Dissolved organic carbon and nitrogen in the Western ... - CiteSeerX
Marine Chemistry 105 (2007) 140 – 150 www.elsevier.com/locate/marchem
Dissolved organic carbon and nitrogen in the Western Black Sea Hugh W. Ducklow a,⁎, Dennis A. Hansell b , Jessica A. Morgan a,1 b
a School of Marine Science, College of William and Mary, P.O. Box 1346, Gloucester Point, VA, 23062, USA Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker, Causeway, Miami, FL 33149, USA
Received 24 June 2006; received in revised form 27 October 2006; accepted 16 January 2007 Available online 3 February 2007
Abstract Dissolved organic carbon and nitrogen (DOC and DON) concentrations were measured in the Black Sea during May–June 2001. Sampling was conducted along a Shelf–Gyre transect, and was focused at the suboxic–anoxic interface at the deep stations; hypotheses were tested regarding trends in these variables across the transect and between sub-surface water layers. DOC and DON concentrations were higher (272 μM and 15 μM, respectively) on the Shelf compared to the Gyre (200 μM and 11 μM, respectively), as a result of terrigenous inputs and in situ net production. The bulk DOC:DON ratio was constant with distance and depth (approximately 15–19). DOM concentrations decreased with depth (average anoxic layer concentrations of 123 μM and 6.1 μM for DOC and DON, respectively), in contrast to earlier observations of increasing DOC concentration with depth. The deep Basin (2000 m) DOC concentrations were high compared to deep open ocean values (120 vs 45 μM). We suggest that the high deep DOC is a product of mixing of terrigenous (300 μM) and Aegean Sea (60 μM) DOC, with some in situ decomposition over the 600 year residence time for the deep water mass. High surface concentrations of DOC and DON and high DOC:DON ratios throughout the sampling region indicate the pervasive influence of remnant terrigenous DOM with some net production. The timescales for DOM Shelf–Basin exchange and decomposition could not be estimated due to a lack of geochemical tracer data. © 2007 Elsevier B.V. All rights reserved. Keywords: Black Sea; DOC; DON
1. Introduction Studies of the distribution and cycling of dissolved organic matter (DOM), consisting of dissolved organic carbon (DOC) and nitrogen (DON) underwent a renaissance in the 1990s following the introduction of high-temperature catalytic oxidation (HTCO) analytical methods (Sugimura and Suzuki, 1988). The method was controversial (Williams and Druffel, 1988) but eventu⁎ Corresponding author. E-mail address: [email protected] (H.W. Ducklow). 1 Present address: NOAA/NESDIS/STAR/SOCD E/RA31, SSMC1, Room 5309, 1335 East West, Hwy., Silver Spring, MD 20910-3226. 0304-4203/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.marchem.2007.01.015
ally became an established element of hydrographic and biogeochemical research in oceanography (Hansell and Carlson, 2002). To date, there have been few modern investigations of the standing stocks and vertical and horizontal distributions of DOC in the Black Sea (Tugrul, 1993; Polat and Tugrul, 1995; Becquevort et al., 2002). Only a single profile of DON has been reported for the Black Sea (Karl and Knauer, 1991). The unique biochemical nature of the Black Sea, the world's largest anoxic Basin, provides an interesting and challenging site for exploration of biogeochemical processes. From a biogeochemical perspective, the distinct redox systems that dominate inorganic nutrient cycling and remineralization of organic matter between the
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surface oxic layer, subsurface suboxic layer, and deep anoxic Basin result in complex interactions between physical and biological processes (Murray et al., 1995, 1999; Oguz et al., 2000). Organic matter cycling and remineralization in the Central Black Sea differs significantly from most other coastal seas and the open ocean due to the presence of a predominantly anoxic water column: bacterial uptake and remineralization is high in surface waters, but may be limited below the oxycline due to metabolic shifts from aerobic to anaerobic respiration modes. Thus, while export of organic matter from the surface is high, bacterial production in deeper waters is likely to be low compared to other deep marine environments (Morgan et al., 2006). In addition to organic matter utilization by bacteria in the surface and subsurface layers, chemosynthetic processes may be net sources, not sinks, or organic matter in the subsurface layers. Indeed, Brewer and Murray (1973) fit observed vertical profiles of C, N and P to a one-dimensional advection–diffusion model and demonstrated net consumption of DIC and inorganic N and P in the suboxic layer. This geochemical diagnosis of net chemosynthetic activity is consistent with many microbiological observations (Yilmaz et al., 2006) and with some early vertical profiles of dissolved organic carbon (DOC) showing increases with depth and subsurface maxima (Deuser, 1971; Torgunova, 1994). The deep maxima in DOC could be the result of in situ net production by
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chemosynthetic activity, or net accumulation facilitated by incomplete oxidation in anaerobic waters. However, these DOC profiles do not match widespread observations of DOC profiles in the world ocean, all of which show surface maxima and stable, low concentrations at depth (Carlson and Ducklow, 1995; Hansell and Carlson, 1998b). The Black Sea is fed by several large rivers originating in Europe and Asia, which carry substantial loads of organic matter, nutrients and anthropogenic contaminants (Murray, 1991; Mee et al., 2005) (Fig. 1). The primary riverine input to the Black Sea is the Danube river, which accounts for 64% of the riverine water discharge (6300 m3 s− 1) (Garnier et al., 2002). Other rivers may contribute large amounts of POM and DOM to the Northwest Shelf, which comprises about 10% of the surface area of the Black Sea and extends from the Ukrainian coast to about 250 km south and has a mean depth of about 24 m (Saliot et al., 2002). The exchange of water, nutrients and organic matter (including DOC) between the shelves and Central Basin in the Black Sea is complex and only starting to be understood (Stanev et al., 2002). The main in situ production mechanisms of dissolved organic matter (DOM) include exudation by phytoplankton, egestion by proto- and metazoan grazers, and viral lysis of phytoplankton and bacterial cells (Nagata, 2000). Dissolved organic matter that is readily utilized by bacteria (i.e., reactive or labile DOM) on time scales
Fig. 1. Map of the Black Sea. Major hydrographical features (Gyres and Main Rim current) are shown, as well as stations occupied during the R/V Knorr research cruises in May 2001 (1, shaded symbols) and June 2001 (2, closed symbols). Bold line indicates Shelf–Gyre transect.
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Table 1 Stations occupied during research cruises to the Black Sea aboard the R/V Knorr Station Northwest Shelf St.2–5, Shelf St.2–7, Shelf St.2–9, Shelf-break St.2–3, Rim Current
Date
Location
Depth
June 5, 2001 June 5, 2001 June 6, 2001
45° 54′N, 31° 07′E 45° 23′N, 31° 03′E 44° 32′N, 30° 58′E
31 m 45 m 105 m
June 4, 2001
44° 08′N, 30° 55′E
480 m
42° 30′N, 30° 46′E 42° 30′N, 30° 46′E
2200 m 2200 m
Southwestern Gyre St.1–6, Gyre May 26–28, 2001 St.2–2, Gyre June 2–3, 2001
data on dissolved organic nitrogen (DON) is required, but it has not previously been measured extensively in the Black Sea. In this paper, we present measurements of dissolved organic carbon and nitrogen in the Black Sea that were made during May–June 2001. Sampling was conducted along a Shelf–Gyre transect and was focused at the suboxic–anoxic interface at the deep stations. Our objectives are to: 1) present data on vertical and horizontal distributions; 2) estimate the proportion of semilabile DOC and 3) outline the processes controlling DOC and DON dynamics and distributions in the region. 2. Materials and methods
of minutes to days is considered to be newly produced, while DOM that is resistant to bacterial degradation (i.e., refractory DOM) may be millennia in age (Carlson et al., 1996). Labile DOM is characterized by low carbon-to-nitrogen (C:N) ratios relative to refractory DOM (Carlson, 2002). A third pool of DOM, termed semilabile, turns over on time scales of weeks to seasons and potentially constitutes an important source of bacterial nutrition and nutrient regeneration (Carlson, 2002). The potential for utilization or remineralization of organic growth substrates is determined primarily by the substrate C:N ratio: bacteria growing on low C:N ratio substrates will conserve carbon and remineralize nitrogen in the form of ammonium (Goldman et al., 1987). In order to better understand these processes,
Research cruises to the Black Sea were conducted in May and June 2001 aboard the R/V Knorr. The majority of sampling was in the Western Gyre (Stations 1–6 and 2–2) and along a transect from the Gyre northward to the Northwest Shelf (Stations 2–3, 2–9, 2–7, and 2–5) Fig. 1, Table 1). Vertical profiles were sampled using a CTD/Rosette equipped with 5- or 30-liter Niskin bottles. Seawater samples for DOC and DON concentrations were taken throughout the water column, with particular focus on the surface layers (mixed layer and sub-mixed euphotic layer), the oxycline, the suboxic layer, and the upper anoxic zone (Fig. 2). Layer depths were determined from hydrographic data (salinity, temperature, and density) and dissolved oxygen (DO) and hydrogen sulfide concentrations following (Tugrul et al., 1992).
