anoxic water column of the Black Sea

ARTICLE IN PRESS

Deep-Sea Research II 53 (2006) 1817–1841 www.elsevier.com/locate/dsr2

Processes controlling the redox budget for the oxic/anoxic water column of the Black Sea S.K. Konovalova,, J.W. Murrayb, G.W. Lutherc, B.M. Tebod a

Marine Hydrophysical Institute, NASU, Kapitanskaya 2, Sevastopol 99011, Ukraine School of Oceanography, University of Washington, Seattle, WA 98195-5351, USA c College of Marine Studies, University of Delaware, Lewes, DE 19958, USA d Marine Biology Research Division, Scripps Institution of Oceanography, University of California, San Diego, CA 92093,USA b

Received 1 November 2003; accepted 27 March 2006 Available online 1 September 2006

Abstract A one-dimensional isopycnal model has been constructed to simulate 16 major components of the Black Sea biogeochemical structure and to discuss processes controlling the redox budget for the oxic/anoxic water column of the Black Sea. The model includes parameterizations of physical exchange in the water column that takes account of vertical advection and diffusion and lateral exchange between the Black Sea and the Bosporus Plume. The model incorporates parameterizations for 25 biogeochemical processes, which we have found to be important to simulate the redox biogeochemical structure over a period of several decades. Parameterizations for biogeochemical processes follow the principles of formal chemical kinetics. Limiting functions are not applied. Neither physical nor biogeochemical processes are limited to depth or density layers of water, making the generated biogeochemical structure flexible. The redox budget and importance of individual processes for the budget of oxygen, sulfide, nitrate, ammonium, organic matter, manganese and iron are discussed in detail. In particular, we demonstrate that the biogeochemical structure of the oxic and suboxic layer strongly depends on export production and climate-induced variations in ventilation. The redox budget and the biogeochemical structure of the anoxic zone highly depend on the lateral exchange between the Black Sea and the Bosporus Plume, which appears to be the major reason for the existence of the suboxic zone. r 2006 Elsevier Ltd. All rights reserved.

1. Introduction During the last decade considerable progress has been made modeling carbon cycling in the euphotic zone (Oguz et al., 1996, 1998) and biogeochemical processes in the oxic/anoxic environment of the Corresponding author.

E-mail addresses: [email protected] (S.K. Konovalov), [email protected] (J.W. Murray), [email protected] (G.W. Luther), [email protected] (B.M. Tebo). 0967-0645/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2006.03.013

Black Sea (Belyaev et al., 1997; Lyubartseva and Lyubartsev, 1998; Yakushev, 1998; Yakushev and Neretin, 1997; Oguz et al., 1999, 2001). A diagnostic model of redox cycling in the Black Sea suboxic zone was presented by Oguz et al. (2001). Here, we present a numerical investigation of the redox budget and evolution of the biogeochemical structure throughout the aphotic oxic/anoxic water column of the Black Sea. The Black Sea is a land-locked marine basin with restricted seawater-exchange through the Bosporus

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and with heavy anthropogenic input. This is a classic 2-layer density stratified system where salinity increases from about 18 to 22 between the upper and the lower layers (Murray et al., 1991) (Fig. 1A). As a result, vertical exchange is restricted and a vertical sequence of oxic, suboxic and anoxic conditions occurs (Fig. 1B). The vertical stratification and depth of the main pycnocline (Fig. 1A) varies spatially, and the density scale is preferred to plot and analyze the biogeochemical structure (Murray et al., 1995). This marine system has become a reference site for studying anthropogenic impacts on oceanography. The reported changes in biological (Yunev et al., 2002; Vinogradov and Simonov, 1989) and biogeochemical structure (Vladimirov et al., 1997; Konovalov et al., 1999a, b; Konovalov and Murray, 2001) reflect dramatic degradation of the Black Sea ecosystem (Mee, 1992; Mee et al., 2005) and have resulted in economic losses of over $500 million per year (Black Sea INCOM Science Plan, 2000). This trend, if not reversed, might result in severe

consequences for people living at the coast, as there is only a 100-m thick oxic lid overlying the 2000 m thick sulfidic zone (Fig. 1B). This marine system serves as a natural laboratory (i) to investigate the influence of climate change and anthropogenic eutrophication on the oxic/anoxic balance, (ii) to understand natural processes supporting the oxic/anoxic balance over several millennia, and (iii) to provide knowledge that can be used in other regions, where anoxic conditions are or tend to be developed due to eutrophication and other human-related activities. Numerous national cruises and international expeditions (summarized by Konovalov and Murray (2001) and Ivanov et al. (1998), including the 1988, 2001 and 2003 KNORR expeditions, have provided fundamental observational data. This has made possible comprehensive analysis (Konovalov and Murray, 2001; Yunev et al., 2002) and modeling (Oguz et al., 1996, 1998; Stanev et al., 2001; Gregoire and Lacroix, 2001) of the Black Sea biogeochemical properties.

Fig. 1. Thermohaline (A) and oxic/anoxic (B) structure of the Black Sea water column. (CIL is the Cold Intermediate Layer with 8 1C for its upper and lower boundary. T–S data and data on oxygen and sulfide are from NATO TU-Black Sea basin-wide cruises in 1991–1994. The depth scale at panel (B) is changed at 300 m.)

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Papers by Oguz et al. (1996, 1998, 1999) and Gregoire and Lacroix (2001) serve as good examples of the Fasham et al. (1990) and Ducklow and Fasham (1992) type food-web models to successfully simulate nitrogen-dependent seasonal variations in the structure, distribution and activity of phytoplankton and zooplankton. In particular, Oguz et al. (1999) obtained a better understanding and explanation of the role of individual biological species and climatic forcing for primary and secondary biological processes through modeling. Gregoire and Lacroix (2001) investigated physical and biogeochemical mechanisms that lead to ventilation (oxygenation) of intermediate and deep anoxic waters. They addressed the impact of winter turbulent mixing, frontal instabilities, cascading along the continental slope of the shelf waters, remineralization of detritus, and processes of nitrification. Ecosystem food-web models have successfully allowed investigations of the influence of the general circulation and associated synoptic and meso-scale structures on the space–time distribution of primary and secondary production. However, models of nitrogen transformations below the euphotic zone have been oversimplified. Oxygen consumption has been parameterized as a nitrogen-dependent and nitrogen-equivalent process. In order to specifically avoid oxygen consumption in the absence of oxygen, a set of limiting functions (Gregoire and Lacroix, 2001; Yakushev, 1998) and restriction of individual processes to certain layers of water (Oguz et al., 1999) were applied to the numerical scheme. Similar procedures were applied to other biogeochemical properties. As a result, budgets and variations in individual biogeochemical properties were independent or weakly cross-linked below the euphotic zone, while the budget and structure of the euphotic layer was simulated realistically (Gregoire and Lacroix, 2001; Oguz et al., 1999). A different approach is required for ‘‘small’’ marine basins or oxic/anoxic basins, such as the Black Sea, where the inventory and annual inputs and outputs of nitrate are of the same order of magnitude (Konovalov et al., 2000). Here nitrogen cycling is not limited to detritus oxidation and physical upward flux, but also includes processes that are characteristic of sub-oxic and anoxic conditions. The goal of this work is to investigate (i) the redox budget of the Black Sea, (ii) the importance of individual processes, and (iii) possible changes in the biogeochemical structure due to variations in

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climate conditions and the level of eutrophication. We will elaborate on a one-dimensional (1-D) biogeochemical model for the Black Sea water column with simulation of horizontal ventilation (i) extending it to the entire aphotic water column of the Black Sea (from 50 m to the bottom), (ii) introducing in the model all important redox budget processes without artificial limiting functions, and (iii) incorporating the latest achievements in parameterization of physical vertical exchange in the Black Sea water column (Ivanov and Samodurov, 2001) and redox transformations (Konovalov et al., 2004; Kahler and Koeve, 2001).

2. Model The present model is based on the previously developed numerical model of physical exchange (Samodurov and Ivanov, 1998; Ivanov and Samodurov, 2001), extended to simulate the basic biogeochemical processes in the Black Sea water column. The vertical distribution of all properties was assumed to be isopycnal (Figs. 1 and 2). This assumption makes it possible to simulate the Black Sea in a 1-D isopycnal simulation. In addition, the Black Sea thermohaline structure is assumed to be at steady state on a long-time scale. Centuryaveraged vertical profiles of temperature and salinity have been utilized to derive the vertical profiles of vertical velocity and diffusivity (Konovalov et al., 2000; Ivanov and Samodurov, 2001), which are applied to calculate cross-isopycnal fluxes and exchange between water from the Bosporus of Mediterranean origin and the Black Sea (Eqs. (1) and (2)). These exchange processes are often referred in recent publications to explain the vertically distributed source of salt and heat (Ozsoy et al., 1995; Ivanov and Samodurov, 2001), the entrainment of water from the Cold Intermediate Layer (Murray et al., 1991; Buesseler et al., 1991), the Bosporus Plume (a mixture of the Mediterranean and Black Sea waters), and intrusions in the layer of the main pycnocline and anoxic zone (Konovalov et al., 2003; Glazer et al., 2006). This input from the Bosporus, modified by entrainment, is the mechanism generating the basin-average vertical advection, which reaches a maximum value of about 3 m yr
1 (Murray et al., 1991; Lee et al., 2002) to 7 m yr
1 (Ivanov and Samodurov, 2001) in the middle pycnocline at density values of st15.7–15.8.

