Behavior of terrestrial dissolved organic matter at the continent-ocean boundary from high-resolution distributions

doi:10.1016/S0016-7037(99)00370-1

Geochimica et Cosmochimica Acta

Volume 64, Issue 16, August 2000, Pages 2765-2774

https://doi.org/10.1016/S0016-7037(99)00370-1 Get rights and content

Abstract

This paper presents distributions of fluorescent dissolved organic matter (FDOM) in the Columbia River estuary and associated coastal plume as determined by in situ measurement with a fiber optic spectrometer. We interpret these data to represent fluorescence from the humic material associated with terrestrial dissolved organic matter (DOM). We were able to convert FDOM data to quantitative estimates of fluorescent dissolved organic carbon (FDOC) by calibrating our instrument against organic carbon measured by high temperature combustion. The concentration of FDOC in the Columbia River is 140–180 μM, in agreement with DOC data from several previous investigations and our own high-temperature measurements. This result supports the view that humic fluorescence can be used as an analog for terrestrial DOC in some circumstances. The distribution of FDOM in the estuary is nonlinear when plotted against salinity indicating a source within the estuary. The flux of DOC from the river was 1040 mols C sec⁻¹ during our October sampling period. Mass balance calculations show that estuarine source(s) added 14% to this amount. Potential sources of this material are groundwater, intertidal embayments, and in situ production associated with the estuary turbidity maximum (ETM). FDOM-absorbance demonstrates that substantial shifts in fluorescence efficiency occur only in the ETM. This result together with other recent studies suggests that microbial attack of particulate organic carbon in the ETM may be the primary source of excess FDOC in the Columbia estuary. Once FDOM leaves the estuary it mixes into the coastal ocean conservatively with little indication of removal. The presence of three water masses explains most of the variability of FDOM on the shelf. There is a reversal in the slope of the FDOM-salt relationship between Columbia River plume water and deeper shelf water masses. We interpret this bilateral distribution to be the net result of two conservative processes: primary dilution of dissolved fluvial material with coastal surface water and mixing of refractory terrestrial material into the ocean.

Introduction

Terrestrial organic matter is an important component of the carbon budget in coastal areas. This study focuses on the behavior of humic-rich dissolved organic carbon in the Columbia River estuary and associated coastal plume. We image this component as fluorescent dissolved organic material (FDOM) which is part of the pool of dissolved organic matter. DOM is responsible for the color, odor, and binding capacity of natural waters and plays a key role in many natural processes. For example, it is well known that the physiochemical makeup of DOM is the primary attractant during fish migration in brackish and fresh water Wisby and Hasler 1954, Cooper and Hirsch 1982. Moreover, terrestrial DOM has high binding for anthropogenic substances (Patterson et al., 1996) and a knowledge of its behavior allows us to better understand the fate of river-borne contaminants.

When humic material enters the oceans it can be remineralized by a combination of photooxidation and microbial activity Kramer 1979, Amador et al 1989, Kieber et al 1990, Mopper et al 1991, Herndl et al 1993, Miller and Zepp 1995, Amon and Benner 1996a. DOM can also be removed by adsorption onto particles during flocculation (Sholkovitz, 1976). Remineralization creates new ligands that can become a food source for heterotrophic bacteria (Amon and Benner, 1996b) as well as sites for adsorption and further reaction (Kieber et al., 1989). The break down of DOM also releases nutrients and metals to the water column thus supporting the production of marine carbon (Lefèvre et al., 1996). Remineralization of even a small percentage of DOM can have a significant impact on the overall organic carbon budget as the mass flux of fluvial carbon is large compared to local productivity.

It is well known that some terrestrial organic material escapes degradation and resides in the oceans for some time. The most direct evidence for this refractory component is the ligneous material found in all ocean basins (Opsahl and Benner, 1997). There are also higher concentrations of extractable lignins in the deep Atlantic than in the deep Pacific, consistent with a fluvial source. Moreover a refractory terrigenous component is often seen as the most likely explanation for the extreme age of DOC in deep waters Williams and Druffel 1987, Bauer et al 1992. In summary, we know that some fluvial DOM is reactive and that another fraction mixes into the oceans. What is not well understood is the proportioning and reactivity of this material as it enters the oceans. This paper attempts to get at this problem by examining the behavior of terrestrial DOM at the continent-ocean boundary using high-resolution optical data from the Columbia River estuary (Fig. 1) and associated coastal waters.

