Dynamics of dissolved organic carbon (DOC) in oceanic environments

Limnol. Oceanogr.. 40(8), 1995, 1 392-1403 0 1995, by the American Society of Limnology and Oceanography, Inc.

Dynamics of dissolved organic carbon (DOC) in oceanic environments

Laodong Guo, Peter H. Santschi, and Kent W. Warnken Department of Oceanography, Texas A&M University, 5007 Avenue U, Galveston 7755 1

Abstract Cross-flow ultrafiltration techniques and a high-temperature combustion (HTC) method were used to

investigate the distributions and fluxes of dissolved (DOC) and colloidal organic carbon (COC) in the Gulf of Mexico and in the Middle Atlantic Bight. Concentrations of DOC in both regions decreased from 580 PM in surface waters to -45 PM in deep waters and showed large vertical gradients in the subsurface layer. The vertical distributions of DOC were oceanographically consistent. A conservative mixing behavior of DOC was observed in slope waters, and water mixing processes were important factors in controlling the distribution of DOC in the ocean. Calculated downward fluxes of DOC are comparable to those measured for particulate organic C. Size fractionation results revealed that COC (1 kDa-0.2 pm) comprised -4O-50% of the total DOC in seawater off Cape Hatteras, while it comprised -30-40% in the Gulf of Mexico. High- molecular-weight COC,, (10 kDa-0.2 pm) represented 4-l 0% of the total DOC in both study areas. Con- centrations of COC, , COC3, and COC,, and their percentages in the total DOC decreased from nearshore to offshore and from surface to deep waters. The COC fractions seem to partition in a predictable way in seawater, with DOC concentration as a master variable. On average, 4-7% of the total DOC was in the COC,, fraction, 7-14% was in the 3-lo-kDa fraction, and -24% was in the I-3-kDa fraction, leaving 55- 65% in the < I-kDa fraction.

Dissolved organic carbon (DOC) is the largest organic carbon pool in the ocean. It plays a central role in the marine biogeochemical cycle of carbon (Amon and Ben- ner 1994; Carlson et al. 1994). However despite decades of research (Hedges 1992), DOC continues to be the least understood reservoir of reduced carbon. The recent DOC controversy has stimulated many efforts to evaluate both the precision and accuracy of DOC measurements using the high-temperature combustion (HTC) method (e.g. Benner and Strom 1993; Sharp et al. 1993; Cauwet 1994). Accurate determination of DOC concentrations in seawater is critical to many biogeochemical studies. How- ever, obtaining a high level of precision in measurements of DOC remains difficult (Sharp et al. 1995). Due to the uncertainties in individual instrument and water blanks, DOC concentrations may be underestimated or overes- timated, depending on how they are corrected for the DOC blanks when the HTC method is used.

Part of the DOC is composed of colloids that can be separated from seawater by cross-flow ultrafiltration or other techniques, resulting in a number of size or molecular weight fractions. There is evidence that marine colloids, which include macromolecules and microparticles, are organic in nature (e.g. Benner et al. 1992b; Guo et al.

Acknowledgments We thank Sarah Oktay, Chris Paternostro, Matt Quigley, and

crew members of the RV Gyre aboard the 93GOl and 93G07 cruises for their assistance during sample collection. Hydro- graphic data (CTD, dissolved oxygen, and nutrients) were collected by members of the Marine Operations Group headed by D. Biggs. Curtis Suttle and two anonymous reviewers provided helpful comments.

This work was funded, in part, by the Department of Energy (DE-FG-05-92ER6 142 l), the National Science Foundation (OCE-90- I2 103), and the Texas Institute of Oceanography.

1994; Bianchi et al. 1995). Furthermore, recent studies have shown that colloidal organic carbon (COC) com- prises a large portion of the total DOC in river water (Whitehouse et al. 1989; Benner and Hedges 1993), seawater (Carlson et al. 1985; Benner et al. 1992b; Guo et al. 1994), and sediment pore water (Krom and Sholkovitz 1977; Chin and Gschwend 199 1). Although colloids play a vital role in the carbon cycle, trace metal scavenging, and biogeochemical processes in aquatic environments (Sigleo and Means 1990; Honeyman and Santschi 1992; Santschi et #al. 1995), the colloidal pool as a whole is poorly understood. Little is known about the molecular weight distribution of DOC due to the lack of reliable and unbiased methods for molecular weight determination (Benner 199 1; Guo 1995).

Most models of global carbon cycling use average prop- erties of the open ocean and ignore processes occurring at ocean margins. However, the role of the continental shelf and slope areas in the global carbon cycle may be of considerable importance (e.g. Christensen 1994). For example, Jahnke et al. (1990) and Walsh (199 1) have argued that continental margins are important areas in the global cycle of carbon and other biogenic elements. However, the dynamics of DOC in ocean margin waters and the fluxes of DOC to the deep ocean in these areas are not well documented. We chose as study areas the continental shelf, the continental slope, and open-ocean stations in the Gulf of Mexico (GOM) off Texas and in the Middle Atlantic Bight (MAB) off Cape Hatteras. We present horizontal and vertical distributions of DOC and COC separated by cross-flow ultrafiltration techniques and analyzed by the HTC method. In addition, vertical and lateral fluxes of DOC were estimated. Our data demonstrate a uniform pattern in the vertical distributions of DOC in the *water column and a consistent trend in the molecular weight distribution of DOC in the ocean.

1392

DOC and colloids in the ocean’ 1393

Temperature (“C) Temperature (“C)

Gulf of Mexico Slation 04

Temperature (“C) 5 10 15 20

ji A--= S.0 ; T

I ’ I’ 0’ 4’

I I

1 I I L+L.I 1 I ( I 36 37 38 Salinity

35 SalirEy

37

NO, 0.W

Salinity

NO, WV -5 0 5 10 i5 20 25 30 35

0 lIII(,llllllll~TITTtI

‘.o-<*------

800--

1200-

1600 2

Gulf of Mexico . . Station

\ \ ‘0

Gulf of Mexico Station 06

‘4

Gulf of Mexico

5 6 0, (ml liter”)

6 7 8 L 5 4 3 b

0, (ml liter”) 0, (ml liter-‘)

Fig. 1. Vertical profiles of temperature, salinity, nutrients, and dissolved oxygen concentrations from stations in the Gulf of Mexico (January 1993).