ARTICLE IN PRESS S.K. Konovalov et al. / Deep-Sea Research II 53 (2006) 1817–1841

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The resulting balance equations include simulation of the result of ventilation by the Bosporus plume and can be written as qC þ wC, qz     q qC qC qw
k ðC b
CÞ, þw ¼Rþ qz qz qz qz

(1)

Flux ¼
k

(2)

where k is the vertical diffusion coefficient and
kðqC=qzÞ is the diffusive flux, w is the vertical velocity and, hence, wC is the advective flux occurring due to displacement of the Black Sea deep waters with the waters from the Bosporus plume, R is the rate of biogeochemical production– consumption, qw=qzðC b
CÞ is rate of changes in the biogeochemical structure due to the Bosporus plume, C b is concentration in the ‘‘Bosporus plume’’, and C is concentration of the same substance in the ambient water (Samodurov and Ivanov, 1998). The role of the Bosporus plume for the biogeochemical structure varies with depth. C b varies between the layers of entrainment of the Black Sea waters to the plume, and it remains constant in each individual layer of intrusions. qw=qz progressively decreases with depth in the layer of intrusions, making the plume-related flux of biogeochemical Ammonium (NH4+), µM

(A)

0

10

20

30

40

components and the rate of changes in the biogeochemical structure undetectable below 800–1000 m. The resulting vertical profiles of the rate of advection and turbulent diffusivity must satisfy two basic constraints: (i) they must simulate the observed thermohaline structure, and (ii) they must realistically simulate temporal variations in the distribution of conservative properties, which are not the subject of biogeochemical transformations. 2.1. Biogeochemical components and terms The 16 components of the biogeochemical structure of the water column we have included in this model are dissolved oxygen, one form of particulate and two forms of dissolved organic matter, nitrate, ammonium, di-nitrogen gas, elemental sulfur, sulfide, dissolved manganese (II), suspended manganese (IV) oxide, suspended manganese (II) carbonate, suspended manganese (II) sulfide, dissolved iron (II), suspended iron (III) oxide and suspended iron (II) sulfide. This list includes the components that appear to be the major biogeochemical elements for the overall redox budget (Fig. 2). Only redox end-members have been considered for parameterization, unless an intermediate Iron (Fe2+), nM

(B)

0

50

100

200

300

0

2

4

6

10

15.0

NO3Suboxic zone

So

Sigma-t

Sigma-t

8 O2

O2

16.0

500

Manganese (Mn2+), µM

Elemental Sulfur (S°), µM 0.0 0.1 0.2

15.0

400

N2 Suboxic zone

16.0

NH4+ 17.0

0 100 200 300 Oxygen (O2) & Sulfide (H2S), µM 0.0

2.0

4.0

Fe2+

17.0

H2S

6.0

8.0

Nitrate (NO3-), µM

10.0

0

0

Mn2+

100 200 Oxygen (O2), µM 10

20

300

30

Nitrogen Excess (N2), µM

Fig. 2. Redox biogeochemical structure of the Black Sea water column (oxygen, nitrate, sulfide data are from NATO TU-Black Sea basinwide cruises in 1991–1994). Ammonium data are from the 1988 KNORR expedition. Manganese data are from the 1988 and 2001 KNORR expeditions. Iron data are from the 1988 KNORR expedition and MHI cruises in 1991–1994. Data on elemental sulfur are from the 2001 KNORR expedition. Data on di-nitrogen gas are from Murray et al. (2003a, b).

ARTICLE IN PRESS S.K. Konovalov et al. / Deep-Sea Research II 53 (2006) 1817–1841

product appears to be important for the redox budget and structure of a layer. Interaction between these components has been simulated using 25 biogeochemical processes listed in Table 1. Essentially all of these are biologically mediated/catalyzed and utilized as a source of energy or elements. Sulfide or iron (II), for example, can be abiotically oxidized by oxygen and these processes are abiotically catalyzed or auto-catalyzed, but these processes are also widely utilized by bacteria as a source of energy in aquatic systems. Biogeochemical processes are usually the result of a number of elementary reactions. Thus, a sequence of at least seven elementary reactions describe the process of iron (II) oxidation (Table 1, process 13) (Stumm and Morgan, 1996) and sulfide is oxidized to sulfate through an extended sequence of inter-

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mediate products, such as thiosulfate, sulfite, polysulfide, elemental sulfur, etc. Every elementary reaction is most likely a binary first-order process with respect to each individual reagent, but kinetic data are usually not available or make the model overly complicated for practical utilization. The experimental rate law expresses only the rate of an overall process and it is as simple as possible to realistically simulate the rate of the overall process, which are widely applied in numerical models. The order for reagents does not have to follow the stoichiometry (Hausecroft and Constable, 1997) and it often becomes a fraction. Variations in the order for reagents allow application of the principles of formal chemical kinetics to typical abiotic process of dissolution (Table 1, process 23), and to typical microbial process, such as of respiration of

Table 1 Biogeochemical processes and their specific rate constants No.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Biogeochemical processes

Specific rate constants (Ki)

POM oxidation by oxygen C106(NH4)16(PO4)+106O2 ¼ 106CO2+16(NH4)+(PO4) DOM(l) (DOC:DON ¼ 23.5) oxidation by oxygen C23.5a(NH4)a(PO4)b+23.5aO2 ¼ 23.5aCO2+a(NH4)+b(PO4) POM oxidation by nitrate + ¼ 530CO2+276N2+256H2O+5(PO4) 5C106(NH4)16(PO4)+472NO
3 +392H 2
POM respiration to sulfide C106(NH4)16(PO4)+53SO2
4 ¼ 106CO2+53S +16(NH4)+(PO4) DOM(l) (DOC:DON ¼ 23.5) respiration to sulfide 2
2C23.5a(NH4)a(PO4)b+23.5aSO2
4 ¼ 47aCO2+23.5aS +2a(NH4)+2b(PO4) POM transformation to DOM(r) (DOC:DON ¼ 5) aC106(NH4)16(PO4)+26aO2 ¼ 26aCO2+16C5a(NH4)a(PO4)b+(a
16b)(PO4) DOM(l) (DOC:DON ¼ 23.5) oxidation to POM 16C23.5a(NH4)a(PO4)b+270aO2 ¼ 270aCO2+aC106(NH4)16(PO4)+(16b
a)(PO4) DOM(l) (DOC:DON ¼ 23.5) ammonification to POM 106C23.5a(NH4)a(PO4)b+270a(NH4) ¼ 23.5aC106(NH4)16(PO4)+(106b
23.5a)(PO4) +
Nitrification NH+ 4 +2O2 ¼ NO3 +H2O+2H + Manganese oxidation by nitrate 5Mn2++2NO
3 +4H2O ¼ 5MnO2+N2+8H Manganese oxidation by oxygen 2Mn2++O2+2H2O ¼ 2MnO2+4H+ + Iron oxidation by nitrate 10Fe2++2NO
¼ 10Fe3++N2+6H2O 3 +12H Iron oxidation by oxygen 4Fe2++O2+2H2O ¼ 4Fe3++4OH
Sulfide oxidation by oxygen S2
+2O2 ¼ SO2
4 2

Sulfide oxidation by nitrate 5S2
+8NO
3 +4H2O ¼ 5SO4 +4N2+8OH Sulfide oxidation by suspended manganese (IV) 0
MnO(OH)2+S2
+CO2
3 +H2O ¼ MnCO3+S +4OH Sulfide oxidation by suspended iron (III) S2
+2Fe3+ ¼ S0+2Fe2+ +
De-nitrification 5NH+ 4 +3NO3 ¼ 4N2+2H +9H2O Ammonium denitrification/oxidation by suspended manganese (IV) + 2NH+ ¼ N2+3Mn2++6H2O 4 +3MnO2+4H 3+ Ammonium denitrification/oxidation by suspended iron (III) 2NH+ ¼ N2+6Fe2++8H+ 4 +6Fe 2+ Cross-oxidation of iron (II) by manganese (IV) MnO(OH)2+2Fe +H2O ¼ Mn2++2FeOOH+2H+ + Elemental sulfur oxidation 2S0+3O2+2H2O ¼ 2SO2
4 +4H 2+ Dissolution of sinking manganese carbonate MnCO3 ¼ Mn +CO2
3 Generation of manganese sulfide MnCO3+2HS
¼ MnS2+CO2
3 +H2 Generation of iron sulfide Fe2++2HS
¼ FeS2+H2

7.5  10
3 5.0  10
8 1.0  10
5 6.35  10
4 5.0  10
6 2  10
3 1.2  10
5 2.75  10
6 6.5  10
3 3.0 2.5 5.0 4.0 2.5  10
2 0.5 2.7 5.0  10
2 2.0 5.0  10
2 5.0  10
2 0.74 0.03 1.7 5.0 1.35  10
3