Fig. 2. Vertical profiles of oxygen, hydrogen sulfide and nitrate at all stations sampled. Properties are plotted against density to regularize the depth distributions. Biogeochemical layers are characterized by the chemical distributions (see text). OL: oxycline; SL: suboxic; AL: anoxic layers.
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Fig. 3. Dissolved organic carbon (DOC) concentrations. ML: mixed; EL: euphotic; OL: oxycline; SL: suboxic; AL: anoxic. Dashed lines indicate minimum deep Basin concentration of 118 μM. A) Gyre, B) Shelf-break, C) Northwest Shelf.
DOC and DON concentrations were measured using a Shimadzu TOC-5000 HTC instrument (Hansell and Carlson, 2001). Seawater samples were prefiltered to remove the nominal N 0.7 μm particulate fraction using inline precombusted 47-mm GF/F filters attached directly to the Niskin bottles via silicone tubing, then frozen at − 80 °C for transfer to Miami after the cruises. Water samples for DOC were acidified with hydrochloric acid to pH 2 and sparged with CO2-free oxygen to remove inorganic carbon. One hundred microliter
subsamples were then manually injected into the combustion tube at 680 °C. The resulting CO2 was measured using a nondispersive infrared detector. DON was calculated as the difference between total dissolved nitrogen (TDN) and dissolved inorganic nitrogen (DIN). DIN (nitrate, nitrite and ammonium) concentrations were measured by auto-analyzer (S. Tugrul, IMS/METU/Turkey). As in the DOC method, 100 μl subsamples for TDN were manually injected into the combustion tube at 900 °C. The resulting nitric oxide was
Fig. 4. Dissolved organic nitrogen (DON) concentrations. ML: mixed; EL: euphotic; OL: oxycline; SL: suboxic; AL: anoxic. Dashed lines indicate minimum deep Basin concentration of 5.7 μM. A) Gyre, B) Shelf-break, C) Northwest Shelf.
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Table 2 Bulk DOC:DON ratios Station
Mixed layer
Euphotic layer
Oxycline layer
Suboxic layer
Upper anoxic zone
Northwest Shelf St.2–5 St.2–7 St.2–9 St.2–3
15.5 ± 1.10 (4) 18.7 ± 1.51 (2) 20.6 ± 0.68 (2) 21.3 ± 1.90 (4)
16.0 ± 0.27 (2) 18.6 ± 0.43 (2) n.d. 16.5 ± 0.75 (2)
19.1 ± 3.16 (3) 21.4 ± 4.22 (3)
15.7 ± 2.58 (5)
18.3 ± 4.65 (2)
Southwestern Gyre St.1–6 18.3 ± 0.84 (5)
18.1 ± 0.26 (3)
18.4 -
20.6 ± 1.86 (8)
20.6 ± 2.23 (3)
Standard deviations are shown in italics, sample sizes are shown in parentheses; dashes indicate n = 1. n.d.: no data.
then reacted with ozone with the resulting signal detected by a chemiluminescence instrument (Garside, 1985). Semilabile upper water column DOM was estimated by subtracting the mean deep water concentrations for DOC and DON (118 and 5.7 mM, respectively; see Figs. 3, 4) from upper water column concentrations. These are referred to as the excess DOC and DON concentrations or stocks. For example, the semilabile or excess DOC and DON at Station 1–6 are 200 − 118 = 82 and 10.9 − 5.7 = 5.2 μM, respectively; and the C:N ratio of the excess DOM is 82/5.2 = 15.9. Differences in variables between sub-surface water layers (oxycline, suboxic, and upper anoxic layers) in the Western Gyre (“water layer analysis”) were examined using pooled data from May and June 2001 (Stations 1–6 and 2–2) by a one-way analysis of variance test (ANOVA), with Tukey's multiple comparisons tests to determine differences between layers (Underwood, 1981; Zar, 1999). 3. Results The DO and hydrogen sulfide data define the boundaries of the oxycline (OL), suboxic (SL), and upper anoxic layers (AL) (Fig. 2). The upper boundary of the oxycline layer (14.25σt) ranged from 51 m the Western Gyre to 87 m at the Shelf-break. Sampled depths located between the base of the euphotic layer and density of 14.5σt were considered to be part of the oxycline layer. The upper boundary of the suboxic layer (15.4σt to 16.2σt) ranged from 74 m to 115 m, and the upper boundary of the anoxic layer (16.2σt) ranged from 117 to 171 m for those same stations. These layers were higher in the water column in the Gyre stations, and deepened shelfward. Surface dissolved organic carbon (DOC) and nitrogen (DON) concentrations were high (170 to 280 μM DOC and 8 to 16 μM DON, respectively) (Figs. 3, 4); and increased landward from the Gyre to Shelf-break
and Shelf regions. Mean mixed layer concentrations were highest on the Northwest Shelf (233 to 272 μM DOC and 14.5 to 15.0 μM DON, Stations 2–5 and 2–7) (Figs. 3C, 4C) and lowest at the Western Gyre (200 μM DOC and 10.9 μM DON, Station 1–6) (Figs. 3A, 4A). Along the Shelf–Gyre transect, mixed layer concentrations of both DOC and DON significantly increased landward (regressions against distance, p-DOC = 0.002, p-DON b 0.0001, R 2 -DOC = 50%, R 2 -DON = 62%). Shelf-break station surface concentrations were intermediate, ranging from 211 to 240 μM DOC and 10.5 to 12.4 μM DON (Figs. 3B, 4B). The DOC and DON concentrations decreased with increasing depth below the mixed layer for all stations (Figs. 3, 4). Below the euphotic layer DOC concentrations decreased through the deeper water layers to about 118 μM. DON concentrations also decreased with depth to about 5.7 μM, but with much greater variability compared to DOC; peaks of DON in the suboxic layer were observed at Station 2–3 (Fig. 4B). Deep values for DON are based on the difference between total dissolved nitrogen (TDN) and ammonium concentrations
Fig. 5. Bulk DOC:DON ratios. OL: oxycline; SL: suboxic; AL: anoxic layers.
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Table 3 DOC:DON ratios for excess DOM above deep concentrations Station
Mixed layer
Euphotic layer
Oxycline layer
Suboxic layer
Upper anoxiczone
Northwest Shelf St.2–5 St.2–7 St.2–9 St.2–3
11.3 ± 1.21 (4) 15.7 ± 2.10 (2) 17.2 ± 1.21 (2) 19.0 ± 2.93 (4)
10.7 ± 0.37 (2) 13.7 ± 0.58 (2) n.d. 11.1 ± 0.86 (2)
12.7 ± 6.04 (3) 17.5 ± 10.2 (3)
4.2 ± 2.18 (5)
4.6 ± 3.97 (2)
Southwestern Gyre St.1–6 13.3 ± 1.08 (5)
12.5 ± 0.37 (3)
9.9 -
10.3 ± 4.96 (8)
5.9 ± 3.85 (3)
The excess fraction is calculated as the difference between the total amount and the deep Basin value (117.5 μM DOC or 4.73 μM DON). Standard deviations are shown in italics, sample sizes are shown in parentheses; dashes indicate n = 1. n.d.: no data.