Study areas Table 1. Sampling locations and ancillary data from surface waters of the Gulf of Mexico and off Cape Hatteras in the Middle Atlantic Bight.

Water depth Temp. Salin-

Sta. Location (m) SPM* (“C) ityt

Gulf of Mexico (January 1993) GBO l$ 29”50’N, 94’42’W 4 - 0 GB09 29”20’N, 94”4O’W 8 3,200 12.1 23.0

01 28”45’N, 94’44’W 18 962 19.1 35.03 03 28”44’N, 94’43’W 105 75 22.4 35.68 04 27”30’N, 94’5O’W 380 70 23.4 35.91 05 26”40’N, 95”OO’W 1,650 117 23.9 36.41 06 27”14’N, 94’59’W 1,200 - 23.7 36.38

Middle Atlantic Bight (May 1993) 01 36”04’N, 74”43’W 850 104 15.9 31.58 02 36”04’N, 74”47’W 400 - 16.2 30.73 10 36”04’N, 74’5O’W 100 109 13.4 32.38 16 36”04’N, 75’2 1’W 25 118 17.8 30.48 11 36”30’N, 74’05’W 2,500 - 14.7 33.72 21 35”25’N, 74’47’W 1,100 83 24.9 36.33

* Concentration of suspended particulate matter in surface waters (gg liter - l).

t Surface water. $ GBOl water sample was taken from the Trinity River.

Gulf of Mexico - Seawater samples were collected along a transect at - 95.OO”W across the shelf and slope areas in the GOM during the January 1993 RV Gyre 93GOl cruise. Sampling locations are listed in Table 1, and ancillary data are shown in Fig. 1. The open-gulf station (station 05) contained mostly oligotrophic Caribbean waters because it was in a warm core ring (Gyre 93G0 1 Cruise Rep.). For example, the vertical distribution pattern (Fig. 1) of nutrients and other hydrographic parameters from this station are similar to those of open-ocean waters (Broecker and Peng 1982).

Middle Atlantic Bight-Seawater samples were collected off Cape Hatteras from four stations along a transect at - 36.04”N across the shelf and slope areas in the MAB during the May 1993 RV Gyre 93607 cruise. In addition, samples for a DOC vertical profile from one open-ocean station, station 11 (36”29’N, 74”05’W), were also collected. Cape Hatteras is a very dynamic area because the Gulf Stream diverges from the coast and mixes with Vir- ginia and cape coastal waters and the western boundary undercurrent (Schmitz et al. 1987). Vertical profiles of temperature, salinity, nutrients, and dissolved oxygen

1394 Guo et al.

Temperature (“C)

Salinity

16

36

? Cape Hatteras : Station 10

0, (ml liter-‘)

- l *- ‘@ :

loo-

**;t ‘Q-?xs I \

\ \

i b 1 I I

200-- ; 0 -- ,

I

I’ , I

,

300 ‘- ‘,- f’

,I is

- I’ Cape Hatteras : Station 02

,,,-; Lt2-g I ‘4; 1 1 ,i;L, I 1.; 3 3

Salinity

NO, (PM)

“* Cape Halteras ;

Station 02 d I

3 4 5 6 7 8 0, (ml liter-‘)

Fig. 2. As Fig. 1, but off Cape Hatteras in the Middle,

Temperature (“C)

I

I a,

0 Q

d

IS I l Cape Hatteras o

Station 01

NO, (FM)

0 2&&&21fwg 1, ;;,,, ?”

o* /g \

200-- .’ ‘8 I \

4:

\ \ . ‘0

400-- I __

. . \

l , 4

600-- 0, i ; NO, -- \ l

1 d \ ’

800-- , ’

_ Cape Hatteras i A

Station 01

0, (ml liter-‘)

Atlantic Bight (May 1993).

concentrations are shown in Fig 2. The variability in the vertical distribution of hydrographic data in the MAB along the transect at - 36.04”N demonstrates dynamic water mixing (Fig. 2), which may significantly affect the distribution of DOC in the water column.

Methods

Sampling- Water-column samples for DOC measurements were collected by Niskin bottles attached to a CTD Rosette. Immediately after collection, water samples were passed through precombusted (450°C) glass-fiber filters (Whatman GF/F). The filtered water was collected directly by an acid-cleaned, precombusted (5 50°C) glass bottle with a Teflon-lined cap. Sample filtration and handling were carried out in a portable clean bench (class- 100) and class- 100 clean room gloves were used during all stages of sample collection and handling to avoid con- tamination. All DOC samples were acidified to pH -2 with HPLC-grade H3P04 (- 3 drops of coned acid in - 20 ml of seawater) and frozen during storage.

For ultrafiltration experiments, surface water was pumped directly through a 0.2~pm Nuclepore prefilter (in- line setup) into an enclosed 200-liter reservoir for each ultrafiltration system (1-, 3-, and 1 0-kDa systems). Deep

water was transferred from Niskin bottles to a 200-liter polypropylene drum by a closed in-line tubing setup and then pumped through the Nuclepore prefilter cartridge into the reservoir for each ultrafiltration system.