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organic matter (Table 1, processes 1, 3, 4, etc.), which are usually parameterized by Michaelis– Menten kinetics. Thus, the parameterizations we use here (Table 2) do not prove that ‘‘chemical’’ parameterizations are better than ‘‘microbiological’’, but they do allow us to formally fit the observed distributions and to simplify calculations. It is extremely important to emphasize that the rate laws are often oversimplified. Thus, q½H2 S ¼ K  ½H2 Sx  ½O2 y , qt where K is a specific rate constant that does not depend on the concentration of sulfide ½H2 S or oxygen ½O2 , is often reduced to q½H2 S ¼ K 1  ½H2 S, qt where K1 is a pseudo-first-order rate constant, and used in this form to simulate sulfide oxidation in oxic/anoxic marine systems. This reduced equation describes kinetics of sulfide oxidation correctly if only the oxygen concentration is high and remains constant. That means that K 1 ¼ K  ½O2   const: and the rate of sulfide oxidation does not formally depend on the concentration and even the presence of oxygen. This assumption does not fit the real conditions in the suboxic/anoxic transition layer of the Black Sea. A similar situation occurs when remineralization of sinking POM and production of ammonium and nitrate are parameterized by a first-order equation. This assumes that the processes of POM and ammonium oxidation do not depend on the ambient concentrations of oxygen and require a limiting function that artificially changes the rate of processes when the concentration of oxygen drops to suboxic levels (Yakushev, 1998; Gregoire and Lacroix, 2001). In fact, limiting functions are rarely needed, if a complete equation   q½NHþ y þ x 4 ¼ K NHþ4  ½NH4   ½O2  qt or   q½POM x y ¼ K POM  ½POM  ½O2  qt is used. The rate of oxidation varies proportionally to the concentration of oxygen, and so equals zero when oxygen is absent. In fact, the sulfide concentration, for example, never equals zero, but remains

at a sub-nanomolar level even in the exclusively oxic marine systems (Luther et al., 1991). Sulfide, as an intermediate product of transformation of organic matter, exists in all marine environments, and oxygen controls its equilibrium concentration. Similarly, almost all redox processes are parameterized in this work by rate-law equations that include the concentration of both reductants and oxidants (Table 2). However, we have simplified equations for the rate of POM and DOM respiration to sulfide (processes 4 and 5, Table 1) because the concentration of sulfate never limits the rate of sulfide production in the Black Sea water column. The specific rate constants of the specific processes are listed as Ki (Table 1). These coefficients were initially derived from published values and/or estimated from redox capabilities of individual reagents. Thus, for example, the specific rate constant (K) of particulate organic matter oxidation in oxygenated waters of 0.05–0.15 d
1 (after Yakushev, 1998), is actually equal to K ¼ K 1  ½O2 y . The final set of specific rate constants (Table 1) has been adjusted in several runs of the model to keep the numerically simulated biogeochemical structure as close as possible to the observed values (Figs. 5 and 6). In total, 25 specific rate constants and several sinking rates drive the model (Tables 1 and 2). The procedure of optimization of these values did not take much effort, as there is a limited range over which to vary the parameters. All budgets of individual components are bounded and crossbounded through the stoichiometry of the processes (Tables 1 and 2) and the resulting rate laws. Every attempt to adjust the profile for an individual component results in equivalent changes in profiles of other components. If an individual specific rate constant is chosen without consideration of other components, the total biogeochemical structure becomes unrealistic in a short run of the model. The result of these parameterizations suggests that the overall scheme of processes is more important than specific values of the rate constants. Thus, for example, data published by Oguz et al. (2001) and Stanev et al. (2001) demonstrate that consideration solely of oxygen interactions with sulfide always results in a layer of co-existence of these components, rather than the observed suboxic zone. Attempts to adjust the specific rate constant of this process would be unsuccessful in simulation of the suboxic zone, because important processes are omitted.

5

4

3

2

Particulate organic matter (POM)

1

q½NO3  472 ¼
 K 3  ½POM  ½NO3  þ 1  K 9  ½NH4   ½O2  qt 80
1  K 10  ½MnðIIÞ  ½NO3 0:4  ½MnðIVÞ
1  K 12  ½FeðIIÞ  ½NO3  3
1  K 15  ½S2
  ½NO3 
 K 18  ½NH4   ½NO3   ½MnðIVÞ 5

Nitrate (NO3)


1  K 20  ½FeðIIIÞ  ½NH4 

q½NH4  ¼ 1  K 1  ½POM  ½O2 0:2 þ 1  K 2  ½DOMðlÞ  ½O2 0:5 þ 1  K 4  ½POM qt 270  K 8  ½DOMðlÞ  ½NH4 
1  K 9  ½NH4   ½O2  þ 1  K 5  ½DOMðlÞ
376
1  K 18  ½NH4   ½NO3   ½MnðIVÞ
1  K 19  ½MnðIVÞ  ½NH4 

Ammonium (NH4)

q½DOMðrÞ ¼ 1  K 6  ½POM  ½O2 0:2 qt

Refractory dissolved organic matter (DOM(r))

q½DOMðlÞ ¼
1  K 2  ½DOMðlÞ  ½O2 0:5
1  K 5  ½DOMðlÞ qt 106  K 8  ½DOMðlÞ  ½NH4 
1  K 7  ½DOMðlÞ0:5  ½O2 
376

Labile dissolved organic matter (DOM(l))

q½POM ¼
1  K 1  ½POM  ½O2 0:2
1  K 3  ½POM  ½NO3 
1  K 4  ½POM qt
1  K 6  ½POM  ½O2 0:2 þ 1  K 7  ½DOMðlÞ0:5  ½O2  þ K 8  ½DOMðlÞ  ½NH4 

Biogeochemical terms

No.

Table 2 Biogeochemical terms and boundary conditions

0.2 mM

0.01 mM

1 mM of DON

15 mM of DON

2.5 mM of PON

Upper boundary concentration

0.054

Lower boundary flux (mM m
2 d
1) S.K. Konovalov et al. / Deep-Sea Research II 53 (2006) 1817–1841

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11

10

9

8

7

Nitrogen gas (N2)

6

Manganese oxide (Mn(IV)), Sinking rate is 14.7 m d
1

q½MnðIIÞ 5 ¼
 K 10  ½MnðIIÞ  ½NO3 0:4
1  K 11  ½MnðIIÞ  ½O2   ½MnðIVÞ qt 2 3 1 þ  K 19  ½MnðIVÞ  ½NH4  þ  K 21  ½MnðIVÞ0:5  ½FeðIIÞ þ 1  K 23  ½MnCO3 1:7 2 2

Dissolved Manganese (II) (Mn(II))

q½S0  ¼ 1  K 16  ½MnðIVÞ  ½S2
0:125 þ 1  K 17  ½FeðIIIÞ  ½S2
0:125
1  K 22  ½S0   ½O2  qt

Elemental Sulfur (S0)

q½S  53 23:5 ¼  K 4  ½POM þ  K 5  ½DOM qt 16 2
1  K 14  ½S2
  ½O2 
1  K 15  ½S2
  ½NO3 
1  K 16  ½MnðIVÞ  ½S2
0:125 1
 K 17  ½FeðIIIÞ  ½S2
0:125
2  K 24  ½MnCO3   ½S2

2  K 25  ½FeðIIÞ  ½S2
 5

2


Sulfide (S2
)

q½O2  106 26 ¼
 K 1  ½POM  ½O2 0:2
23:5  K 2  ½DOM  ½O2 0:5
 K 6  ½POM  ½O2 0:2 qt 16 16 270 1 0:5  K 7  ½DOMðlÞ  ½O2 
2  K 9  ½NH4   ½O2 
 K 11  ½MnðIIÞ  ½O2   ½MnðIVÞ
16 2 1 3 2

 K 13  ½FeðIIÞ  ½O2 
2  K 14  ½S   ½O2 
 K 22  ½S0   ½O2  4 2

Oxygen (O2)

q½N2  276 ¼  K 3  ½POM  ½NO3  qt 80 1 1 þ  K 10  ½MnðIIÞ  ½NO3 0:4  ½MnðIVÞ þ  K 12  ½FeðIIÞ  ½NO3  2 2 1 4 þ  K 15  ½H2 S  ½NO3  þ  K 18  ½NH4   ½NO3   ½MnðIVÞ 2 5 1 1 þ  K 19  ½MnðIVÞ  ½NH4  þ  K 20  ½FeðIIIÞ  ½NH4  2 2

Biogeochemical terms

No.