(nitrate and nitrite are absent), which are very high and in some cases exceeded TDN, causing low sensitivity among DON measurements. In the Western Gyre, DOC in the suboxic layer (SL) was significantly higher than in the anoxic layer (AL; one-way ANOVA, p = 0.47), while DON did not significantly vary among the OL, SL and AL (Fig. 4A). Because only one depth was sampled in the oxycline layer, differences between the oxycline and deeper layers could not be determined. Molar ratios of bulk (total) DOC to DON concentrations (DOC: DON) in surface waters were high (15.5 to 21.3) and not significantly different along the Shelf–Gyre transect (Table 2). Below the euphotic layer, bulk DOC:DON ratios were variable but no significant difference between water layers was detected (Table 2). No trends were observed in the vertical profiles of C:N ratios Fig. 5). The excess concentrations of DOC and DON in the surface layers represent the semilabile DOM stocks accumulated above the refractory DOM in the deep water (Carlson, 2002). In the Black Sea the deepwater values in the Western Gyre are 118 μM and 5.7 μM DOC and DON, respectively. In Figs. 3 and 4, the portion of the DOM profiles to the right of the dashed lines indicates the excess fraction. Ratios of excess DOC:DON (Table 3) were lower than those of the bulk DOM (Table 2) for all samples. In surface waters, ratios ranged from 13.3 to 19.0 and did not significantly different along the Shelf–Gyre transect (Table 3). Below the euphotic layer, semilabile DOC:DON ratios were variable (12.7 to 17.5) but no significant difference between water layers was detected (Table 3). 4. Discussion 4.1. Components of the DOM pools The composition of the marine DOM pool is complex, and potentially composed of dozens or hundreds of poorly-characterized compounds (Mopper and Kieber,
1991; Saliot et al., 2002). At a cruder level, several operationally-defined components can be identified: a biologically labile fraction that turns over in hours to days with concentrations of 10s–100s nM (Kirchman et al., 2001); a semilabile fraction turning over on seasonal to interannual timescales (Hansell and Carlson, 2001; Church et al., 2002) with concentrations 1– 10 μM; and a long-lived (centuries–millennia), refractory component, averaging about 45 μM in the deep ocean (Hansell and Carlson, 1998b). The semilabile fraction is estimated by incubating water samples and monitoring changes in DOM concentrations over ca 10 days (Hansell et al., 1995). Selifonova and Lukina (2001) estimated the annual average labile DOC concentration along the Russian coast of the Black Sea to be about 108 μM, comprising about 40% of the total DOC. This value must be an artifact of the incubation conditions. In the Danube mixing zone, Becquevort et al. (2002) estimated the semilabile fraction of the DOC to be 9% of the bulk DOC pool (ca. 25 μM). They also observed a seasonal (April to July) increase in DOC from 167–250 mM (i.e., 30% of the bulk pool), but this change is too large to be due solely to local production (Hansell and Carlson, 1998a). Characterization of the locally-produced semilabile fraction (sensu Hansell and Carlson, 2001) is further complicated in estuaries, nearshore regions and enclosed basins like the Black Sea by allochthonous contributions from land with a spectrum of reactivities (Amon and Benner, 1996; McCallister et al., 2004). Simply subtracting deepwater background concentrations overestimates the locally-produced semilabile component in systems influenced by terrigenous inputs. The excess DOM in the Black Sea is not strictly analogous to the semilabile fraction of oceanic DOM as observed at Bermuda, Hawaii (Hansell and Carlson, 2001) and elsewhere. Rather it is some mixture of remnant riverine DOM with high C:N, and locally-produced semilabile DOM of unknown C:N.
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In the Danube River mixing zone, Saliot et al. (2002) found low values of δ13C (− 25.7 to − 26.9‰) for the DOC pool, indicating a predominately terrestrial humic source (Williams and Gordon, 1970). The same authors also found non-conservative mixing of DOC over the Danube mixing regime, indicating DOC sources in both the upper estuary (5 to 10 psu) and at the Shelf–Basin mixing zone, which they attributed to enhanced autochthonous primary production. The C:N ratio of bulk DOM in the surface layers ranged from 15.5 to 18.7 (Table 2), similar to refractory marine DOM (∼15) and well within the range bracketed by semilabile marine DOM (∼ 7) and terrestrial humic DOM (N20) (Nagata, 2000; Carlson, 2002). Compared to the bulk pool, the excess DOC:DON ratios for the Northwest Shelf were lower (ranging from 10.7 to 19.0) (Table 3), suggesting some local production of N-enriched DOM. Both the bulk and excess DOM ratios did not vary significantly along the Shelf–Gyre transect. The excess ratios were more similar to refractory marine DOM (∼15) than semilabile DOM values, indicating that the surface DOM was primarily derived from N-poor terrestrial sources or regenerative processes, rather than recent phytoplankton exudation (Nagata, 2000). 4.2. DOM on the Northwest Shelf Shipboard research and satellite imagery suggest that the NW Shelf region is extremely productive (Yilmaz et al., 2006), with high phytoplankton standing stocks (1 to 4 mg Chl-a m− 3) (Barale et al., 2002; Oguz et al., 2002), high rates of photosynthesis (20 to 350 mg C m− 3 d− 1) (Becquevort et al., 2002) and bacterial production (15 to 60 mg C m− 3 d− 1) (Vinogradov et al., 1998; Sorokin, 2002). The Northwest Shelf also receives riverine and estuarine inputs of inorganic nutrients and organic matter supporting high production and adding to the organic enrichment (Aubrey et al., 1996; Kideys, 2002). Torgunova (1994) found very high surface DOC concentrations (N 750 μM) at the Danube river mouth compared to coastal regions (420 to 580 μM), while Becquevort et al. (2002) reported lower values for the Danube mouth (192 to 325 μM) and Shelf (183 to 300 μM), respectively. In our study, mixed layer DOC concentrations ranged from 226 to 284 μM, and DON concentrations ranged from 14.0 to 16.4 μM. These DOC values are much lower than the older published values, but fall well within the range of more recent research; this difference over time is probably due to problems with DOC analytical methods (Tugrul, 1993) and should not be interpreted as a significant decrease in
the concentration of DOC across the entire Northwest Shelf. It is difficult to specify the source of high DOM concentrations (N 200 μM observed in the Central Basin) over the NW Shelf, because several rivers and a complex circulation (Oguz et al., 1995) contribute to the observed distributions. Plots of DOC and DON on density and salinity (Fig. 6) indicate riverine sources of DOM at Stations 2–5 and 2–7. The Dneiper, Dneister and Danube Rivers are the most likely sources of high DOM concentrations, with the Danube being the largest contributor (Saliot et al., 2002). 4.3. DOM in the Central Gyre Deuser (1971) reported surface DOC concentrations of 208 μM in the Central Eastern Basin, comparable to measurements by Tugrul (1993) in the Eastern and Western Gyres (160 to 210 μM). Karl and Knauer (1991) found much lower DOC values (90 μM) in the Western Gyre; they also reported DON values of about 10 μM. Their single profile represents the only other known DON result prior to this report. Torgunova (1994) found much higher surface DOC concentrations in the Central Basin (580 to 670 μM). In our study, mean mixed layer dissolved organic matter concentrations in the Western Gyre were 200 μM and 10.9 μM for DOC and DON, respectively (Figs. 3, 4), lower than the Shelf values. The timescales for exchanges between the Shelf and Gyres are not well-known, partly because of the complex circulation, and because geochemical tracers are difficult to apply in this region. For example, Hansell et al. (2004) used Radium 228/226 ratios to estimate mean watermass ages and Shelf–Basin exchange and decomposition rates of riverine DOC in the Arctic Ocean. They obtained a half-life of 7 years for the mineralization of terrigenous DOC from 550 to 150 μM in the Shelf circulation. However Ra-226 concentrations in the Black Sea have declined by a factor of 5 over the past 3 decades (Falkner et al., 1991; O'Neill et al., 1992). These isotopic tracers are not in steady state, preventing their use for estimating horizontal exchanges. The shorter-lived isotopes Ra223 and 224 have offshore sources (possibly methane seeps), complicating their application in mixing analyses (W.S. Moore III, personal communication). However, given the size of the Basin, its vigorous circulation and mesoscale exchanges, and warmer temperatures, the half-life for riverine DOM is probably less than the 7 years estimated by Hansell et al. (2004) in the Arctic Ocean.