Ultrafiltration -Three ultrafiltration systems were used to extract COC from seawater to investigate the molecular weight distributions of DOC: an Amicon DC-30 with nine 1-kDa cutoff cartridges (Amicon, SlONl); an Amicon DC 1 OL with either two 1 -kDa cutoff cartridges (Amicon, S 1 ONl) or a 3-kDa cutoff cartridge (Amicon, H 1 OP3); a home-made ultrafiltration system with a lo-kDa cutoff cartridge (Amicon, H 1 OP lo), which included a peristaltic pump and Teflon tubing. Ultrafiltration cartridges were thoroughly cleaned with laboratory detergent (Micro), NaOH, HC1, and other cleaning solutions before sampling as recommended by the manufacturer (Guo 1995). In addition, large volumes (>20 liters) of Nanopure water were used to rinse the cartridges several times between cleaning solutions, and then a small volume of prefiltered seawater was used to condition the cartridges before ultrafiltration. Cartridges were also cleaned between samples with NaOH and HCl solutions. All ultrafiltration cartridges were calibrated and checked for integrity before use with macromolecules of known molecular weight (e.g. vitamin B1:! for 1-kDa cartridges). Further details about

DOC and colloids in the ocean

DOC (PM) DOC CUM) DOC (PM)

Gulf of Mexico Station 06

40 45 50 55 60 65 70 75 80

400

600

800

1395

Fig. 3. DOC vertical profiles.

these calibration procedures are described elsewhere (Guo 1995). Since a 0.2-pm Nuclepore cartridge was chosen as the prefilter, COC is operationally defined here as the size fraction of 1 kDa-0.2 pm (COC), 3 kDa-0.2 pm (COQ, or 10 kDa-0.2 pm (CO&J. Thus, COC10 and COC3 are subfractions of COCl, and COCw3 and COC3-10 represent the 1-3-kDa and 3-lo-kDa fractions.

All ultrafiltration experiments were carried out on-board ship immediately after sample collection. For COCl, colloids were concentrated from up to 1,000 liters of seawater with the DC-30 system over lo-12 h and then reduced to 2-4 liters by the DC-10 system. COC10 and COC3 were extracted from 1200 liters of seawater over 6-8 h with the home-made ultrafiltration system or the DC- 10 system with 1 0-kDa or 3-kDa ultrafilters. Dupli- cate samples of prefhltered seawater, integral ultrafiltrate, and colloidal fractions were sampled for DOC analysis. Results of the DOC mass balance for our ultrafiltration systems were usually within 5% (Guo et al. 1994; Guo 1995), which are equal to or better than values reported in the literature (Carlson et al. 1985; Whitehouse et al. 1990; Benner et al. 19923). High concentration factors (usually 100-250) were used for the ultrafiltration of seawater. Colloids were then desalted by diafiltration, and subsequently freeze-dried for further characterization (Guo et al. in prep.; Santschi et al. 1995). The values reported here are those before diafiltration.

DOC measurements- Concentrations of DOC and COC were measured by the HTC method with a Shimadzu TOC-5000 analyzer with platinized alumina catalyst (Guo et al. 1994). The determination of the instrument and water blanks of DOC is critical for DOC measurements of seawater, especially for deep-water samples. A 10 PM total DOC blank (including instrument and water blanks) would result in a > 20% overestimate if it is assumed that deep-ocean water has a DOC concentration of -45 PM, and no blank corrections are made. On the other hand, DOC could be underestimated by subtracting the total DOC blank rather than only the instrument blank, since “carbon-free” water is not available at present. Thus, a distinction between the water blank and instrumental blank from the total DOC blank is necessary (Benner and Strom 1993; Guo et al. 1994). In addition, a low system blank and low carbon water are important prerequisites for obtaining accurate and precise DOC concentrations. Unfortunately, a total DOC blank > 10 PM is still com- mon for HTC methods (Sharp et al. 1995). We were able to reduce the total DOC blank, which includes the instrument and water blanks, to < 5-6 PM C and to produce low DOC (5 2 PM) Nanopure water (Guo 1995). There- fore, our instrument blank was of the order of 3-4 PM. This instrument blank was assessed every day because it varied with time and catalysts. A standard calibration curve was run before measuring samples. The DOC concentration was calculated from the slope of a calibration

1396 Guo et al.

DOC (pM)

40 50 60 70 80 90

2 lOOO-

a

- I)

-

s4 ; 1500- :

2000-- Station 11(36”29’N, 74”05’W)-- Middle Atlantic Bight :

(May 1993)

2500-- ’ ’ ’ 1h 1 ’ ’ ’ ’ 1 ’ ’ ’ ’ / ’ ’ ’ ’ 1 ’ ’ ’ ’ Fig. 4. A DOC profile from open-ocean station 11 in the

Atlantic.

curve by using a sample’s net peak area obtained by subtracting only the instrument blank from the sample’s total peak area (Guo et al. 1994). Our results of DOC measurements from stages 1 and 2 of the DOC intercompar- ison exercises confirmed these low blank values and agreed well with recommended values (Sharp pers. comm.).

Results and discussion

Distributions of DOC-The distributions of DOC in seawater from the GOM and off Cape Hatteras in the MAB are shown in Fig. 3. Another vertical DOC profile from an open Atlantic station (Sta. 11, 2,500-m water depth) is shown in Fig. 4.

DOC concentrations in the upper water column of the GOM ranged from - 70 to 80 PM. In the upper 100 m, DOC concentrations were quite uniform, which charac- terizes a winter mixed layer and is consistent with other hydrographic features shown in Fig. 1. In the deep waters, DOC concentrations were fairly constant at - 45 PM (Fig. 3), even when DOC concentrations in the upper water column were elevated and varied from station to station. Between surface and bottom waters, DOC concentrations exhibited a sharp gradient. If we consider surface, subsurface, and deep water as three layers distinguishable by our DOC data, the steep DOC gradients in the subsurface layer probably produce the real carbon flux to the deep

ocean. For example, DOC profiles from stations 4 and 6 in the GOM and stations 01 and 02 in the MAB show consistent and steep declines of DOC concentrations in the 100-500-m depth range (Fig. 3). Hence, the downward transport of DOC could be important in the cycling of carbon in the study areas (see discussion below).