Table 2 (continued )

3.0  10
5 mM

330 mM

Concentration oversaturation 5 mM

Upper boundary concentration

0.002

0.213

Lower boundary flux (mM m
2 d
1)

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ARTICLE IN PRESS

16

15

14

13

12

q½FeS2  ¼ 1  K 25  ½FeðIIÞ  ½S2
0 qt

Iron sulfide (FeS2), Sinking rate is 0.82 m d
1

þ 1  K 21  ½MnðIVÞ0:5  ½FeðIIÞ

q½FeðIIIÞ ¼ 5  K 12  ½FeðIIÞ  ½NO3  þ 1  K 13  ½FeðIIÞ½O2  qt
1  K 17  ½FeðIIIÞ  ½S2
0:125
3  K 20  ½FeðIIIÞ  ½NH4 

Iron oxide (Fe(III)), Sinking rate is 5.3 m d
1


1  K 21  ½MnðIVÞ0:5  ½FeðIIÞ
1  K 25  ½FeðIIÞ  ½S2
0

q½FeðIIÞ ¼
5  K 12  ½FeðIIÞ  ½NO3 
1  K 13  ½FeðIIÞ½O2  qt þ 1  K 17  ½FeðIIIÞ  ½S2
0:125 þ 3  K 20  ½FeðIIIÞ  ½NH4 

Dissolved Iron (Fe(II))

q½MnS ¼ 1  K 24  ½MnCO3 1:7  ½S2
0 qt

Manganese sulfide (MnS2), Sinking rate is 0.82 m d
1

q½MnCO3  ¼ 1  K 16  ½MnðIVÞ  ½S2
0:125
1  K 23  ½MnCO3 1:7 qt
1  K 24  ½MnCO3 1:7  ½S2
0

Manganese carbonate (MnCO3), Sinking rate is 193 m d
1

q½MnðIVÞ 5 ¼  K 10  ½MnðIIÞ  ½NO3 0:4  ½MnðIVÞ qt 2 þ 1  K 11  ½MnðIIÞ  ½O2   ½MnðIVÞ
1  K 16  ½MnðIVÞ  ½S2
0:125 3 1
 K 19  ½MnðIVÞ  ½NH4 
 K 21  ½MnðIVÞ0:5  ½FeðIIÞ 2 2

6.0  10
3 mM

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2.1.1. Inorganic components The presence of dissolved oxygen, nitrate, ammonium, sulfide, dissolved manganese (II), and suspended hydrous manganese (IV) oxide have been discussed as important redox components of the Black Sea water column in a number of publications (Codispoti et al., 1991; Murray et al., 1995; Konovalov and Murray, 2001; Oguz et al., 2001; Konovalov et al., 2004; etc.). Data for di-nitrogen gas, as the end-product of denitrification, which occurs in the suboxic zone, was discussed Murray et al. (2003b, 2005). Recent work by Jorgensen et al. (1991), Luther et al. (1991), Luther (1991), and Glazer et al. (2006) have provided data on the vertical distribution of elemental sulfur. Speciation and cycling of manganese and iron in the Black Sea water column has been investigated by Lewis and Landing (1991) and Tebo (1991) and parameterized for the purpose of numerical modeling by Konovalov et al. (2004). The forms of manganese and iron included here are sufficient to simulate numerically the observed distribution of the dissolved and suspended forms of these metals (Tables 1 and 2). 2.1.2. Particulate and dissolved organic matter Organic matter is produced in the euphotic layer, and the export production serves as a source of organic carbon, nutrient elements and energy to drive biogeochemical processes. Oxidation of organic matter under oxic conditions results in consumption of oxygen and production of nitrate. (A)

4

8

POC, µM 12

PON, µM 0.4 0.8 1.2 1.6 2

(B)

16

Sulfide and ammonium, which support chemosynthesis (Brewer and Murray, 1973), are the result of respiration of organic matter under anoxic conditions. Thus, organic matter is the most important substance that drives the biogeochemical processes in the oxic/anoxic water column of the Black Sea. While the distribution of particulate organic matter (POM), the level of primary production, and new production and temporal (seasonal to decadal) variations in primary production are somewhat known for the Black Sea (Vedernikov and Demidov, 1997; Burlakova et al., 1997, 2003; Ylmaz et al., 1998; Coban-Yildiz et al., 2000, 2006), the distribution and cycling of DOM have been poorly investigated (Polat and Tugrul, 1995; Morgan and Ducklow, 2000; Cauwet et al., 2002). Very little is known about the importance of DOM for carbon and nitrogen cycling in the Black Sea water column and its DOC:DON ratio. Data from 52 stations of nine cruises by MHI for different seasons of 1987 to 1994 have been used to construct average profiles of POC and PON (Fig. 3). The average POC:PON ratio of 7.2 varies little and differs from the Redfield value of 6.7 by only 10%. High values tend to occur at certain locations during warmer periods (Burlakova et al., 2003). POM sinking in the Black Sea water column (mostly marine snow aggregates) has been estimated to vary in size (0.5–5.5 mm diameter) and settling speeds (1.3–280 m d
1) (Diercks and Asper, 1997).

20

0

0

50

50

(C)

0

2.4 2.8

W (POM), m/day 2 4

6

14

100

150

150

200

200

Sigma-t

100

Depth, m

Depth, m

15

16

17

Fig. 3. Vertical distribution of POC (A), PON (B) and the rate of sinking of POM (C) below the euphotic zone. (POC and PON data are from MHI and IBSS cruises in 1984–1993. The sinking rate of POM has been derived from the 1988 KNORR data on the flux and concentration of POM (Karl and Knauer, 1991). Explanations are in the text.

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sumption of oxygen in the layer above the nitrate maximum at st15.6, suggesting that respiration of carbon-enriched DOM is important for the Black Sea. For these calculations we used data for the vertical distribution of nitrate and oxygen and Eq. (1) to calculate the profiles of the vertical flux (Konovalov et al., 2000). The derivatives of the vertical flux in respect to depth (Eq. (2)) were used to calculate the rate of net production minus consumption. The rate of oxygen consumption was calculated based on the rate of nitrate production, assuming that oxygen is consumed oxidizing POM with POC:PON ¼ 106:16. The resulting deficit of nitrate (shaded area) cannot be attributed to active processes of nitrate consumption like denitrification. Nitrate consumption, inferred from data for nitrite (Fig. 4), occurs primarily in the euphotic layer above st14.6 and in the suboxic layer from st15.6 to 16.0, but not in the layer of active consumption of oxygen. Osterroht and Thomas (2000) have reported a similar situation for the Baltic Sea, where AOU suggests production of excess DIC, while equivalent production of inorganic nitrogen or phosphorus does not occur. Following Ogawa et al. (1999) and Kahler and Koeve (2001), we assume that DOM in the Black Nitrate Production-Consumption, M.m-1.y-1 9

-4x10

-2x109

0x100

2x109

4x109

From O2 data

14.5

15.0 Sigma-t

The rate of sinking is a complex function of the density of both seawater and POM, of morphological characteristics of POM and ballast content, which significantly vary in the Black Sea water column. Thus, while parameterization of the rate of POM sinking with a constant value may work in rather thin layers (Oguz et al., 1999; Gregoire and Lacroix, 2001), it may be insufficient if one wants to simulate the biogeochemical structure and redox budget for the pycnocline and the entire aphotic zone to about 2000 m. As a first step, we derived the profile of the sinking rate of POM from data on its distribution and flux in the water column (Karl and Knauer, 1991). The ratio of the flux to the concentration gives an average rate of sinking (Fig. 3C) that is used in the model. Data are not available to distinguish between suspended, slow and fast sinking POM. We fit the calculated values to get a high-resolution profile (dashed line, Fig. 3C) and modified the most upper part of the calculated profile to limit the rate of sinking to an arbitrary value of 0.2 m d
1 at the upper boundary (solid line, Fig. 3C). Investigation of DOM has been a fast-developing area of oceanography over the last decade (Hansell and Carlson, 2001; Lefevre et al., 1996; Ogawa et al., 1999). However, few publications have addressed the distribution of DOM in the Black Sea (Polat and Tugrul, 1995; Morgan and Ducklow, 2000; Cauwet et al., 2002). The vertical distribution and seasonal variations for the Black Sea are similar to other regions of the World Ocean, but the concentrations of DOC in the Black Sea are higher. The importance of DOM cycling in the ocean has been uncertain until recently, while the ability of DOM to transform to POM has been known since at least 1963 (Baylor and Sutcliffe, 1963). Lefevre et al. (1996) claimed that POC could support only 20% of the overall organic matter remineralization in the aphotic layer (200–1000 m). However, Kahler and Koeve (2001) concluded that DOM was needed only to explain seasonal over-consumption of oxygen in the euphotic zone, while its impact on long-term carbon and nitrogen balance of the sea was small. Simultaneous measurements of DOM, POM, oxygen consumption and ammonium and/or nitrate production for the Black Sea are not available, but the distributions of oxygen and nitrate are well known (Fig. 2). The net production of nitrate calculated from vertical distributions of nitrate and oxygen (Fig. 4) reveals persistent over-con-

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From NO3 data

15.5

NO2-

16.0

0.00

0.20

0.40 Nitrite, µM

0.60

0.80

Fig. 4. Profiles (i) of the rate of nitrate production-consumption derived from data on the distribution of nitrate and oxygen (solid lines) and (ii) concentrations of nitrite (individual dots). (The shaded area shows the layer and intensity of over-consumption of oxygen due to respiration of carbon-enriched DON.)