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Fig. 6. DOC and DON plotted against density and salinity, showing mixing and potential sources of elevated DOC. OL: oxycline; SL: suboxic; AL: anoxic layers. The layers are defined on density surfaces, and the salinities corresponding to those surfaces at Station 1–6 are shown here.
4.4. DOM in subsurface layers In this study, DOC concentrations decreased with depth, ranging from ∼ 150 μM in the oxycline to ∼ 120 μM in the anoxic zone (Fig. 3), much greater than the values reported by Karl and Knauer (1991; ∼ 60 μM). Previous researchers found a DOC minimum at the suboxic-anoxic interface and increases in DOC concentration with depth below 200 m: up to 500 μM in the Central Basin (Deuser, 1971) and up to 1500 μM in the Western Gyre (Torgunova, 1994). Conversely, Tugrul (1993) found no evidence of increased DOC concentration at depth in the Western and Eastern Gyres, with deep concentrations ranging from 100 to 120 μM, more similar to our observations. The present observations indicate high deep water values (about 120 μM) compared to ∼45 μM in the deep open ocean (Hansell and Carlson, 1998b) and the anoxic layers of the Arabian Sea (Hansell and Peltzer, 1998c). Black Sea water is a mixture of freshwater from several rivers and water inflowing at depth (∼ 75 m) through the Dardenelles and Bosphorus Strait from the Aegean Sea (Oguz et al., 1993, 1994, 1995). Fig. 7 shows a mixing diagram for these endmembers. For the
riverine source we use 300 μM (Becquevort et al., 2002). Dardenelles DOC and salinity values obtained from Polat and Tugrul (1996), were 64 μM and 39, respectively. Surface water in the Western Gyre (Stations 1–6, 2–3) lies above the mixing line, suggesting net in situ production of DOC or another unidentified source. DOC concentrations on the Shelf also lie above the mixing line. The mixing diagram also suggests that nonconservative processes influence the
Fig. 7. Mixing diagram showing endmember values for DOC and salinity in the Black Sea. The endmembers are the major water sources in the Basin: the Danube River and the Aegean Sea (entering via the Dardanelles and Bosphorus).
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deep (2000 m) water in the SW Gyre. About 50 μM of DOC is removed during the ageing of this watermass (relative to the value anticipated with conservative mixing). Lee et al. (2002) estimate the residence time for the deep water layer to be ∼ 600 years. They also estimated that the age of suboxic zone waters (80–120 m) was ∼ 5 years, suggesting that reduction of DOC from surface concentrations of 200 μM to ∼ 150 μM in the suboxic zone proceeds over that time interval (Fig. 7). Dissolved organic nitrogen concentrations decreased with depth, ranging from 8.3 μM in the oxycline to 5.7 μM in the anoxic zone (Fig. 4); these data agree well with values reported by Karl and Knauer (1991) ranging from 5 to 7 μM. They are no different from oceanic concentrations at the same depths (e.g., http://hahana. soest.hawaii.edu/hot/hot-dogs/interface.html). Coble et al. (1991) observed DOM and flavin fluorescence profiles that increased steadily with depth through the upper 200 m. These profiles suggested a locallyproduced, rather than terrestrial humic source. Mopper and Kieber (1991) found that the majority (52%) of the DOM was comprised of low molecular weight carboxylic acids (formate, acetate, and lactate), which may be the products of anaerobic fermentations and may serve as substrates for sulfate reducers and methanogens. Other important constituents included thiols, dissolved free amino acids, and dissolved sugars; the profiles of these compounds were highly variable throughout the upper water column. In our study, subsurface bulk DOC: DON ratios were similar (15.7–21.4; Table 2, Fig. 4) across water layers and indicative of refractory DOM. Excess DOC:DON ratios were more variable (4.2–12.7; Table 3) although no significant difference between water layers was detected. The very low ratios in the suboxic and anoxic layers probably reflect the analytical uncertainty in the deep DON values; nonetheless, it is likely that the subsurface excess DOM pool is relatively nitrogen-rich and may indicate in situ organic matter production (Coban-Yildiz et al., 2006). Approximately 95% of the excess DOC was consumed above the suboxic–anoxic interface (Fig. 3), suggesting that much of the semilabile fraction is efficiently utilized in the upper (oxygenated) water column. Organic matter consumption and production in the anoxic Basin appears to be low, probably due to the less efficient metabolic modes of the anaerobic bacteria. 5. Conclusions Dissolved organic carbon and nitrogen (DOC and DON) concentrations were higher on the NW Shelf, while the C:N ratio was high (15) and constant,
indicating the predominance of terrestrial organic matter inputs. Concentrations were high in the surface waters of the Central Gyre, possibly reflecting a remnant pool of terrigenous DOM. DOC and DON concentrations generally decreased with depth. Approximately 95% of the excess DOC was consumed above the suboxic– anoxic interface. DOC concentrations in deep water were much lower than previously reported values but were high compared to open ocean values. This DOC is possibly derived from mixing of terrigenous and Aegean Sea endmember sources, with some in situ decomposition. The timescales for decomposition of the terrigenous DOM input to the Central Basin are unknown due to a lack of geochemical tracer information. CFC-estimated ages the for suboxic and anoxic layers suggest surface concentrations are reduced by 25 μM in a few years, with a further 25 μM reduction during the residence time of the deep water in the Central Gyre (600 yr). Acknowledgements This work was supported by NSF Grants OCE9908092 and OCE-9616305 to H.W.D. We thank Temel Oguz for comments and discussion and Jim Murray and W. S. Moore III for advice on watermass ages. We thank Suleyman Tugrul (IMS-Turkey) for inorganic nitrogen data. References Amon, R.M.W., Benner, R., 1996. Bacterial utilization of different size classes of dissolved organic matter. Limnology and Oceanography 41, 41–51. Aubrey, D., Moncheva, S., Demirov, E., Diaconu, V., Dimitrov, A., 1996. Environmental changes in the western Black Sea related to anthropogenic and natural conditions. Journal of Marine Systems 7, 411–425. Barale, V., Cipollini, P., Davidov, A., Mellin, F., 2002. Water constituents in the north–western Black Sea from optical remote sensing and in situ data. Estuarine, Coastal and Shelf Science 54, 309–320. Becquevort, S., Bouvier, T., Lancelot, C., Cauwet, G., Deliat, G., Egorov, V.N., Popovichev, V.N., 2002. The seasonal modulation of organic matter utilization by bacteria in the Danube–Black Sea mixing zone. Estuarine, Coastal and Shelf Science 54, 337–354. Brewer, P.G., Murray, J.W., 1973. Carbon, nitrogen and phosphorus in the Black Sea. Deep-Sea Research 20, 803–818. Carlson, C.A., 2002. Production and removal processes. In: Hansell, D.A., Carlson, C.A. (Eds.), Biogeochemistry of Marine Dissolved Organic Matter. Elsevier Science, USA, New York, pp. 91–151. Carlson, C.A., Ducklow, H.W., 1995. Dissolved organic carbon in the upper ocean of the central equatorial Pacific Ocean, 1992: daily and finescale vertical variations. Deep-Sea Research II 42, 639–656. Carlson, C.A., Ducklow, H.W., Sleeter, T.D., 1996. Stocks and dynamics of bacterioplankton in the northwestern Sargasso Sea. Deep-Sea Research II 43, 491–506.
