Res.

Geochim. Cosmochim. Acta

Org. Geochem.

Earth Planet. Sci. Lett.

Science

Mar. Chem.

J. Geochem. Explor.

Mar. Chem.

Geochim. Cosmochim. Acta

Org. Geochem.

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

J. Hydrol.

Mar. Chem.