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Sea is a mixture of a labile freshly produced fraction (DOM(l)) with DOC:DON ¼ 23.5 and a refractory fraction (DOM(r)) with DOC:DON ¼ 5. We also assume after Baylor and Sutcliffe (1963) that labile DOM can undergo transformation to POM (process 7, Table 1). This implies bacterial consumption of labile DOM by partially oxidizing it for energy supply and partially to use it to construct bacterial biomass. Simplifying the scheme, we assume the presence of only one fraction of POM with POC:PON ¼ 106:16. POM can be respired to refractory DOM (process 6, Table 1). Consumption of refractory DOM (with DOC:DON ¼ 5) is not considered in this scheme, because data on the possibility of this process below the euphotic zone in the Black Sea are not available. Thus, a physical upward flux is the only sink of DOM(r). 2.2. Initial and boundary conditions and numerical procedure Average vertical profiles from the observed data were used to initiate and calibrate the model, which is to numerically simulate the biogeochemical structure at steady state on a time scale of decades. These profiles (Fig. 5) result from averaging tens to hundreds of observations for POM, oxygen, sulfide, nitrate, ammonium versus sigma-t scale obtained from the central part of the Black Sea in the late

0

10

50

Di-nitrogen gas, µM 10 15 20

5

(A)

Ammonium, µM 20 30 40

1980s to early 1990s (KNORR-88, NATO TU-Black Sea, NATO Ocean Data Base Management System (ODBMS), etc.). Data on dissolved and suspended manganese (Fig. 6) were pooled from the 1988 KNORR cruise (Lewis and Landing, 1991; Tebo, 1991), from several NATO TU-Black Sea cruises in the early 1990s, and from the 2001 KNORR cruise (unpublished data of B. Tebo). Data for dissolved iron originated from the 1988 KNORR cruise (Lewis and Landing, 1991) and NATO TU-Black Sea cruises (unpublished data of E. Ovsyaniy, MHI, Ukraine). Data on suspended iron are rare and the initial profile originates from Lewis and Landing (1991). Data of the vertical distribution of dinitrogen gas were pooled from several NATO ODBMS cruises and the 2001 KNORR cruise (Murray et al., 2003a, b). The profile of elemental sulfur was simulated in numerical experiments to compare to results from the 2001 KNORR expedition (unpublished data of G. Luther; Glazer et al., this volume; Konovalov et al., 2003). The total profile of DOM was drawn to follow Morgan and Ducklow (2000) and Cauwet et al. (2002) (Fig. 5B). The boundary conditions were set using a flux at the bottom boundary (st ¼ 17:236, 2132 m) and a concentration at the upper boundary (st ¼ 14:4, 50 m). All values are listed in Table 2. The flux of all solutes, except for dissolved Mn(II), ammonium and sulfide, were set equal to zero at the bottom

25

(B)

0

0.3

PON, µM 0.6 0.9

1.2

1.5

16

20

O2 15.0

PON 15.0

16.0

N2

DON 17.0

H2S 0

0.0

16.0

NH4+

S°

17.0

Suboxic zone

Sigma-t

Sigma-t

NO3-

100 200 Oxygen & Sulfide, µM 2.0

4.0 6.0 Nitrate, µM

8.0

300

0

4

8 12 DON, µM

10.0

Fig. 5. Numerically simulated (solid lines and open symbols) vs. initial profiles (dashed lines with solid symbols) of oxygen, sulfide, nitrate, di-nitrogen, ammonium (A) and PON and DON (B) in the simulated water column.

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Iron dissolved, µM

(A) 0

0.1

0.2

(B) 0

0.3

0.05

15.0

Sigma-t

15.0 Sigma-t

Iron particulate, µM 0.025

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Suboxic zone

16.0

16.0

Mnpart

Fediss Mndiss

17.0 0

4 8 Manganese dissolved, µM

Fepart

17.0 12

0

0.01 0.02 0.03 Manganese particulate, µM

Fig. 6. Numerically simulated (solid lines with open symbols) vs. average profiles (dashed lines with solid symbols) of dissolved (A) or arbitrary profiles (dashed lines) of particulate (B) manganese and iron in the simulated water column.

boundary. The flux for dissolved manganese (II), sulfide and ammonium were adjusted to keep the concentration of these solutes in the bottom layer of water at steady state. These fluxes are discussed below. The concentrations at the upper boundary were set to correspond to the average values for the late 1980s–1990s or the published values for input of suspended Mn(IV) and Fe(III). The water column from st ¼ 14:4 (50 m) to the bottom (st ¼ 17:236 at 2132 m) was divided into intervals of Dst ¼ 0:01 (the three deepest levels were set at st ¼ 17:233, 17.235 and 17.236), which vary in depth from 0.25 m in the middle pycnocline (st15.5) to 300 m near the bottom. The time step of numerical integration was set to 0.0025 d. The present version of the model allows simulation of about 5 years of the Black Sea evolution per hour of numerical integration on a PC equipped with a Pentium 2.2 GHz processor. 2.3. Numerical simulation of the biogeochemical structure The rates of the processes depend on the vertical distribution of individual components and these rates affect these distributions at every step of the integration. Distributions of biogeochemical properties are generated as the model runs forward in time. Vertical advection and diffusion influence all properties. Biogeochemical processes are linked and coupled through those substances that take part in multiple processes. Thus, the profile of oxygen, for example, is affected by physical processes of advection, diffusion, entrainment (st o15:5) and

intrusions (st 415:5) of the Bosporus plume, and by nine biogeochemical processes. The generated distribution of oxygen affects both physical fluxes of oxygen and the intensity of all nine biogeochemical processes. Initial experiments were conducted to calibrate the model and to optimize the set of processes in order to keep the numerically simulated biogeochemical structure as close to the average profiles as possible (Figs. 5 and 6). An initial attempt to simulate the biogeochemical structure without including DOM cycling revealed that either consumption of POM and production of nitrate became unrealistically high or modeled consumption of oxygen in the upper pycnocline was too low. This resulted in a downward movement of the oxycline with time or in a distortion of the profile of nitrate with a few-fold increase in the maximum concentrations and an unobserved shift of this maximum to st15.1, where the maximum of oxygen consumption was observed (Fig. 4). When DOM cycling was included, the distribution, flux and rate of production–consumption of oxygen, POM and nitrate became reasonably close to the observed values (Fig. 5). The difference between the simulated and initial vertical distribution of nitrate may be related to our poor understanding the distribution and reactivity of DOM and its C:N ratio in the Black Sea. Similarly, we could not accurately simulate the distribution of sulfide assuming that it was oxidized by suspended manganese (IV) to sulfate. The deeper part of the profile of nitrate, the upper part of the profile of sulfide, and the suboxic/anoxic part of the

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profile of dissolved manganese (II) were distorted and shifted relative to observed data. Consideration and parameterization of elemental sulfur cycling, as an intermediate product of oxidation of sulfide, resulted in a simulated peak of elemental sulfur (Fig. 5A), whose magnitude and depth fit precisely the results of the 2001 KNORR expedition (Glazer et al., 2006; Konovalov et al., 2003). The anoxic part of the simulated distribution of elemental sulfur predicted higher concentrations than observed probably because biogeochemical processes governing elemental sulfur consumption under anoxic conditions were not considered. Thus, elemental sulfur appears to be an essential redox component that needs to be considered to correctly simulate the oxic/anoxic structure and the redox budget of the Black Sea water column. The model predictions of the biogeochemical structure of the oxic/anoxic water column (Figs. 5 and 6) successfully reproduce the observed system and remains close to steady state on a time scale of several decades. It did drift slowly, so that the detectable changes were observed over time scales of a century. We used T–S data averaged for about 70 years to parameterize vertical exchange in the water column, and biogeochemical data averaged for about one decade to parameterize and calibrate redox processes, so any deviations from steady state in excess of a century could reflect natural trends or limitations in the applied parameterizations.

3. Discussion 3.1. Redox budget and importance of individual processes When physical and biogeochemical processes are in balance, the budget and distributions of properties are at steady state. The model provides an independent way to estimate the importance of individual processes, as they cannot be modified and fixed individually, but have to be adjusted simultaneously to keep all simulated profiles as close to the initial average distributions, as possible (Figs. 5 and 6). Figs. 7, 9 and 10 present data on the numerically simulated budgets of individual biogeochemical components. We believe that the budget simulated in this way reveals the importance of specific physical and individual biogeochemical processes.

3.1.1. Oxygen The simulated budget of oxygen for the early 1990s (Fig. 7A) strongly depends on the diffusive flux of oxygen from the Cold Intermediate Layer (CIL, Fig. 1), as this flux is the only source of oxygen for the layer of the oxycline, where oxygen is mostly consumed to oxidize organic matter. About 30% of this consumption is spent to oxidize POM, while 40% is used to oxidize DOM. The important role of DOM for O2 consumption explains the excess-consumption of oxygen in the upper oxycline (Fig. 4). Lefevre et al. (1996) reported that POM could account for 20% of oxygen consumption in the Mediterranean. However, the Mediterranean is less biologically productive and the ratio of POM/DOM production is lower than in the Black Sea. About 10% of oxygen is consumed to oxidize ammonium to nitrate, mostly in the layer of the oxycline above the suboxic zone (the vertical distributions of the rate of individual physical and biogeochemical processes are not shown). Only 4% is utilized in redox processes in the suboxic zone itself. The other biogeochemical processes we consider here play a minor role and do not affect the budget of oxygen to any significant extent. Only about 0.1% of the vertical flux of oxygen is spent to oxidize sulfide and other reduced species coming up from the anoxic zone. The specific rate constants of oxidation of sulfide and ammonium or manganese (II) and iron (II) are much higher than similar values for oxidation of POM and DOM (Table 1), but the vertical flux of oxygen does not reach the lower part of the suboxic zone and therefore cannot play any important role in suboxic biogeochemistry (Murray et al., 1995). The sum of all biogeochemical processes in the oxic and suboxic zones appears to be responsible for consumption of 88% of the vertical flux of oxygen. The remaining 12% is entrained into the Bosporus plume to generate the lateral flux of oxygen into the suboxic and anoxic zones. Entrainment of Black Sea water into the Bosporus plume and intrusion of the plume waters in the anoxic zone has been discussed by Murray et al. (1991) and Ozsoy et al. (1995) for physical properties and by Buesseler et al. (1991) for cesium-137, but intrusions of the plume waters also should generate the lateral flux of oxygen. This flux of oxygen has been previously deduced (Konovalov and Murray, 2001) from deviations of the ratio of sulfide to ammonium in the anoxic zone from the expected value (Fig. 8B) and it has been recently