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Dissolved organic carbon and nitrogen in the Western Black Sea Hugh W. Ducklow a,⁎, Dennis A. Hansell b , Jessica A. Morgan a,1 b
a School of Marine Science, College of William and Mary, P.O. Box 1346, Gloucester Point, VA, 23062, USA Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker, Causeway, Miami, FL 33149, USA
Received 24 June 2006; received in revised form 27 October 2006; accepted 16 January 2007 Available online 3 February 2007
Abstract Dissolved organic carbon and nitrogen (DOC and DON) concentrations were measured in the Black Sea during May–June 2001. Sampling was conducted along a Shelf–Gyre transect, and was focused at the suboxic–anoxic interface at the deep stations; hypotheses were tested regarding trends in these variables across the transect and between sub-surface water layers. DOC and DON concentrations were higher (272 μM and 15 μM, respectively) on the Shelf compared to the Gyre (200 μM and 11 μM, respectively), as a result of terrigenous inputs and in situ net production. The bulk DOC:DON ratio was constant with distance and depth (approximately 15–19). DOM concentrations decreased with depth (average anoxic layer concentrations of 123 μM and 6.1 μM for DOC and DON, respectively), in contrast to earlier observations of increasing DOC concentration with depth. The deep Basin (2000 m) DOC concentrations were high compared to deep open ocean values (120 vs 45 μM). We suggest that the high deep DOC is a product of mixing of terrigenous (300 μM) and Aegean Sea (60 μM) DOC, with some in situ decomposition over the 600 year residence time for the deep water mass. High surface concentrations of DOC and DON and high DOC:DON ratios throughout the sampling region indicate the pervasive influence of remnant terrigenous DOM with some net production. The timescales for DOM Shelf–Basin exchange and decomposition could not be estimated due to a lack of geochemical tracer data. © 2007 Elsevier B.V. All rights reserved. Keywords: Black Sea; DOC; DON
1. Introduction Studies of the distribution and cycling of dissolved organic matter (DOM), consisting of dissolved organic carbon (DOC) and nitrogen (DON) underwent a renaissance in the 1990s following the introduction of high-temperature catalytic oxidation (HTCO) analytical methods (Sugimura and Suzuki, 1988). The method was controversial (Williams and Druffel, 1988) but eventu⁎ Corresponding author. E-mail address: [email protected] (H.W. Ducklow). 1 Present address: NOAA/NESDIS/STAR/SOCD E/RA31, SSMC1, Room 5309, 1335 East West, Hwy., Silver Spring, MD 20910-3226. 0304-4203/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.marchem.2007.01.015
ally became an established element of hydrographic and biogeochemical research in oceanography (Hansell and Carlson, 2002). To date, there have been few modern investigations of the standing stocks and vertical and horizontal distributions of DOC in the Black Sea (Tugrul, 1993; Polat and Tugrul, 1995; Becquevort et al., 2002). Only a single profile of DON has been reported for the Black Sea (Karl and Knauer, 1991). The unique biochemical nature of the Black Sea, the world's largest anoxic Basin, provides an interesting and challenging site for exploration of biogeochemical processes. From a biogeochemical perspective, the distinct redox systems that dominate inorganic nutrient cycling and remineralization of organic matter between the
H.W. Ducklow et al. / Marine Chemistry 105 (2007) 140–150
surface oxic layer, subsurface suboxic layer, and deep anoxic Basin result in complex interactions between physical and biological processes (Murray et al., 1995, 1999; Oguz et al., 2000). Organic matter cycling and remineralization in the Central Black Sea differs significantly from most other coastal seas and the open ocean due to the presence of a predominantly anoxic water column: bacterial uptake and remineralization is high in surface waters, but may be limited below the oxycline due to metabolic shifts from aerobic to anaerobic respiration modes. Thus, while export of organic matter from the surface is high, bacterial production in deeper waters is likely to be low compared to other deep marine environments (Morgan et al., 2006). In addition to organic matter utilization by bacteria in the surface and subsurface layers, chemosynthetic processes may be net sources, not sinks, or organic matter in the subsurface layers. Indeed, Brewer and Murray (1973) fit observed vertical profiles of C, N and P to a one-dimensional advection–diffusion model and demonstrated net consumption of DIC and inorganic N and P in the suboxic layer. This geochemical diagnosis of net chemosynthetic activity is consistent with many microbiological observations (Yilmaz et al., 2006) and with some early vertical profiles of dissolved organic carbon (DOC) showing increases with depth and subsurface maxima (Deuser, 1971; Torgunova, 1994). The deep maxima in DOC could be the result of in situ net production by
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chemosynthetic activity, or net accumulation facilitated by incomplete oxidation in anaerobic waters. However, these DOC profiles do not match widespread observations of DOC profiles in the world ocean, all of which show surface maxima and stable, low concentrations at depth (Carlson and Ducklow, 1995; Hansell and Carlson, 1998b). The Black Sea is fed by several large rivers originating in Europe and Asia, which carry substantial loads of organic matter, nutrients and anthropogenic contaminants (Murray, 1991; Mee et al., 2005) (Fig. 1). The primary riverine input to the Black Sea is the Danube river, which accounts for 64% of the riverine water discharge (6300 m3 s− 1) (Garnier et al., 2002). Other rivers may contribute large amounts of POM and DOM to the Northwest Shelf, which comprises about 10% of the surface area of the Black Sea and extends from the Ukrainian coast to about 250 km south and has a mean depth of about 24 m (Saliot et al., 2002). The exchange of water, nutrients and organic matter (including DOC) between the shelves and Central Basin in the Black Sea is complex and only starting to be understood (Stanev et al., 2002). The main in situ production mechanisms of dissolved organic matter (DOM) include exudation by phytoplankton, egestion by proto- and metazoan grazers, and viral lysis of phytoplankton and bacterial cells (Nagata, 2000). Dissolved organic matter that is readily utilized by bacteria (i.e., reactive or labile DOM) on time scales
Fig. 1. Map of the Black Sea. Major hydrographical features (Gyres and Main Rim current) are shown, as well as stations occupied during the R/V Knorr research cruises in May 2001 (1, shaded symbols) and June 2001 (2, closed symbols). Bold line indicates Shelf–Gyre transect.
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Table 1 Stations occupied during research cruises to the Black Sea aboard the R/V Knorr Station Northwest Shelf St.2–5, Shelf St.2–7, Shelf St.2–9, Shelf-break St.2–3, Rim Current
Date
Location
Depth
June 5, 2001 June 5, 2001 June 6, 2001
45° 54′N, 31° 07′E 45° 23′N, 31° 03′E 44° 32′N, 30° 58′E
31 m 45 m 105 m
June 4, 2001
44° 08′N, 30° 55′E
480 m
42° 30′N, 30° 46′E 42° 30′N, 30° 46′E
2200 m 2200 m
Southwestern Gyre St.1–6, Gyre May 26–28, 2001 St.2–2, Gyre June 2–3, 2001
data on dissolved organic nitrogen (DON) is required, but it has not previously been measured extensively in the Black Sea. In this paper, we present measurements of dissolved organic carbon and nitrogen in the Black Sea that were made during May–June 2001. Sampling was conducted along a Shelf–Gyre transect and was focused at the suboxic–anoxic interface at the deep stations. Our objectives are to: 1) present data on vertical and horizontal distributions; 2) estimate the proportion of semilabile DOC and 3) outline the processes controlling DOC and DON dynamics and distributions in the region. 2. Materials and methods
of minutes to days is considered to be newly produced, while DOM that is resistant to bacterial degradation (i.e., refractory DOM) may be millennia in age (Carlson et al., 1996). Labile DOM is characterized by low carbon-to-nitrogen (C:N) ratios relative to refractory DOM (Carlson, 2002). A third pool of DOM, termed semilabile, turns over on time scales of weeks to seasons and potentially constitutes an important source of bacterial nutrition and nutrient regeneration (Carlson, 2002). The potential for utilization or remineralization of organic growth substrates is determined primarily by the substrate C:N ratio: bacteria growing on low C:N ratio substrates will conserve carbon and remineralize nitrogen in the form of ammonium (Goldman et al., 1987). In order to better understand these processes,
Research cruises to the Black Sea were conducted in May and June 2001 aboard the R/V Knorr. The majority of sampling was in the Western Gyre (Stations 1–6 and 2–2) and along a transect from the Gyre northward to the Northwest Shelf (Stations 2–3, 2–9, 2–7, and 2–5) Fig. 1, Table 1). Vertical profiles were sampled using a CTD/Rosette equipped with 5- or 30-liter Niskin bottles. Seawater samples for DOC and DON concentrations were taken throughout the water column, with particular focus on the surface layers (mixed layer and sub-mixed euphotic layer), the oxycline, the suboxic layer, and the upper anoxic zone (Fig. 2). Layer depths were determined from hydrographic data (salinity, temperature, and density) and dissolved oxygen (DO) and hydrogen sulfide concentrations following (Tugrul et al., 1992).