Section snippets

Methods

Several years ago our laboratory constructed a fiber optic spectrometer that we use as a flow-through chemical analyzer and UV fluorometer (Klinkhammer, 1994). ZAPS (zero angle photon spectrometer) is a fixed-filter instrument that couples a photomultiplier tube and xenon flash lamp through a short (18 cm) bifurcated bundle of fused silica fibers potted in stainless steel tubes. The fiber assembly is external to the pressure case that holds interference filters, xenon lamp, flash power supply,

Arctic calibration

A recently published study in the Arctic demonstrates that we can use a fiber optic spectrometer to estimate DOC in some environments (Guay et al., 1999). This work was done as part of the SCICEX Program that brings together oceanographic research and annual patrols carried out by the US Navy using a nuclear submarine. During 1997 we mounted a ZAPS instrument on the hull of the USS ARCHERFISH. Included in this patrol was a transit at 58 meters water depth between the Beaufort Sea (Canada

Mass-balance of dissolved humic material in the estuary

The distribution of fluorescence in the estuary is nonlinear when plotted against salinity Fig. 5, Fig. 6 and suggests that fluorescent material was being added within the estuary during both years of this study. We can calculate the amount of FDOC added by considering balances for water and salt during this period. Since the input of fresh water from small rivers is negligible, discharge from the estuary (Q_E) equals Columbia river flow (Q_R) plus coastal water entering the estuary (Q_C), $Q_{E} =Q_{R} +Q_{C}$

Conclusions

FDOM measured with a fiber optic spectrometer can be used as an analog for estimating dissolved organic carbon in some environments. Such data show that the flux of DOC from the Columbia estuary at low flow in the fall of 1997 was 2100 mols C sec⁻¹. A large portion (36%) of this flux was recycled material from the coastal ocean with 50% coming from the river. The high-resolution distribution of FDOM across the estuary measured on several occasions indicates that there is an additional source of

Acknowledgements

We wish to thank the captain and crews of the RV Wecoma and to Fred Jones, the Marine Superintendent. We also thank Paul Stoffregen, Joe Bussell, Kathryn Brooksforce, John Prins, Jay Simpkins, Bob Collier, and Erwin Suess (GEOMAR). A special thanks to Paula Coble and Ron Benner for providing supporting data. ONR contract N00014-92J1299 to GPK funded this work.