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(A)

(B)

DOM(l) to POM POM to NH4 NH4 to NO3 S(0) to SO4 POM to DOM(r) DOM(l) to NH4 H2S to SO4 Mn(II) to Mn(IV) Fe(II) to Fe(III)

1831

Sulfide by Mn(IV) Sulfide by NO3 Sulfide by Fe(III) Sulfide by O2 DOM(l) respiration POM respiration Intrusion Entrainment Advection

Intrusions Entrainment Advection

Diffusion

Diffusion Biogeochemical processes

Biogeochemical processes Physical processes

Physical processes Balance

-12

Balance

-8 -4 0 4 8 12 Consumption or Production of Oxygen, µM.m-2.day-1 (C)

-1.0

-0.5 0.0 0.5 Consumption or Production of Sulfide, µM.m-2.day-1

(D)

H2S by NO3 to N2 Mn(II) by NO3 to Mn(IV) NH4 & NO3 to N2 POM by NO3 Fe(II) by NO3 to Fe(III)

DOM(l) by O2 to NH4 DOM(l) respiration POM respiration POM by O2 to NH4

NH4 by O2 to NO3 Intrusion Entrainment Advection Diffusion

1.0

NH4 by O2 to NO3 NH4 & NO3 to N2 DOM(l) & NH4 to POM NH4 by Fe(III) NH4 by Mn(IV)

Intrusions Entrainment Advection Diffusion

Biogeochemical processes

Biogeochemical processes

Physical processes Balance

-0.6

Physical processes Balance

-0.3 0 0.3 0.6 Consumption or Production of Nitrate, µM.m-2.day-1

-1.0 -0.5 0.0 0.5 1.0 Consumption or Production of Ammonium, µM.m-2.day-1

Fig. 7. The oxygen (A), sulfide (B), nitrate (C), and ammonium (D) simulated budget in the water column. (Bars are in three groups at every diagram to show the importance of individual biogeochemical processes (upper group), physical processes (middle group), and an integrated result (lower group) of bars.)

7.6

7.8

Temperature, C 8 8.2

8.4

(A) 12

(B) 16.0 2001 KNORR data Voltammetric Data Volumetric Data Temperature, C

16.2 Sigma-t

Sigma-t

13 14

16.4 16.6

15 1988 KNORR data The ratio 16/53 equation 4, Table 1

16.8 16 17.0 0

100

200 300 Oxygen, µM

400

0.0

1.0 2.0 Ammonium/Sulfide Ratio

3.0

Fig. 8. Lateral intrusions of oxygenated Bosporus plume waters (A) and the result of these intrusions for the ammonium/sulfide ratio (B). Data are reproduced from Konovalov et al. (2003) and Konovalov and Murray (2001).

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confirmed (Konovalov et al., 2003; Glazer et al., 2006) by direct observations (Fig. 8A). 3.1.2. Sulfide The simulated budget of sulfide (Fig. 7B) reveals quantitative estimates of individual processes that can be hardly resolved from direct measurements. Only 8% of sulfide can be produced from respiration of DOM, while 74% of sulfide production is generated due to respiration of POM. Any attempt to increase the specific rate constant of anoxic respiration of DOM alters the entire biogeochemical structure for the worse. The minor role of DOM in sulfide production fits well the conclusions from other studies, that DOM is important for biogeochemical processes in the upper part of the water column, while POM basically drives biogeochemistry of deep layers of water (e.g., Kahler and Koeve, 2001). The fact that 82% of the sulfide is produced due to respiration of organic matter in the anoxic water column is not surprising, as the microbiological origin of sulfide is presently widely accepted. The other 18% of sulfide production is estimated to flux from sea-bottom respiration of organic matter sinking to the sediments. Our model estimated value of 0.2 mmole m
2 d
1 fits well the published rate of sulfide production in Black Sea sediments of 0.6 mmole m
2 d
1 (Weber et al., 2001), as only a fraction of sulfide produced reaches the water column. Most of the sulfide produced is buried in the sediments (see review by Rickard et al., 1995). Based on these model results oxidation of sulfide by the vertical flux of oxygen accounts for less than 0.5% of the total upward flux of sulfide. This process is not artificially limited in the model by, for example, a low specific rate constant of this process (Table 1). The applied specific rate constant of oxidation of sulfide suggests its t1/2 of about 0.3 h for oxygen saturated conditions, which agrees with data published by Millero (1991). Rather, oxidation of sulfide by the vertical flux of oxygen does not happen because sulfide and oxygen are consumed by other processes and concentrations of these components are too small at the onset of sulfide to support oxidation of any importance. Sinking manganese (IV) oxides, generated in the suboxic zone, oxidize almost 25% of the vertical flux of sulfide. About 8% of the flux of sulfide is oxidized by nitrate, but only 2% is oxidized by the vertical flux of nitrate, while 6% is oxidized inside the anoxic zone due to the flux of nitrate injected with the Bosporus plume. In a

similar way the lateral flux of oxygen (Fig. 8) oxidizes over 60% of sulfide inside the anoxic zone. The mechanism of this process includes mediation and catalysis by manganese (II)–manganese (IV) redox transformations (Luther et al., 1991), but catalytic cycles do not affect the overall redox budget and they are not parameterized in this work, although the catalytic effect is taken into account by the value of specific rate constants. The simulated budget of sulfide reveals that about 5% of the sulfide remains available to increase the inventory of sulfide in the anoxic zone. This value may demonstrate the presence of some uncertainty in the rates of different processes, but this also is consistent with observations that the inventory and concentrations of sulfide are increasing with time inside the anoxic zone (Konovalov et al., 1999a, b; Konovalov and Murray, 2001).

3.1.3. Nitrate The budget of nitrate (Fig. 7C) is linked to the budgets of oxygen, sulfide and ammonium (Fig. 7), organic matter (Fig. 9), manganese and iron (Fig. 10). Nitrate is the ultimate product of oxidation of ammonium. The rate of this process increases towards the maximum of nitrate around st15.5 and then decreases abruptly in the suboxic zone towards the onset of sulfide (data are not shown). The very low concentrations of oxygen in the suboxic zone result in low rates of organic matter oxidation and ammonium production. Neither oxidation of organic matter nor dissolved iron by nitrate appears to play an important role in the Black Sea but for different reasons. Ottley et al. (1997) have demonstrated that direct chemical reduction of nitrate by dissolved iron (II) is feasible, but oxidation of iron cannot play an important role for nitrate in the Black Sea because the upward flux of iron (II) is too small to impact the nitrate budget (this may not be true for other oxic/anoxic marine systems). Oxidation of organic matter by nitrate does occur in the Black Sea and the rate of this process is proportional to the concentration of organic matter and nitrate, but the concentrations of organic matter and nitrate, as well as the concentration of N2O (data of M. Westley from the 2001 and 2003 KNORR cruises), and the thickness of the suboxic layer in the Black Sea are much smaller than in the regions of the Arabian Sea where denitrification is important for the redox budget.

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(A)

(B)

POM by O2 to NH4

1833

DOM(l) by O2 to POM

POM respiration

DOM(l) & NH4 to POM

POM by O2 to DOM(r)

DOM(l) respiration

POM by NO3

DOM(l) by O2

DOM(l)&NH4 to POM DOM(l) by O2 to POM Intrusion

Intrusion

Entrainment

Entrainment

Advection

Advection

Sinking

Diffusion Biogeochemical processes

Biogeochemical processes

Physical processes

Physical processes

Balance

-1

Balance

-0.5 0 0.5 Consumption or Production of POM, µM.m-2.day-1

1

-1

-0.5 0 0.5 Consumption or Production of DOM(l), µM.m-2.day-1

1

Fig. 9. The POM (A) and DOM(l) (B) simulated budget in the water column.

(A)

(B)

Mn(II) by NO3 Mn(II) by O2 Mn(IV) by NH4 Mn(IV) by Fe(II) MnCO3 dissolution

Fe(II) to FeS2 Fe(II) by Mn(IV) Fe(II) by NO3 Fe(II) by O2 Fe(III) by NH4 Fe(III) by Sulfide

Intrusion

Intrusion

Entrainment

Entrainment Advection

Advection

Diffusion

Diffusion Biogeochemical processes

Biogeochemical processes

Physical processes

Physical processes Balance

Balance

-0.40

-0.20 0.00 0.20 Consumption or Production of Mn(II), µM.m-2.day-1

(C)

0.40

-0.06 -0.04 -0.02 0 0.02 0.04 Consumption or Production of Fe(II), µM.m-2.day-1 (D)

Mn(IV) by Sulfide Mn(IV) by Fe(II) Mn(IV) by NH4

0.06

Fe(III) by Sulfide Fe(III) by NH4 Fe(II) by O2 Fe(II) by NO3 Fe(II) by Mn(IV)

Mn(II) by O2 Mn(II) by NO3

Entrainment

Entrainment Sinking Advection

Sinking Advection Diffusion

Diffusion Biogeochemical processes

Physical processes

Biogeochemical processes Physical processes

Balance

-0.30 -0.20 -0.10 0.00 0.10 0.20 Consumption or Production of Mn(IV), µM.m-2.day-1

Balance

0.30

-0.06 -0.04 -0.02 0 0.02 0.04 Consumption or Production of Fe(III), µM.m-2.day-1

0.06

Fig. 10. The dissolved manganese (A) and iron (B), and suspended manganese (C) and iron (D) simulated budget in the water column.