Fig. 2. Vertical profiles of oxygen, hydrogen sulfide and nitrate at all stations sampled. Properties are plotted against density to regularize the depth distributions. Biogeochemical layers are characterized by the chemical distributions (see text). OL: oxycline; SL: suboxic; AL: anoxic layers.
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Fig. 3. Dissolved organic carbon (DOC) concentrations. ML: mixed; EL: euphotic; OL: oxycline; SL: suboxic; AL: anoxic. Dashed lines indicate minimum deep Basin concentration of 118 μM. A) Gyre, B) Shelf-break, C) Northwest Shelf.
DOC and DON concentrations were measured using a Shimadzu TOC-5000 HTC instrument (Hansell and Carlson, 2001). Seawater samples were prefiltered to remove the nominal N 0.7 μm particulate fraction using inline precombusted 47-mm GF/F filters attached directly to the Niskin bottles via silicone tubing, then frozen at − 80 °C for transfer to Miami after the cruises. Water samples for DOC were acidified with hydrochloric acid to pH 2 and sparged with CO2-free oxygen to remove inorganic carbon. One hundred microliter
subsamples were then manually injected into the combustion tube at 680 °C. The resulting CO2 was measured using a nondispersive infrared detector. DON was calculated as the difference between total dissolved nitrogen (TDN) and dissolved inorganic nitrogen (DIN). DIN (nitrate, nitrite and ammonium) concentrations were measured by auto-analyzer (S. Tugrul, IMS/METU/Turkey). As in the DOC method, 100 μl subsamples for TDN were manually injected into the combustion tube at 900 °C. The resulting nitric oxide was
Fig. 4. Dissolved organic nitrogen (DON) concentrations. ML: mixed; EL: euphotic; OL: oxycline; SL: suboxic; AL: anoxic. Dashed lines indicate minimum deep Basin concentration of 5.7 μM. A) Gyre, B) Shelf-break, C) Northwest Shelf.
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Table 2 Bulk DOC:DON ratios Station
Mixed layer
Euphotic layer
Oxycline layer
Suboxic layer
Upper anoxic zone
Northwest Shelf St.2–5 St.2–7 St.2–9 St.2–3
15.5 ± 1.10 (4) 18.7 ± 1.51 (2) 20.6 ± 0.68 (2) 21.3 ± 1.90 (4)
16.0 ± 0.27 (2) 18.6 ± 0.43 (2) n.d. 16.5 ± 0.75 (2)
19.1 ± 3.16 (3) 21.4 ± 4.22 (3)
15.7 ± 2.58 (5)
18.3 ± 4.65 (2)
Southwestern Gyre St.1–6 18.3 ± 0.84 (5)
18.1 ± 0.26 (3)
18.4 -
20.6 ± 1.86 (8)
20.6 ± 2.23 (3)
Standard deviations are shown in italics, sample sizes are shown in parentheses; dashes indicate n = 1. n.d.: no data.
then reacted with ozone with the resulting signal detected by a chemiluminescence instrument (Garside, 1985). Semilabile upper water column DOM was estimated by subtracting the mean deep water concentrations for DOC and DON (118 and 5.7 mM, respectively; see Figs. 3, 4) from upper water column concentrations. These are referred to as the excess DOC and DON concentrations or stocks. For example, the semilabile or excess DOC and DON at Station 1–6 are 200 − 118 = 82 and 10.9 − 5.7 = 5.2 μM, respectively; and the C:N ratio of the excess DOM is 82/5.2 = 15.9. Differences in variables between sub-surface water layers (oxycline, suboxic, and upper anoxic layers) in the Western Gyre (“water layer analysis”) were examined using pooled data from May and June 2001 (Stations 1–6 and 2–2) by a one-way analysis of variance test (ANOVA), with Tukey's multiple comparisons tests to determine differences between layers (Underwood, 1981; Zar, 1999). 3. Results The DO and hydrogen sulfide data define the boundaries of the oxycline (OL), suboxic (SL), and upper anoxic layers (AL) (Fig. 2). The upper boundary of the oxycline layer (14.25σt) ranged from 51 m the Western Gyre to 87 m at the Shelf-break. Sampled depths located between the base of the euphotic layer and density of 14.5σt were considered to be part of the oxycline layer. The upper boundary of the suboxic layer (15.4σt to 16.2σt) ranged from 74 m to 115 m, and the upper boundary of the anoxic layer (16.2σt) ranged from 117 to 171 m for those same stations. These layers were higher in the water column in the Gyre stations, and deepened shelfward. Surface dissolved organic carbon (DOC) and nitrogen (DON) concentrations were high (170 to 280 μM DOC and 8 to 16 μM DON, respectively) (Figs. 3, 4); and increased landward from the Gyre to Shelf-break
and Shelf regions. Mean mixed layer concentrations were highest on the Northwest Shelf (233 to 272 μM DOC and 14.5 to 15.0 μM DON, Stations 2–5 and 2–7) (Figs. 3C, 4C) and lowest at the Western Gyre (200 μM DOC and 10.9 μM DON, Station 1–6) (Figs. 3A, 4A). Along the Shelf–Gyre transect, mixed layer concentrations of both DOC and DON significantly increased landward (regressions against distance, p-DOC = 0.002, p-DON b 0.0001, R 2 -DOC = 50%, R 2 -DON = 62%). Shelf-break station surface concentrations were intermediate, ranging from 211 to 240 μM DOC and 10.5 to 12.4 μM DON (Figs. 3B, 4B). The DOC and DON concentrations decreased with increasing depth below the mixed layer for all stations (Figs. 3, 4). Below the euphotic layer DOC concentrations decreased through the deeper water layers to about 118 μM. DON concentrations also decreased with depth to about 5.7 μM, but with much greater variability compared to DOC; peaks of DON in the suboxic layer were observed at Station 2–3 (Fig. 4B). Deep values for DON are based on the difference between total dissolved nitrogen (TDN) and ammonium concentrations
Fig. 5. Bulk DOC:DON ratios. OL: oxycline; SL: suboxic; AL: anoxic layers.
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Table 3 DOC:DON ratios for excess DOM above deep concentrations Station
Mixed layer
Euphotic layer
Oxycline layer
Suboxic layer
Upper anoxiczone
Northwest Shelf St.2–5 St.2–7 St.2–9 St.2–3
11.3 ± 1.21 (4) 15.7 ± 2.10 (2) 17.2 ± 1.21 (2) 19.0 ± 2.93 (4)
10.7 ± 0.37 (2) 13.7 ± 0.58 (2) n.d. 11.1 ± 0.86 (2)
12.7 ± 6.04 (3) 17.5 ± 10.2 (3)
4.2 ± 2.18 (5)
4.6 ± 3.97 (2)
Southwestern Gyre St.1–6 13.3 ± 1.08 (5)
12.5 ± 0.37 (3)
9.9 -
10.3 ± 4.96 (8)
5.9 ± 3.85 (3)
The excess fraction is calculated as the difference between the total amount and the deep Basin value (117.5 μM DOC or 4.73 μM DON). Standard deviations are shown in italics, sample sizes are shown in parentheses; dashes indicate n = 1. n.d.: no data.