In the upper water column off Cape Hatteras, the DOC distribution was much more variable and dynamic (Fig. 3). DOC pr,ofiles in the MAB matched well with the hydrographic features of this area, as shown in Fig. 2. For example, lower DOC concentrations and higher water temperatures were found in surface and bottom water samples at station 19, while a maximum of DOC and a minimum of water temperature existed at a depth of -40 m (Figs. 2 and 3). This relationship is clear evidence that water mixing regulates the distribution of DOC in the water column in this highly dynamic area even though biological plrocesses are responsible for the production and consumption of DOC, but on distinctly different time scales. In ad.dition, the difference in DOC concentrations between seasons is very small (Carlson et al. 1994). Thus, without othler reliable biological parameters, it is difficult to examine quantitatively the cycling of DOC related to biological processes. Concentrations of DOC in the upper water column off Cape Hatteras can vary depending on the water masses present and differences in rates of biological processes. However, the bottom-water DOC concentrations in the MAB all approach a low value (46-48 PM) similar to that at the GOM stations (Figs. 3 and 4). Therefore, DOC seems to have a similar vertical distribution pattern at open-ocean stations in both study areas.

At present, there are few reliable vertical DOC profiles in the ocean. Figure 4 demonstrates a typical DOC profile from an oplen-ocean station in the MAB. This profile shows some interesting features in the subsurface layer (Fig. 4). Be‘low the summer mixed layer, a steep DOC concentration gradient was followed by a decline in the subsurface layer as observed at stations 01 and 02 in the MAB. Interestingly, the minimum DOC concentration in the water column at a depth of 500 m (rather than at the oxygen minimum 150 m at Sta. 11) was followed by a consistent and subtle increase toward the bottom. With- out high-precision DOC data, such a feature could not have been observed.

Estimate c,fDOCfluxes- Large horizontal and vertical gradients in DOC concentrations have been reported for shelf and slope areas (e.g. Benner et al. 1992a; Guo et al. 1994). Moreover, recent studies have shown that the downward transport of DOC can be comparable to or greater than that of particulate organic carbon (POC) as measured by sediment traps (e.g. Copin-Montegut and Avril 1993; Carlson et al. 1994). Using DOC profiles from both study areas, we can estimate the vertical diffusive fluxes of DOC [FcPOc,] from

&x3c.) = -K, x A[DOC]/AZ.

K, is the vertical eddy diffusion coefficient, and A[DOC]/ AZ is the DOC concentration gradient. The calculated downward fluxes from the upper water column are listed

DOC and colloids in the ocean 1397

Table 2. DOC downward fluxes (mol C m-2 d-l) from the upper water column (UWC).

uwc Sta. (m) (m%I)

A[DOC]/AZ (mol m-4) Flux

Gulf of Mexico (January 1993) 05 O-250 5.08 0.72 x 1O-4

Middle Atlantic Bight (May 1993)

3.6 x 1O-4

01 O-100 0.855 2.1 x 10-d 1.7 x 1o-4 02 O-100 0.685 2.6 x 1O-4 1.8 X 1o-4 11 O-100 0.994 1.7 x 10-4 1.7 x 10-4

in Table 2. K, calculations are based on seawater density gradients (e.g. Gargett 1984), and K, values for each station are also listed in Table 2. K, values calculated for the MAB area are similar to those reported previously under summer stratified conditions (e.g. Kemp et al. 1994; Wallace 1994).

The estimated DOC diffusive flux was 3.6 x 1O-4 mol C mm2 d-l from the upper 250 m water column at station 05 in the GOM and 1.7-1.8 x 1O-4 mol C m-2 d-l from the upper 100-m water column at stations of the MAB (Table 2), assuming that the DOC concentration at the surface is constant over the time period of mixing in the upper water column. The DOC vertical flux estimate as- sumes lateral homogeneity and steady state, which might not always be the case. Thus, the DOC diffusive fluxes represent lower limits of the total flux because horizontal water mixing and other lateral transport processes have been ignored but could have been important (see below).

When these DOC fluxes were compared with the sediment trap POC fluxes of 8.1 x 1O-4 mol C m-2 d-l reported previously for the GOM (Walsh 1990) and of 40 x 1 OB4 mol C 111~~ d-l measured off Cape Hatteras during the same cruise at station 01 (Walsh and Santschi unpubl. results) or 54 x 1O-4 mol C m-2 d-l measured during SEEP-2 (Anderson et al. 1994), the downward flux of DOC corresponded to - 44% of the POC flux at station 05 in the GOM and only 3-5% for stations off Cape Hatteras. However, the strong lateral processes at station 01 significantly enhanced particulate fluxes (Walsh and Santschi unpubl. results). Therefore, lateral processes may also enhance fluxes of DOC to the deep ocean, as previously suggested by Walsh (1994). For example, with a net movement of the bottom nepheloid layer of l-2 cm s-l (Houghton et al. 1994) and a DOC concentration difference (- 10 PM) in the MAB bottom waters between shelf and slope stations, we can estimate a lateral advective DOC flux of 8- 17 mol C m-2 d- l from the shelf into the MAB or Gulf Stream. This lateral flux is much higher than the estimated vertical DOC flux as well as the POC flux measured at the MAB. Due to the sloping isopycnals in the western Atlantic Ocean caused by the Gulf Stream and other boundary currents, lateral transport could in- deed inject DOC deeper into the ocean interior.

Another way to estimate DOC fluxes is from seasonal changes in the DOC inventory of the upper water column, assuming these changes are produced by vertical mixing.

Table 3. Concentrations of colloidal organic carbon (COC) in seawater from the Gulf of Mexico. GBOl and GB-TR water samples were collected from the Trinity River during different sampling trips and thus had different DOC concentrations. High concentration factors (100-250) were used for ultrafiltration of seawater. For two estuarine samples, the concentration factor was N 20-40. Fractions 1, 3, and 10 correspond to COC in the size range of 1 kDa-0.2 pm, 3 kDa-0.2 vrn, and 10 kDa-0.2 pm.

Sta.