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The model predicts more than 12% of nitrate consumption to oxidize the upward flux of ammonium from the anoxic zone. This process is known as the anammox reaction between nitrite and ammonium (Kuypers et al., 2003). Murray et al. (1995, 2003b) discussed this process in regard to denitrification and production of di-nitrogen gas in the suboxic zone. In addition, over 19% of nitrate consumption appears to be utilized to oxidize dissolved manganese in the suboxic zone, while other 24% occurs during oxidation of sulfide inside the anoxic zone. In total, 57% of nitrate produced in the Black Sea is consumed in redox processes in the suboxic and anoxic layers. The flux of nitrate to the Black Sea with the Mediterranean waters (Ozsoy et al., 1995) is equivalent to as much as 3% of nitrate production and the upward flux of nitrate at the upper boundary of the simulated water column accounts for 46% of nitrate production. (57% of nitrate consumption and 46% of the upward flux equal 100% of production and 3% of influx with the Mediterranean waters.) The upward flux of nitrate to the euphotic zone equals 39%, when calculated for the initial profile of nitrate (Konovalov et al., 2000), and we consider 7% difference to be a good result, as very little is known about the POM:DOM ratio and DOC:DON composition of DOM in the Black Sea. Among all components of the budget of nitrate, the amount of nitrate that is consumed to oxidize the upward flux of dissolved manganese is the most intriguing. This process has never been confirmed by direct microbiological measurements (Tebo, 1991), but it is often referred to (e.g., Murray et al., 1995; Oguz et al., 2001; Luther et al., 1997). A similar process of oxidation of iron (II) by nitrate has been reported by Ottley et al. (1997). It has been proposed (Konovalov et al., 2004) that oxidation of dissolved manganese by nitrate is the process that can balance the redox budget of the suboxic zone and explain the observed vertical profile of dissolved manganese. This process might involve Mn(II)– Mn(III)–Mn(IV) redox transformations, a catalytic cycle of trace metals (Luther et al., 1997) or hydrogen peroxide (detected from the results of voltammetric profiling; unpublished data of B. Tebo and G. Luther from the 2001 and 2003 KNORR expedition to the Black Sea). The results of numerical experiments are not proof of oxidation of dissolved manganese by nitrate, but they demonstrate that the budget of oxygen in the

suboxic zone cannot support the redox budget of manganese, while nitrate can. Experimental microbiological investigations of manganese redox cycling are thus very important. 3.1.4. Ammonium POM oxidation in the oxic layer and POM respiration in the sulfidic zone appear to be the major sources of ammonium in the Black Sea water column (Fig. 7D) and account for 61% and 32% of its production. These processes are most important to sustain a steady-state distribution of ammonium in the oxic, the suboxic and the upper to middle part of the anoxic layer. An attempt to simulate aerobic or anaerobic oxidation of DOM with an equivalent release of ammonium has revealed that these processes are not of major importance for the budget of ammonium. This is consistent with data showing that ammonium and amino acids are consumed by microorganisms, rather than released, in the presence of DOM (Amon and Benner, 1996). As with sulfide, we predict there should be substantial flux of ammonium from the sediments to support the distribution of ammonium in, and to balance the physical upward flux out of, the deepest anoxic layers. This flux of ammonium should be close to 6% of all sources of ammonium. Sources of ammonium are balanced by the processes of ammonium oxidation in the oxic layer (72%) and de-nitrification (including anammox) in the suboxic layer (15%). Choe et al. (2000) discussed denitrification of nitrate by iron to dinitrogen gas, and Luther et al. (1997) demonstrated that oxidation of ammonium by manganese (IV) is thermodynamically possible. However, attempts to numerically simulate oxidation of ammonium by manganese (IV) or iron (III) oxides show that these processes are probably not important for the budget and distribution of ammonium. Utilization of DOM and ammonium to produce POM (mimicking bacterial DOM and ammonium utilization) can be responsible for consumption of about 12% of produced ammonium. The average upward flux of ammonium to the euphotic zone is extremely small because both the concentration and the vertical gradient of concentration are very low, as compared to nitrate (Fig. 5A). 3.1.5. DOM and POM The simulated budget of POM (Fig. 9A) suggests that sinking from the euphotic zone provides 55% of POM that is oxidized and respired in the water

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column. The reminder of the POM supply seems to be derived by bacterial conversion of labile DOM(l) to POM. If production of POM from DOM(l) is not included in the model, it becomes impossible to generate (i) the observed over-consumption of oxygen and a lack of nitrate in the upper oxycline (Fig. 4), and (ii) the major sink of labile DOM(l) (Fig. 9B). The simulated bacterial DOM(l) utilization, which is responsible for 98% of the overall consumption of DOM (Fig. 9B), is parameterized by two major processes. The most important one (86%) represents oxidation of a large portion of organic carbon in DOM(l) to produce energy. This agrees with the published low efficiency of bacterial consumption of DOM (Kahler et al., 1997; Ducklow et al., 2002). A minor part of carbon and all nitrogen is converted by heterotrophic microorganisms into biomass. Incorporation of DOM(l) and ammonium from ambient waters to produce POM is also significant (12%). While the flux, production and utilization of energy have not been calculated and parameterized in the present version of the model, the results for the importance of individual processes obtained by fitting the observed distributions agrees with those for an optimized bioenergetic model of bacterial DOM utilization (Vallino et al., 1996). For several possible processes, the most energy-efficient process becomes the major way for both bacterial growth and geochemical transformations. Production and sinking of POM is balanced by its oxidation in the oxic layer (71%) and respiration in the anoxic layer (29%). This partitioning of overall consumption of POM between oxic and anoxic layers fits well the published data by Lein and Ivanov (1991) on the sulfur and carbon balances in the Black Sea. 3.1.6. Manganese and iron The budgets of reduced (dissolved) and oxidized (suspended) forms of manganese and iron (Fig. 10) reveal several features that make these elements similar in one way and different in another. Thus, physical processes redistribute dissolved iron (II) between individual layers of the water column, but do not affect the overall budget of dissolved iron (II), as fluxes of this solute at the boundaries of the water column are equal or close to zero. The flux of dissolved manganese (II) from sediments has been adjusted to be about 1% of the overall production of this form of manganese, and model results suggest this flux cannot be larger at steady state.

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Results of initial numerical experiments demonstrate that the flux of dissolved iron (II) from sediments is equal to zero. This demonstrates that pyritization in Black Sea sediments prevents the net flux of dissolved iron (II) to the water column (Rickard et al., 1995). The flux of manganese at the upper boundary of the modeled water column (st ¼ 14:4, 50 m) is 20% of its flux from sediments and it is below 0.2% of produced in the water column (Fig. 10A), making the budget of manganese basically dependant on recycling inside the water column. Temporal variations are very slow, and the residence time of manganese in the Black Sea water column is on the time scale of millennia (Lewis and Landing, 1991; Konovalov et al., 2004). The flux of iron at the upper boundary of the modeled water column (st ¼ 14:4, 50 m) is about 70 times the flux for manganese, while the inventory of iron in the water column is about 0.005 times that for manganese. In contrast to manganese, the external supply of iron supports 54% of the sources of iron (III). Thus, we predict a short residence time and fast temporal variations in the distribution of iron in response to changes in external fluxes of iron to the Black Sea. Both dissolved manganese (II) and iron (II) are oxidized in the lower part of the suboxic zone, but the model predicts that 97% of the dissolved manganese is oxidized by nitrate (Fig. 10C), while 39% of dissolved iron (II) is oxidized by nitrate and the rest is oxidized by suspended manganese (IV) (Fig. 10D). The latter process has been investigated by Postma (1985) and Postma and Appelo (2000) for sediments and in laboratory experiments and parameterized for the Black Sea conditions by Konovalov et al. (2004). The main evidence for considering this process is that oxidation of dissolved iron (II) occurs both in the lower part of the suboxic zone and in the upper part of the anoxic layer. This process appears to be important for the budget of iron (Fig. 10D), but it consumes less than 3% of suspended manganese (IV) (Fig. 10C). Suspended manganese (IV) and iron (III) are primarily used to oxidize sulfide (Fig. 10C and D). This accounts for 96% of the sink of manganese (IV) and results in oxidation of up to 25% of the upward flux of sulfide (Fig. 7B). A similar process accounts for consumption of 84% of the particulate iron (III) flux, but it has a minor effect for the budget of sulfide because the absolute flux of iron (III) is much smaller, compared to manganese (IV). Bottcher and Thamdrup (2001) have reported on