(nitrate and nitrite are absent), which are very high and in some cases exceeded TDN, causing low sensitivity among DON measurements. In the Western Gyre, DOC in the suboxic layer (SL) was significantly higher than in the anoxic layer (AL; one-way ANOVA, p = 0.47), while DON did not significantly vary among the OL, SL and AL (Fig. 4A). Because only one depth was sampled in the oxycline layer, differences between the oxycline and deeper layers could not be determined. Molar ratios of bulk (total) DOC to DON concentrations (DOC: DON) in surface waters were high (15.5 to 21.3) and not significantly different along the Shelf–Gyre transect (Table 2). Below the euphotic layer, bulk DOC:DON ratios were variable but no significant difference between water layers was detected (Table 2). No trends were observed in the vertical profiles of C:N ratios Fig. 5). The excess concentrations of DOC and DON in the surface layers represent the semilabile DOM stocks accumulated above the refractory DOM in the deep water (Carlson, 2002). In the Black Sea the deepwater values in the Western Gyre are 118 μM and 5.7 μM DOC and DON, respectively. In Figs. 3 and 4, the portion of the DOM profiles to the right of the dashed lines indicates the excess fraction. Ratios of excess DOC:DON (Table 3) were lower than those of the bulk DOM (Table 2) for all samples. In surface waters, ratios ranged from 13.3 to 19.0 and did not significantly different along the Shelf–Gyre transect (Table 3). Below the euphotic layer, semilabile DOC:DON ratios were variable (12.7 to 17.5) but no significant difference between water layers was detected (Table 3). 4. Discussion 4.1. Components of the DOM pools The composition of the marine DOM pool is complex, and potentially composed of dozens or hundreds of poorly-characterized compounds (Mopper and Kieber,
1991; Saliot et al., 2002). At a cruder level, several operationally-defined components can be identified: a biologically labile fraction that turns over in hours to days with concentrations of 10s–100s nM (Kirchman et al., 2001); a semilabile fraction turning over on seasonal to interannual timescales (Hansell and Carlson, 2001; Church et al., 2002) with concentrations 1– 10 μM; and a long-lived (centuries–millennia), refractory component, averaging about 45 μM in the deep ocean (Hansell and Carlson, 1998b). The semilabile fraction is estimated by incubating water samples and monitoring changes in DOM concentrations over ca 10 days (Hansell et al., 1995). Selifonova and Lukina (2001) estimated the annual average labile DOC concentration along the Russian coast of the Black Sea to be about 108 μM, comprising about 40% of the total DOC. This value must be an artifact of the incubation conditions. In the Danube mixing zone, Becquevort et al. (2002) estimated the semilabile fraction of the DOC to be 9% of the bulk DOC pool (ca. 25 μM). They also observed a seasonal (April to July) increase in DOC from 167–250 mM (i.e., 30% of the bulk pool), but this change is too large to be due solely to local production (Hansell and Carlson, 1998a). Characterization of the locally-produced semilabile fraction (sensu Hansell and Carlson, 2001) is further complicated in estuaries, nearshore regions and enclosed basins like the Black Sea by allochthonous contributions from land with a spectrum of reactivities (Amon and Benner, 1996; McCallister et al., 2004). Simply subtracting deepwater background concentrations overestimates the locally-produced semilabile component in systems influenced by terrigenous inputs. The excess DOM in the Black Sea is not strictly analogous to the semilabile fraction of oceanic DOM as observed at Bermuda, Hawaii (Hansell and Carlson, 2001) and elsewhere. Rather it is some mixture of remnant riverine DOM with high C:N, and locally-produced semilabile DOM of unknown C:N.
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In the Danube River mixing zone, Saliot et al. (2002) found low values of δ13C (− 25.7 to − 26.9‰) for the DOC pool, indicating a predominately terrestrial humic source (Williams and Gordon, 1970). The same authors also found non-conservative mixing of DOC over the Danube mixing regime, indicating DOC sources in both the upper estuary (5 to 10 psu) and at the Shelf–Basin mixing zone, which they attributed to enhanced autochthonous primary production. The C:N ratio of bulk DOM in the surface layers ranged from 15.5 to 18.7 (Table 2), similar to refractory marine DOM (∼15) and well within the range bracketed by semilabile marine DOM (∼ 7) and terrestrial humic DOM (N20) (Nagata, 2000; Carlson, 2002). Compared to the bulk pool, the excess DOC:DON ratios for the Northwest Shelf were lower (ranging from 10.7 to 19.0) (Table 3), suggesting some local production of N-enriched DOM. Both the bulk and excess DOM ratios did not vary significantly along the Shelf–Gyre transect. The excess ratios were more similar to refractory marine DOM (∼15) than semilabile DOM values, indicating that the surface DOM was primarily derived from N-poor terrestrial sources or regenerative processes, rather than recent phytoplankton exudation (Nagata, 2000). 4.2. DOM on the Northwest Shelf Shipboard research and satellite imagery suggest that the NW Shelf region is extremely productive (Yilmaz et al., 2006), with high phytoplankton standing stocks (1 to 4 mg Chl-a m− 3) (Barale et al., 2002; Oguz et al., 2002), high rates of photosynthesis (20 to 350 mg C m− 3 d− 1) (Becquevort et al., 2002) and bacterial production (15 to 60 mg C m− 3 d− 1) (Vinogradov et al., 1998; Sorokin, 2002). The Northwest Shelf also receives riverine and estuarine inputs of inorganic nutrients and organic matter supporting high production and adding to the organic enrichment (Aubrey et al., 1996; Kideys, 2002). Torgunova (1994) found very high surface DOC concentrations (N 750 μM) at the Danube river mouth compared to coastal regions (420 to 580 μM), while Becquevort et al. (2002) reported lower values for the Danube mouth (192 to 325 μM) and Shelf (183 to 300 μM), respectively. In our study, mixed layer DOC concentrations ranged from 226 to 284 μM, and DON concentrations ranged from 14.0 to 16.4 μM. These DOC values are much lower than the older published values, but fall well within the range of more recent research; this difference over time is probably due to problems with DOC analytical methods (Tugrul, 1993) and should not be interpreted as a significant decrease in
the concentration of DOC across the entire Northwest Shelf. It is difficult to specify the source of high DOM concentrations (N 200 μM observed in the Central Basin) over the NW Shelf, because several rivers and a complex circulation (Oguz et al., 1995) contribute to the observed distributions. Plots of DOC and DON on density and salinity (Fig. 6) indicate riverine sources of DOM at Stations 2–5 and 2–7. The Dneiper, Dneister and Danube Rivers are the most likely sources of high DOM concentrations, with the Danube being the largest contributor (Saliot et al., 2002). 4.3. DOM in the Central Gyre Deuser (1971) reported surface DOC concentrations of 208 μM in the Central Eastern Basin, comparable to measurements by Tugrul (1993) in the Eastern and Western Gyres (160 to 210 μM). Karl and Knauer (1991) found much lower DOC values (90 μM) in the Western Gyre; they also reported DON values of about 10 μM. Their single profile represents the only other known DON result prior to this report. Torgunova (1994) found much higher surface DOC concentrations in the Central Basin (580 to 670 μM). In our study, mean mixed layer dissolved organic matter concentrations in the Western Gyre were 200 μM and 10.9 μM for DOC and DON, respectively (Figs. 3, 4), lower than the Shelf values. The timescales for exchanges between the Shelf and Gyres are not well-known, partly because of the complex circulation, and because geochemical tracers are difficult to apply in this region. For example, Hansell et al. (2004) used Radium 228/226 ratios to estimate mean watermass ages and Shelf–Basin exchange and decomposition rates of riverine DOC in the Arctic Ocean. They obtained a half-life of 7 years for the mineralization of terrigenous DOC from 550 to 150 μM in the Shelf circulation. However Ra-226 concentrations in the Black Sea have declined by a factor of 5 over the past 3 decades (Falkner et al., 1991; O'Neill et al., 1992). These isotopic tracers are not in steady state, preventing their use for estimating horizontal exchanges. The shorter-lived isotopes Ra223 and 224 have offshore sources (possibly methane seeps), complicating their application in mixing analyses (W.S. Moore III, personal communication). However, given the size of the Basin, its vigorous circulation and mesoscale exchanges, and warmer temperatures, the half-life for riverine DOM is probably less than the 7 years estimated by Hansell et al. (2004) in the Arctic Ocean.