GBO 1 GB09

01 03 03 04 04 05 05 05 05 05

GB-TR GB06

01 03 04 05 05

Depth cm>

1

2 2

100 2

400 2

250 1,600

2 1,600

1 1 2 2 2 2

1,600

DOC Fraction cot (PM) &Da) (PM)

575 1 392 214 1 131

86 1 36 84 1 31 71 1 24 82 1 29 47 1 16 70 1 28 67 1 23 45 1 14 70 3 11.3 45 3 4.4

495 10 55 145 10 12 86 10 4.5 84 10 3.0 82 10 3.1 70 10 2.6 45 10 1.3

When we compared the DOC profile (January 1993) at station 05 in the GOM with that measured in June 1992 (Guo et al. 1994), we found a significant change (- 1 mol C m-2) in the DOC inventory of the upper water column (O-250 m). The change in the DOC inventory between June and January corresponds to a maximal DOC downward flux of -2 x 10e3 mol C m-2 d-l (or -0.7 mol C m-2 yr-l). Th is fl ux is similar to the value measured in the northwestern Sargasso Sea (Carlson et al. 1994). Thus, on the seasonal time scale mentioned above, a significant portion of the DOC pool in the upper water column could have been transported to and consumed in the deeper waters.

Distribution of colloidal organic carbon - Concentra- tions of DOC and different COC fractions (COC, , COC3, and COC& in seawater from the Gulf of Mexico and off Cape Hatteras are listed in Tables 3 and 4. Horizontal distributions of the different COC fractions are shown in Fig. 5.

.

Concentrations of the COC fractions (COCr and COC,,,) decreased from nearshore to offshore (Fig. 5). In the Gulf of Mexico, concentrations of COC, in the surface waters decreased from 36 PM at a nearshore station (Sta. 0 1; water depth, - 18 m) to 28 PM at an open station (Sta. 05; water depth, - 1,650 m). Similarly, surface-water COC10 concentrations varied from 4.5 PM at station 01 to 2.6 PM at station 05. For comparison purposes, two COC samples in Galveston Bay, one from the Trinity

1398 Guo et al.

-l+I~T+r Gulf of Mexico

-50 0 50 100 150 200 250 300 Distance from shore (km)

Cape Hatteras (MB)

._ 15

Distance from station 16 (km)

Fig. 5. Variations of COC concentrations from nearshore to offshore stations in the Gulf of Mexico and off Cape Hatteras.

River and the other from the mouth of the bay, were also measured. Galveston Bay is used as an analog of a freshwater source, although the Mississippi River is the most important freshwater end-member for the gulf. COC in the freshwater end-member was up to 392 PM for COC, and 5 5 PM for COClO, and then decreased to 13 1 PM for COC, and - 12 PM for COC10 at the mouth of the bay, where the salinity was - 23. The decrease of COC from freshwater to seawater implies that terrestrial inputs could be important sources of high-molecular-weight (HMW) colloidal organic matter (COM) to the ocean. This con- clusion is consistent with recent results showing a linear inverse relationship between A14C and 613C values of colloidal fractions in both study areas (Guo et al. in prep.; Santschi et al. 1995). In seawater off Cape Hatteras, surface-water COCl concentrations were similar, likely because of the physical complexity of waters and short salinity gradient in the MAB area. However, bottom-water COC concentrations in the MAB demonstrated the same tendencies found in the GOM, decreasing from nearshore to offshore (Fig. 5).

Data from both study areas show consistent features in both the.ir vertical and horizontal distributions for all COC fractions, (i.e. decrease from nearshore to offshore and from surface to deep waters) in both concentration and percentage of the total DOC (see below). This trend suggests that COM may be transported from nearshore to offshore, being produced in the upper water column and then consumed during its transport to the deep ocean. On average., concentrations of surface-water COCl at offshore stations (Tables 3 and 4) correspond to mass concentrations of organic colloids ranging from 672 to 864 yg liter-’ in the GOM and 930-960 pg liter-l in the MAB, which are much higher than the mass concentration of POC and SPM (suspended particulate matter) in these areas (Table 1).

MoleculaS~ weight distribution of DOC- The molecular weight distribution of DOC has been previously exam- ined (e.g. Carlson et al. 1985 and references therein; Ben- ner et al. 19’323; Guo et al. 1994). However, data showing detailed pictures of the molecular weight distribution of DOC in seawater remain limited. In our study, DOC was fractionated by three ultrafiltration cartridges with different molecular weight cutoffs (1, 3, and 10 kDa).

Vertical distributions of the different COC fractions are shown in Fig. 6. At station 05 in the GOM, concentrations

COC fractions expressed as a percentage of the total DOC (J& ==

of all three COC fractions decreased from surface to deep [COC]/[DOC]) are listed in Table 5. In the

GOM, surfzlce-water COC1 comprised - 35-42% of the

Table 4. As Table 3, but in seawater off Cape Hatteras in the Middle Atlantic Bight.

Sta.

01 01 01 02 02 10 16 21 01 10 16 01 01 10 16 21

Depth DOC Cm) 6-N

2 84 100 67 800 47

2 90 400 48

95 69 24 90

2 77 2 85 2 69

24 90 2 85

800 50 95 69 24 90

2 77

Fraction (kD4

1 1 1 1 1 1 1

: 3 3

10 10 10 10 10

cot (PM)

40 31 21 40 20 32 44 39 17 13 22

5 2.5 3.B4

10 6.9

water. (Fig. 6 and Table 3). Similarly, at the same station, concentrations of COC3 were 11 PM in the surface water and decreased to 4.4 PM in the deep water, while COC10 decreased from 2.6 PM to 1.3 PM. In seawater off Cape Hatteras, at station 01, the concentration of COCl decreased with depth (Fig. 6 and Table 4). The COCiO concentration was 5 IAM in surface water and 2.5 PM in water 800 m deep in the MAB. More detailed COC profiles at station 2 1 i:n the MAB demonstrate the same distribution pattern as was observed at station 05 in the gulf and station 01 in the MAB.