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Dissolved organic matter (DOM) is a major component of the organic matter pool, playing a key role in the global ocean functioning. However, studies on DOM in waters of many ocean regions, such as the Gulf of Cadiz (GoC), are poorly known. Advanced aquatic sensors enable autonomous for long-term deployments in situ collection of high frequency DOM data using fluorescent dissolved organic matter (FDOM) as a proxy. The present study evaluates the relevance of FDOM, the estuarine influence and the environmental factors that determine its spatial distribution in the GoC. Our results suggest that the GoC water mass, under the estuarine influence of three main rivers, is receiving large amounts of DOM transported mainly by Guadalquivir and Guadiana rivers and much less from Tinto-Odiel. Salinity is the main factor explaining the FDOM variability within the Guadalquivir and Guadiana rivers and in the inner shelf of the GoC. In the outer shelf of the GoC, plankton-produced DOM could explain the persistent spatial pattern of FDOM, playing an important role in the dynamics of FDOM from the North area of the GoC through the persistent low-salinity Eastern North Atlantic Central Water. The oceanographic dynamics and the spatial pattern of FDOM concentration in the continental shelf of the GoC suggest a net transport of FDOM through the GCC (Gulf of Cadiz Current) to the Mediterranean Sea.
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Citation Excerpt :
DOC often exhibits an apparent conservative mixing in the estuary (Hung and Huang, 2005; Laane, 1980; Mantoura and Woodward, 1983). Conversely, the non-conservative behavior for DOC is often observed in more turbid estuaries (Harvey and Mannino, 2001; Klinkhammer et al., 2000; Uher et al., 2001) or high productive areas (Benner and Opsahl, 2001; Saliot et al., 2002) where processes like DOM flocculation and sorption/desorption, or in situ production and excretion are more likely to occur. Similarly, the non-conservative DIN behavior is usually due to sorption/desorption, precipitation/re-dissolution, and biological processes (Zhu et al., 2014).
Concentrations of dissolved organic carbon (DOC), dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON) and its major compound classes—total hydrolysable amino acids (THAA) were measured at 4 cross-shelf transects of the East China Sea in July 2011. Surface concentrations of DOC, DIN, DON and THAA at the nearshore stations were mostly in excess of those found at the offshore sites, indicating either substantial autochthonous production or allochthonous inputs from the Changjiang River. The vertical distributions of DOC, DON and THAA showed similar trends with higher values in the surface layer, whereas the elevated concentrations of DIN were observed in the bottom layer. Major constituents of THAA presented in the study area were glycine, serine, alanine, glutamic acid, aspartic acid and valine. The mole percentages of neutral amino acids increased from surface water to bottom water, whereas acidic and hydroxy amino acids decreased with the water depth. Concentrations of DOC and THAA were negatively correlated to the ΔDIN values (the difference between the real concentration and theoretical concentration), respectively, indicating the coupling relation between dissolved organic matter (DOM) remineralization and nutrient regeneration in the water column. The C/N ratios in the water column exhibited different characteristics with elevated values appearing in the surface and bottom layers. Box and whisker plots showed that both degradation index (DI) values and THAA yields displayed a decreasing trend from the surface layer to the bottom layer, implying increasing degradation with the water depth. Our data revealed that glycine and alanine increased in relative abundance with decreasing DI, while tyrosine, valine, phenylalanine and isoleucine increased with increasing DI.
Application of an in situ fluorometer to determine the distribution of fluorescent organic matter in the open ocean
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The application of in situ fluorometers to determine the high resolution vertical profiles of oceanic fluorescent organic matter (FOM) enhances the study of FOM distributions in the open ocean and will lead to better understandings of the environmental dynamics of FOM, including the distribution/fate of terrestrial FOM in offshore waters. Here, we applied a commercially available in situ FOM fluorometer that was connected to a conductivity, temperature and depth (CTD) sensors with a carousel multi-sampling system to determine full vertical profiles of the FOM in oceanic environments. It was shown that temperature standardization was necessary to produce robust depth profiles and was performed preceding calibration with a benchtop fluorometer. The calibrated output of the in situ fluorometer was linearly related to apparent oxygen utilization when corrected for differences in fluorometer response over the length of the cruise. The results of this study indicated that (1) in situ FOM fluorometers that are mounted on CTD systems can be used to study the distribution of FOM in the open ocean, and (2) temperature standardization and appropriate calibration are crucial to evaluate the distribution of FOM in the open ocean. Although we used a window filter (weighted average of the output) to overcome the low S/N ratio of the output of the in situ fluorometer, better S/N ratios for in situ fluorometers are expected for open ocean studies.
The contribution of macroalgae to the coastal dissolved organic matter pool
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Dissolved organic matter (DOM) in coastal environments has various origins; one of the most intensely studied sources is terrestrial DOM input via rivers. On the other hand, contributions from other significant DOM sources, such as macroalgae, to the coastal DOM pool have not been extensively studied. The present study quantified the contribution of macroalgae to the DOM pool in the coastal environment using, firstly, a bag-covering experiment on a brown alga, Ecklonia cava, and identifying fluorescent DOM components by parallel factor analysis of three-dimensional excitation–emission matrix spectra. Using the fluorescent DOM as an indicator, we evaluated the horizontal distribution of macroalgal DOM in the coastal area, showing that the fluorescent DOM component had a synchronous gradient with dissolved organic carbon (DOC) concentrations along the transect line from the coast to offshore. On the basis of the correlation between DOC and fluorescent DOM, we evaluated concentrations of DOC originating from macroalgae, accounting for up to 20% of total DOC concentrations. This implies that in contrast to previous studies, macroalgae do make a measurable contribution to the coastal DOM pool.
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This chapter reviews and synthesizes the current state of knowledge on sources, transformations, cycling, and fluxes of dissolved organic carbon (DOC) in river-estuarine and coastal ocean systems. Because estuarine waters represent a complex interface between terrestrial, marine, seafloor, and even atmospheric environments, significant consideration is given to the diverse sources of organic matter contributing to estuarine and coastal marine DOC pools. Distributions of bulk DOC, its stable (δ¹³C) and radioactive (Δ¹⁴C) isotopes, and its biochemical components are reviewed with representative estuaries globally highlighted as examples of the variable types of DOC mixing and processing dynamics in different systems. The significant microbial, photochemical, and physical transformation processes affecting both the amounts and characteristics of DOC during estuarine transport are also evaluated. The chapter concludes by providing some examples of important and compelling recent studies of land–river estuary–coastal ocean DOC fluxes and transfers and their observed long-term changes, both natural and anthropogenically driven.

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^☆

Present address: Large Lakes Observatory, University of Minnesota, 10 University Dr., 109 RLB, Duluth, MN 55812.

Present address: Department of Earth Sciences, Cambridge University, Downing Street, Cambridge CB2 3EQ, UK.

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ArticlesBehavior of terrestrial dissolved organic matter at the continent-ocean boundary from high-resolution distributions☆

Abstract

Introduction

Section snippets

Methods

Arctic calibration

Mass-balance of dissolved humic material in the estuary

Conclusions

Acknowledgements

Geochim. Cosmochim. Acta

Mar. Chem.

Mar. Chem.

Mar. Chem.

Mar. Chem.

Estuar. Coast. Shelf Sci.

Mar. Geol.

Mar. Chem.

Mar. Chem.

Nether. J. Sea Res.

J. Mar. Systems

Mar. Chem.

Mar. Chem.

Geochim. Cosmochim. Acta

Geochimica. Cosmochimica. Acta

Estuar. Coast. Shelf Sci.

Sequential photochemical and microbial degradation of organic molecules bound to humic acid

Appl. Environ. Microbiol.

Bacterial utilization of different size classes of dissolved organic matter

Limnol. Oceanogr.

14C activity of dissolved organic carbon fractions in the north-central Pacific and Sargasso Sea

Nature

Articles
Behavior of terrestrial dissolved organic matter at the continent-ocean boundary from high-resolution distributions☆

¹⁴C activity of dissolved organic carbon fractions in the north-central Pacific and Sargasso Sea