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Fig. 6. DOC and DON plotted against density and salinity, showing mixing and potential sources of elevated DOC. OL: oxycline; SL: suboxic; AL: anoxic layers. The layers are defined on density surfaces, and the salinities corresponding to those surfaces at Station 1–6 are shown here.
4.4. DOM in subsurface layers In this study, DOC concentrations decreased with depth, ranging from ∼ 150 μM in the oxycline to ∼ 120 μM in the anoxic zone (Fig. 3), much greater than the values reported by Karl and Knauer (1991; ∼ 60 μM). Previous researchers found a DOC minimum at the suboxic-anoxic interface and increases in DOC concentration with depth below 200 m: up to 500 μM in the Central Basin (Deuser, 1971) and up to 1500 μM in the Western Gyre (Torgunova, 1994). Conversely, Tugrul (1993) found no evidence of increased DOC concentration at depth in the Western and Eastern Gyres, with deep concentrations ranging from 100 to 120 μM, more similar to our observations. The present observations indicate high deep water values (about 120 μM) compared to ∼45 μM in the deep open ocean (Hansell and Carlson, 1998b) and the anoxic layers of the Arabian Sea (Hansell and Peltzer, 1998c). Black Sea water is a mixture of freshwater from several rivers and water inflowing at depth (∼ 75 m) through the Dardenelles and Bosphorus Strait from the Aegean Sea (Oguz et al., 1993, 1994, 1995). Fig. 7 shows a mixing diagram for these endmembers. For the
riverine source we use 300 μM (Becquevort et al., 2002). Dardenelles DOC and salinity values obtained from Polat and Tugrul (1996), were 64 μM and 39, respectively. Surface water in the Western Gyre (Stations 1–6, 2–3) lies above the mixing line, suggesting net in situ production of DOC or another unidentified source. DOC concentrations on the Shelf also lie above the mixing line. The mixing diagram also suggests that nonconservative processes influence the
Fig. 7. Mixing diagram showing endmember values for DOC and salinity in the Black Sea. The endmembers are the major water sources in the Basin: the Danube River and the Aegean Sea (entering via the Dardanelles and Bosphorus).
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deep (2000 m) water in the SW Gyre. About 50 μM of DOC is removed during the ageing of this watermass (relative to the value anticipated with conservative mixing). Lee et al. (2002) estimate the residence time for the deep water layer to be ∼ 600 years. They also estimated that the age of suboxic zone waters (80–120 m) was ∼ 5 years, suggesting that reduction of DOC from surface concentrations of 200 μM to ∼ 150 μM in the suboxic zone proceeds over that time interval (Fig. 7). Dissolved organic nitrogen concentrations decreased with depth, ranging from 8.3 μM in the oxycline to 5.7 μM in the anoxic zone (Fig. 4); these data agree well with values reported by Karl and Knauer (1991) ranging from 5 to 7 μM. They are no different from oceanic concentrations at the same depths (e.g., http://hahana. soest.hawaii.edu/hot/hot-dogs/interface.html). Coble et al. (1991) observed DOM and flavin fluorescence profiles that increased steadily with depth through the upper 200 m. These profiles suggested a locallyproduced, rather than terrestrial humic source. Mopper and Kieber (1991) found that the majority (52%) of the DOM was comprised of low molecular weight carboxylic acids (formate, acetate, and lactate), which may be the products of anaerobic fermentations and may serve as substrates for sulfate reducers and methanogens. Other important constituents included thiols, dissolved free amino acids, and dissolved sugars; the profiles of these compounds were highly variable throughout the upper water column. In our study, subsurface bulk DOC: DON ratios were similar (15.7–21.4; Table 2, Fig. 4) across water layers and indicative of refractory DOM. Excess DOC:DON ratios were more variable (4.2–12.7; Table 3) although no significant difference between water layers was detected. The very low ratios in the suboxic and anoxic layers probably reflect the analytical uncertainty in the deep DON values; nonetheless, it is likely that the subsurface excess DOM pool is relatively nitrogen-rich and may indicate in situ organic matter production (Coban-Yildiz et al., 2006). Approximately 95% of the excess DOC was consumed above the suboxic–anoxic interface (Fig. 3), suggesting that much of the semilabile fraction is efficiently utilized in the upper (oxygenated) water column. Organic matter consumption and production in the anoxic Basin appears to be low, probably due to the less efficient metabolic modes of the anaerobic bacteria. 5. Conclusions Dissolved organic carbon and nitrogen (DOC and DON) concentrations were higher on the NW Shelf, while the C:N ratio was high (15) and constant,
indicating the predominance of terrestrial organic matter inputs. Concentrations were high in the surface waters of the Central Gyre, possibly reflecting a remnant pool of terrigenous DOM. DOC and DON concentrations generally decreased with depth. Approximately 95% of the excess DOC was consumed above the suboxic– anoxic interface. DOC concentrations in deep water were much lower than previously reported values but were high compared to open ocean values. This DOC is possibly derived from mixing of terrigenous and Aegean Sea endmember sources, with some in situ decomposition. The timescales for decomposition of the terrigenous DOM input to the Central Basin are unknown due to a lack of geochemical tracer information. CFC-estimated ages the for suboxic and anoxic layers suggest surface concentrations are reduced by 25 μM in a few years, with a further 25 μM reduction during the residence time of the deep water in the Central Gyre (600 yr). Acknowledgements This work was supported by NSF Grants OCE9908092 and OCE-9616305 to H.W.D. We thank Temel Oguz for comments and discussion and Jim Murray and W. S. Moore III for advice on watermass ages. We thank Suleyman Tugrul (IMS-Turkey) for inorganic nitrogen data. References Amon, R.M.W., Benner, R., 1996. Bacterial utilization of different size classes of dissolved organic matter. Limnology and Oceanography 41, 41–51. Aubrey, D., Moncheva, S., Demirov, E., Diaconu, V., Dimitrov, A., 1996. Environmental changes in the western Black Sea related to anthropogenic and natural conditions. Journal of Marine Systems 7, 411–425. Barale, V., Cipollini, P., Davidov, A., Mellin, F., 2002. Water constituents in the north–western Black Sea from optical remote sensing and in situ data. Estuarine, Coastal and Shelf Science 54, 309–320. Becquevort, S., Bouvier, T., Lancelot, C., Cauwet, G., Deliat, G., Egorov, V.N., Popovichev, V.N., 2002. The seasonal modulation of organic matter utilization by bacteria in the Danube–Black Sea mixing zone. Estuarine, Coastal and Shelf Science 54, 337–354. Brewer, P.G., Murray, J.W., 1973. Carbon, nitrogen and phosphorus in the Black Sea. Deep-Sea Research 20, 803–818. Carlson, C.A., 2002. Production and removal processes. In: Hansell, D.A., Carlson, C.A. (Eds.), Biogeochemistry of Marine Dissolved Organic Matter. Elsevier Science, USA, New York, pp. 91–151. Carlson, C.A., Ducklow, H.W., 1995. Dissolved organic carbon in the upper ocean of the central equatorial Pacific Ocean, 1992: daily and finescale vertical variations. Deep-Sea Research II 42, 639–656. Carlson, C.A., Ducklow, H.W., Sleeter, T.D., 1996. Stocks and dynamics of bacterioplankton in the northwestern Sargasso Sea. Deep-Sea Research II 43, 491–506.
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