Table 5. Molecular weight distribution of DOC in seawater in the Gulf of Mexico and off Cape Hatteras in the MAB. DOC was fractionated by size using ultrafiltration cartridges with different molecular weight cutoffs (1,3, and 10 kDa). COC, , COC3, and COC,, designate the COC fractions in the size range of 1 kDa-0.2 pm, 3 kDa-0.2 pm, and 10 kDa-0.2 pm. CO& and COC,-,, are the COC fractions of molecular weight of l-3 and 3-l 0 kDa, and UOC, is the < 1 -kDa fraction.

Depth Percentage -

Sta. (m) cot, coc3 COClO coc,&oc,,, uoc,

Gulf of Mexico 01 2 42 - 5 - - 58 03 2 37 - 4 - - 63

100 34 - - - - 66 04 2 35 - 4 - - 65

400 34 - - - - 66 05 2 4-O 16 4 24 12 60

250 34 - - - - 66 1,600 31 10 3 21 7 69

Middle Atlantic Bight 01 2 48 20 6 28 14 52

100 47 - - - - 53 800 43 - 5 - - 57

02 2 44 - 10 - - 56 400 42 - - - - 58

10 95 46 - 5 - - 54 16 24 49 24 - 25 - 51 21 2 51 - 9 - - 49

1,050 49 - 8 - - 51

total DOC. Concentrations of surface-water COCi decreased from nearshore to offshore, but the percentage of COC, in the total DOC changed little between stations 0 1 and 05. For example, Fc,* of COCl first decreased from 42% at station 0 1 to 35% at station 04 and then increased again to -40% at station 05 in the gulf (Table 5). This result is consistent with our previous observations in the gulf during a different season (Guo et al. 1994) and similar to other observations reported in the northern COM

(Benner et al. 1992a). Relatively higher concentrations of DOC and SPM at station 05 than those at station 04 (Table 1) may explain the variation in the COC fractions along the transect. In addition, surface-water COClo comprised 4-5% of the total DOC pool in the gulf (Table 5). These numbers seem significantly different from those previously reported for COClo (8-12%, Guo et al. 1994). However, when total DOC concentrations, sampling seasons (winter vs. summer), and prefilter pore sizes (0.2 pm vs. 0.4 pm) are taken into account, this difference is plau- sible. For COC3, two samples from station 05 in the GOM were measured (Table 5). Surface-water COC3 represented - 16% of the total DOC and decreased to - 10% in deep water (Fig. 6).

The molecular weight distribution in both study areas is further depicted in Fig. 7. In the GOM, a major portion (> 60%) of the DOC pool is in the low-molecular-weight (LMW) fraction (< 1 kDa). In addition, surface water contained more HMW COC than deep water, indicating a COC source in the upper ocean. In seawater off Cape Hatteras, COC fractions showed similar trends, with concentrations and Fc,d decreasing from surface to deep waters (Tables 4 and 5). On average, -54% of the DOC pool was in the UOC, fraction in the MAB (Fig. 7).

In general, while HMW COClo comprised only a small portion of the DOC pool (3-7%), UOCI was the most important fraction of the total DOC (54-67%), and COCI-3 had a similar value (23-25%) in both study areas. Per- centages of all HMW COC fractions decreased from surface to deep waters. In contrast, the LMW UOCI fraction increased from surface to deep waters. The difference between surface and deep waters implies that HMW COC fractions may be preferentially consumed (or aggregated) during their transport to deep waters. Thus, HMW COC seems to have higher reactivities compared with the LMW UOC in seawater.

Mixing behavior of DOC and COC- In order to further investigate the mixing behavior of DOC, we plotted surface-water DOC concentrations as a function of salinity

COC (PM)

0 5 IO 15 20 25 30

600.

800.

COC (PM) COC (PM)

i MAB (May 1993) Station 0 1

I

: cot, 4 , i , I

,

A MAB (May 1993)

Station 2 1 L L L -1 I I 1 ) )ILII.

Fig. 6. Vertical distribution of COC concentrations in seawater from the Gulf of Mexico and off Cape Hatteras (MAB).

1400 Guo et al.

Surface water (GOM) Deep water (GOM) All data (MAB)

Fig. 7. Molecular weight distributions of DOC in seawater in the Gulf of Mexico (GOM) and off Cape Hatteras (MAB).

-r+r-+-+~-,-

Gulf of Mexico (Surfac& water)

Gulf of Mexico (Surface water) Gulf of Mexico (Surface water)

120

0

i__

cot,, (R=0.86) .

l l +

-%--%-2k8- I I I

3b 3b I 1

38

Salinity

>IOkD r-l (-7%)

for the GOM data set in Fig. 8A. DOC concentrations were closely correlated with salinity in the surface waters of the gulf, indicative of conservative mixing behavior. As was shown previously for this area (Guo et al. 1994), this quasi-conservative behavior of DOC implies that DOC distributions are largely controlled by the mixing of different water masses or that production and consumption rates of DOC are of similar magnitude in the upper water column. Similarly, an inverse relationship existed between COC concentration and salinity in surface waters of the GOM (Fig. 8B). The same relationship between COC and salinity was observed for both colloidal fractions (COC1 and CO&) in the MAB (Fig. 8C).

Conservative behavior of DOC has been reported for estuarine waters when salinity was used as a mixing index (e.g. Mantoura and Woodward 1983). However, when the correlation between DOC and salinity is used for the whole-water column at ocean margins, too many different hydrographic regions with different density layers are involved. Thus, (T, should be a more valid mixing index for examining the mixing behavior of DOC in the ocean.

For example, when all DOC data from all slope stations in both study areas, including those measured in June 1992 in the gulf (Guo et al. 1994), are plotted, the relationship between DOC and salinity is highly scattered in both the GOM and the MAB (Fig. 9A). In contrast, a significant correlation between DOC and c’t is found in these slope waters, with no difference between study areas or seasons (Fig. 9B). This correlation again illustrates that water mixing is the most important factor in controlling the distribution of DOC in these ocean margin areas, although biological processes could be important in surface waters where nutrients are largely depleted (Figs. 1, 2, and 9C). Thus, the overall conservative behavior of DOC is corkstent with the relatively old 14C ages in COCl observed in both study areas (Guo et al. in prep.; Santschi

t Fig. 8. Relationship between DOC or COC concentrations

and salinity in the Gulf of Mexico and off Cape Hatteras (MAB).


- Gulf of Mexico

80

60

s 200 - u - s

---L-J-4

0 600

33 34 55 36 3;7 Salinity

DOC (CIW -rtrrrrl

Cape Hatteras

+--I

B

Q

80

60- L

24 25 26 27 i8 Sigma-t

- 4-O 5’0 60 i0 ti0 90 100 110

DOC WV

Fig. 10. Relationship between DOC and COC concentrations in seawater from the Gulf of Mexico and off Cape Hatteras. Data points from the Gulf of Mexico include those from Guo et al. (1994).

in a predictable way, with DOC concentration as a master variable. For example, in seawater off Cape Hatteras, the slope of the correlation line is -0.5 for COCl, - 0.3 for CO& and -0.1 for COC10 (Fig. IOB). These slope values imply that - 50% of the variation in the total DOC pool is from the COC, fraction, - 30% from the COC3 fraction, and - 10% from the COC10 fraction, consistent with values observed in the MAB (Table 5). In comparison, the slope of the correlation line in the GOM is - 0.6 for COC, and -0.08 for COC10 (Fig. 10A). These slope values in- dicate that, on average, - 60% of the variation in the total DOC in the GOM is from the COC, fraction and -8% is from the COC& fraction. The relatively higher slope for the COC, fraction in the gulf could be because the data points from this region include more nearshore stations (Table 1).

3 IL- i 30 35

- ‘.

0 io NO, (PM)

Fig. 9. Correlations between DOC and salinity, Q~, and NO, in slope waters from the Gulf of Mexico (GOM) and the Middle Atlantic Bight (MAB). Data here also include those measured in June 1992 in the gulf (Guo et al. 1994). There is no evidence for any diffcrencc between the two study areas nor between summer and winter seasons in these correlations.

et al. 1995) and in the whole DOC pool in the Pacific (Williams and Druffel 1987).

Conclusions Relationship between COC and DOC-Plots of COC vs. DOC concentrations show a positive linear relationship for all COC fractions (COC, COC3, and COClO) in both study areas (Fig. 10). Thus, COC seems to partition

The observed profiles of DOC in the Gulf of Mexico and off Cape Hatteras in the Middle Atlantic Bight showed

1402 Guo et al.

a similar distribution pattern within three observable layers: surface water, characterized by a seasonal mixed layer, with high DOC concentrations; a subsurface layer, from 100-500-m depth, with a steep DOC gradient showing a consistent decline of DOC concentration; and deep water, with fairly low and constant DOC concentrations.

The consistent relationship between DOC and gt in both study areas is indicative of conservative mixing behavior of DOC in the ocean. Thus, water mixing and lateral transport processes are important factors in controlling the distribution of DOC in these slope areas, even though biological processes are responsible for the production and consumption of DOC.

The calculated downward fluxes of DOC from the upper water column represent a significant fraction of the POC flux and may play an important role in the carbon cycle of the ocean. Lateral advective DOC fluxes, however, could be orders of magnitude higher than POC fluxes, depending on hydrodynamic conditions.

Concentrations of COC fractions and their percentages of the total DOC concentrations decreased from nearshore to offshore and from surface to bottom waters. This decrease suggests that HMW COC fractions have higher reactivities compared with the bulk DOC pool in the ocean.

BIANCHI, T. S., C. LAMBERT, P. H. SANTSCHI, M. BASKARAN, AND L. Guo. 1995. Plant pigments as biomarkers of high- molecular-weight dissolved organic carbon. Limnol. Oceanc,gr. 40: 422-428.

BROECKER, ‘W. S., AND T.-H. PENG. 1982. Tracers in the sea. Eldigio.

CARLSON, C. A., H. W. DUCKLOW, AND A. F. MICHAELS. 1994. Annual flux of dissolved organic carbon from the euphotic zone in the northwestern Sargasso Sea. Nature 371: 405- 408.

CARLSON, D. J., M. L. BRANN, T. H. MAGUE, AND L. M. MAYER. 1985. Molecular weight distribution of dissolved organic materials in seawater determined by ultrafiltration: A re- examination. Mar. Chem. 16: 155-l 71.

CAUWET, G. 1994. HTCO method for dissolved organic carbon analysis in seawater: Influence of catalyst on blank estimation. Mar. Chem. 47: 55-64.

CHIN, Y.-P. AND P. M. GSCHWEND. 1991. The abundance, distribution, and configuration of porewater organic colloids in recent sediments. Geochim. Cosmochim. Acta 55: 1309-1317.

CHRISTENSEN, J. P. 1994. Carbon export from continental shelves, denitrification and atmospheric carbon dioxide. Cont. Shelf Res. 14: 547-576.

Results of DOC molecular weight distribution show that large fractions of the DOC pool are heterogeneous in terms of size or molecular weight. Thus, different fractions of the DOC pool may have varying reactivities and turnover times in the ocean. COC and DOC concentrations were linearly correlated in both study areas.

COPIN-MONYEGUT, G., AND B. AVRIL. 1993. I Vertical distribution and temporal variation of dissolved organic carbon in the north-western Mediterranean Sea. Deep-Sea Res. 40: 1963-l 972.

References

AMON, R. M. W., AND R. BENNER. 1994. Rapid cycling of high-molecular-weight dissolved organic matter in the ocean. Nature 369: 549-552.

ANDERSON, R. F., G. T. ROWE, P. F. KEMP, S. TRUMBORE, AND P. E. BISCAYE. 1994. Carbon budget for the mid-slope depocenter of the Middle Atlantic Bight. Deep-Sea Res. 41: 669-703.

BENNER, R. 199 1. Ultra-filtration for the concentration of bac- teria, viruses, and dissolved organic matter, p. 18 l-l 85. In D. C. Hurd and D. W. Spencer [eds.], Marine particles: Analysis and characterization. Am. Geophys. Union.

-, C. CHIN-LEO, W. GARDNER, B. EADIE, AND J. COTNER. 1992a. The fates and effects of riverine and shelf-derived DOM on Mississippi River plume/gulf shelf processes, p. 84-94. In Nutrient enhanced coastal ocean productivity. NECOP Workshop Proc. NOAA Coastal Ocean Program. TAMU-SG-92- 109.

-, AND J. I. HEDGES. 1993. A test of the accuracy of fresh water DOC measurements by high-temperature catalytic oxidation and UV-promoted persulfate oxidation. Mar. Chem. 41: 161-165.

GARGETT, A. E. 1984. Vertical eddy diffusivity in the ocean interior. J. Mar. Res. 42: 359-393.

Guo, L. 1995. Cycling of dissolved and colloidal organic carbon in oceanic environments: Evidence from carbon and thorium isotopes. Ph.D. thesis, Texas A&M Univ.

-, C. H. COLEMAN, JR., AND P. H. SANTSCHI. 1994. The distribution of colloidal and dissolved organic carbon in the Gulf of Mexico. Mar. Chem. 45: 105-l 19.

HEDGES, J. I. 1992. Global biogeochemical cycles: progress and problems. Mar. Chem. 39: 67-93.

HONEYMAN, B. D., AND P. H. SANTSCHI. 1992. The role of particles and colloids in the transport of radionuclides and trace metals in the oceans, p. 379-423. In J. Buffle and H. P. van Leeuwen [eds.], Environmental particles. IUPAC Environ. Anal. Chem. Ser. V. 1. Lewis.

HOUGHTON, Ii. W., C. N. FLAGG, AND L. J. PIETRAFESA. 1994. Shelf-slope water frontal structure, motion and eddy heat flux in the southern Middle Atlantic Bight. Deep-Sea Res. 41: 273-306.

JAHNKE, R. A.., C. E. REIMERS, AND D. B. CRAVEN. 1990. In- tensification of recycling of organic carbon at the sea floor near ocean margins. Nature 348: 50-54.

KEMP, P. F., P.ND OTHERS. 1994. Modeling vertical oxygen and carbon flux during stratified spring and summer conditions on the continental shelf, Middle Atlantic Bight, eastern U.S.A. Dleep-Sea Res. 41: 629-655.

KROM, M. D., AND E. R. SHOLKOVITZ. 1977. Nature and re- actions ol’dissolved organic matter in the interstitial waters of marine sediments. Geochim. Cosmochim. Acta 41: 1565- 1573.

-, J. D. PAKULSKA, M. MCCARTHY, J. I. HEDGES, AND P. MANTOURA, R. F. C., AND E. M. S. WOODWARD. 1983. Con- G. HATCHER. 1992b. Bulk chemical characteristics of dis- servative behavior of riverine dissolved organic carbon in solved organic matter in the ocean. Science 255: 1561- the Scvern estuary: Chemical and geochemical implica- 1564. tions. Gelochim. Cosmochim. Acta 47: 1293-1309.

-, AND M. STROM. 1993. A critical evaluation of the SANTSCHI, P. H., AND OTHERS. 1995. Isotopic evidence for analytical blank associated with DOC measurements by contemporary origin of high molecular weight organic mat- high-temperature catalytic oxidation. Mar. Chem. 41: 153- ter in oceanic environments. Geochim. Cosmochim. Acta 160. 59: 625-63 1.


SCHMITZ, W. Z., T. M. JOYCE, W. R. WRIGHT, AND N. G. HOGG. 1987. Physical oceanography, p. 27-54. In J. D. Milliman and W. R. Wright [eds.], The marine environment of the U.S. Atlantic continental slope and rise. Jones and Bartlett.

SHARP, J. H., AND OTHERS. 1995 Analyses of dissolved organic carbon in seawater: The JGOFS EQPAC methods comparison. Mar. Chem. 48: 91-108.

-, R. BENNER, C. A. CARLSON, R. Dow, AND S. E. FITZWATER. 199 3. Re-evaluation of high temperature combustion instruments for measurement of dissolved organic carbon in seawater. Limnol. Oceanogr. 39: 1774- 1782.

SIGLEO, A. C., AND J. C. MEANS. 1990. Organic and inorganic components in estuarine colloids: Implications for sorption and transport of pollutants. Rev. Environ. Contam. Tech- nol. 112: 123-147.

WALLACE, D. W. R. 1994. Anthropogenic chlorofluorome- thanes and seasonal mixing rates in the Middle Atlantic Bight. Deep-Sea Res. 41: 307-324.

WALSH, I. D. 1990. Project CATSTIX: Camera, transmissom-

eter, and sediment trap integration experiment. Ph.D. thesis, Texas A&M University: 96 p.

WALSH, J. J. 199 1. Importance of continental margins in the marine biogeochemical cycling of carbon and nitrogen. Na- ture 350: 53-55.

-. 1994. Particle export at Cape Hatteras. Deep-Sea Res. 41: 603-628.

WHITEHOUSE, B. G., R. W. MACDONALD, K. ISEKI, M. B. YUNKER, AND F. A. MCLAUGHLIN. 1989. Organic carbon and colloids in the Mackenzie River and Beaufort Sea. Mar. Chem. 26: 37 l-378.

-, P. A. YEATS, AND P. M. STRAIN. 1990. Cross-flow filtration of colloids from aquatic environments. Limnol. Oceanogr. 35: 1368-l 375.

WILLIAMS, P. M., AND E. R. M. DRUFFEL. 1987. Radiocarbon in dissolved organic matter in the central North Pacific Ocean. Nature 330: 246-248.

Submitted: 22 December 1994 Accepted: 27 June 1995

Revised: 10 July 1995

Dynamics of dissolved organic carbon (DOC) in oceanic environments

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