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oxidation of sulfide by manganese (IV) and iron (III) to sulfate, but our results fit the data published by Yao and Millero (1993, 1996) which suggest elemental sulfur is the main product. The presence of elemental sulfur in the lower suboxic and upper anoxic zone was reported by Jorgensen et al. (1991) and Luther et al. (1991) and more recently by Luther (unpublished data from the 2001 KNORR expedition), Konovalov et al. (2003), and Glazer et al. (2006). The simulated profile of elemental sulfur fits quite well the maximum values and the location of the maximum in the very upper anoxic zone, but the simulated peak of elemental sulfur is much broader compared with observations (Fig. 5A), because the model does not include consumption of elemental sulfur in the anoxic zone of the sea. Manganese is largely recycled within the water column. The upward flux of dissolved manganese (II) increases towards the suboxic/anoxic boundary, as dissolved manganese (II) is oxidized in the suboxic zone to ultimately produce suspended hydrous manganese (IV) oxide. The latter sinks back to the anoxic zone facilitating oxidation of sulfide and production of suspended manganese (II) carbonate. The MnCO3 sinks into deeper layers and is dissolved, compensating the upward flux and keeping the vertical distribution of Mn(II) at a steady state. The flux of manganese both at the upper (st ¼ 14:4, 50 m) and lower (st ¼ 17:236, 2132 m) boundaries of the water column is very small, and it cannot result in annual-to-decade variations in the distribution of manganese. Unlike manganese, the budget and distribution of iron crucially depend on the external flux to the water column. This flux is required to balance the loss (precipitation) of iron sulfide from the water column (Cutter and Kluckhohn, 1999; Rickard and Luther, 1997), which is equal to 54% of the iron (II)—iron (III) redox transformations. This makes the distribution of iron sensitive to temporal variations in external supply of iron that can result in at least 2-fold changes in the inventory of dissolved iron (II) in the water column over a period of 5–10 years (Konovalov et al., 2004). While the distribution of manganese is sensitive to the basic redox structure, recycling and redistribution of manganese within the water column are the primary processes in its budget, but iron reveals a sequence of redox and dissolved-suspended transformations from the upper boundary of the water column towards sediments, rather than cycling within the water column.

3.2. Evolution of the biogeochemical structure The calculated budget of oxygen (Fig. 7A) suggests that the vertical distribution of the main redox species and the structure of the oxic and suboxic zone depend strongly on the concentration of oxygen in the CIL (the main source) (Fig. 1A) and export production of POM (the main sink). The concentration of oxygen in the CIL is important because this concentration determines the overall gradient of oxygen through the oxycline, which in turn drives the vertical diffusive flux. Export production appears to be responsible for consumption of up to 85% of the vertical flux of oxygen. This makes the vertical distribution of oxygen (and thus the structure of the oxycline and location of the upper boundary of the suboxic zone) very sensitive to any climate- and human-driven changes in eutrophication. After a sequence of years with mild winters, which result in weakening of ventilation of the CIL, the oxygen concentration can vary by a factor of 2, from 320 mM in 1993 (Konovalov and Murray, 2001; data from the TU-Black Sea database) to 180 mM, in 2001 (data from the cruise of R.V. KNORR 2001). Some numerically generated results for decreasing concentration of oxygen in the CIL are presented in Fig. 11A. All parameterizations of biogeochemical processes and the export production of organic matter are the same as described above, while the concentration of oxygen in the CIL has been set at 180 mM simulating the observed warming and weakening of ventilation from 1993 to 2001. The model realistically reproduces the observed changes, as it is demonstrated by comparison with the cruise data from the central and northern deep part of the sea, which are directly not affected by the lateral flux of oxygen (Murray et al., 2003a). The most prominent result of a reduction in ventilation of the CIL is that the oxycline moves up and the thickness of the suboxic zone increases. A decrease in the maximum concentration of nitrate, and a slight shoaling of the onset of dissolved Mn(II) (Fig. 11A) and sulfide (Murray et al., 2003a) also occur after this perturbation. The fact that variations in the suboxic zone thickness primarily result from changes in the depth of the upper boundary of this layer, whereas changes in the location of the onset of sulfide are minor, also is consistent with published data on the temporal changes in the structure of the suboxic zone

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Fig. 11. Numerically simulated (dashed lines) evolution vs. initial (solid lines) biogeochemical structure and vs. 2001 KNORR data (individual points) due to variations in the concentration of oxygen in CIL (A) and export production (B).

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Fig. 12. Numerically simulated evolution of the biogeochemical structure and transformation of the suboxic zone in absence of the lateral flux of oxygen in 5 (A) and 12 (B) years. (Solid lines—simulated profiles, dashed lines—initial data.)

(Buesseler et al., 1994; Konovalov and Murray, 2001). The influence of eutrophication on the biogeochemical structure of the Black Sea water column, as compared to the effects of climate change on the ventilation of the CIL, was investigated by increasing the export production 2-fold and keeping all other applied parameterizations and boundary conditions unchanged (Fig. 11B). While the upper oxycline does not change much, the suboxic zone broadens, nitrate concentrations increase, and the onset of sulfide shoals slightly fitting observations for the 1980s (Konovalov et al., 1999b; Konovalov and Murray, 2001). The most fascinating result of these numerical experiments is related to the nature of the suboxic

zone. Konovalov and Murray (2001) suggested that the suboxic zone could exist when the flux of organic matter is enough to consume the vertical flux of oxygen, if the upward flux of sulfide does not exceed the oxidative capacity of the Bosporus plume. If this is true, the suboxic zone would not exist if the Bosporus plume waters were oxygen-free. We tested this scenario in an experiment where the parameterizations and boundary conditions were kept unchanged, but the concentration of oxygen in the Bosporus plume was set to zero (Fig. 12). The simulated ‘‘shrinking’’ of the suboxic zone and appearance of a layer with overlapping O2 and H2S (the ‘‘C-layer’’) is another indication of the critical role played by the lateral flux of oxygen from the Bosporus Plume.

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This experiment also allows prediction of the possible consequences of eutrophication, which exceeds the oxidative capacity of the Bosporus Plume. With an increase in the export flux of organic matter, the oxic/anoxic boundary moves upward towards the upper boundary of the suboxic zone. Nitrate is consumed during remineralization of organic matter and compensation of the upward flux of reducers from the anoxic layer. Concentrations of nitrate decrease several-fold but nitrate alone cannot sustain the suboxic zone, which disappears in about 5–10 years (Fig. 12). The oxycline moves up to the depth where the vertical flux of oxygen balances the downward flux of organic matter and upward flux of sulfide and other reduced species. The suboxic zone disappears and a layer of co-presence of oxygen and sulfide appears, making this virtual situation in the Black Sea similar to the presently observed structure in Mariager Fjord (Zopfi et al., 2001), Framvaren Fjord (Velinsky and Fogel, 1999) and frequently in the Cariaco Trench (Scranton et al., 2001). Acknowledgments This work was funded by the NATO Collaborative Linkage grant to SK and JM. SK acknowledges partial support from CRDF Projects #UG1-2432-SE02 and #UG2-2080. JWM acknowledges NSF Grants OCE 0081118 and MCB 0132101. GWL acknowledges NSF Grant OCE-0096365. BMT acknowledges NSF Grants OCE-0221500 and EAR-9725845. Data on POM have been generously provided by Z.P. Burlakova and L.V. Eremeeva from Marine Hydrophysical Institute, Ukraine. References Amon, R.M.W., Benner, R., 1996. Bacterial utilization of different size classes of dissolved organic matter. Limnology and Oceanography 41, 41–51. Baylor, E.R., Sutcliffe, W.H., 1963. Dissolved organic matter in seawater as a source of particulate food. Limnology and Oceanography 8, 369–371. Belyaev, V.I., Sovga, E.E., Lyubartseva, S.P., 1997. Modelling the hydrogen sulfide zone of the Black Sea. Ecological Modelling 13, 51–59. Black Sea Integrated Coastal and Shelf Zone Monitoring and Modeling (INCOM) Program Science Plan, NATO, CCMS, 2000. Report No. 248, pp. 1–49. Bottcher, M.E., Thamdrup, B., 2001. Anaerobic sulfide oxidation and stable isotope fractionation associated with bacterial sulfur disproportionation in the presence of MnO2. Geochimica et Cosmochimica Acta 65 (10), 1573–1581.

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Trouwborst, R.E., Clement, B., Murray, K.J., Romanov, A.S., 2003. Lateral injection of oxygen with the Bosporus plume-fingers of oxidizing potential in the Black Sea. Limnology and Oceanography 48, 2369–2376. Konovalov, S., Samodurov, A., Oguz, T., Ivanov, L., 2004. Parameterization of iron and manganese cycling in the Black Sea suboxic and anoxic environment. Deep-Sea Research I 51, 2027–2045. Kuypers, M.M.M., Sliekers, A.O., Lavik, G., Schmid, M., Jørgensen, B.B., Kuenen, J.G., Damste, J.S.S., Strous, M., Jetten, M.S.M., 2003. Anaerobic ammonium oxidation by anammox bacteria in the Black Sea. Nature 422, 608–611. Lee, B.-S., Bullister, J.L., Murray, J.W., Sonnerup, R., 2002. Anthropogenic chlorofluorocarbons in the Black Sea and the Sea of Marmara. Deep-Sea Research I 49, 895–913. Lefevre, D., Denis, M., Lambert, C.E., Miquel, J.-C., 1996. Is DOC the main source of organic matter remineralization in the ocean water column? Journal of Marine Systems 7, 281–291. Lein, A.Yu., Ivanov, M.V., 1991. On the sulfur and carbon balances in the Black Sea. In: Izdar, E., Murray, J.W. (Ed.), Black Sea Oceanography, pp. 307–318. Lewis, B.L., Landing, W.M., 1991. The biogeochemistry of manganese and iron in the Black Sea. Deep-Sea Research 38 (Suppl. 2A), S773–S804. Luther, III, G.W., 1991. Sulfur and iodine speciation in the water column of the Black Sea. In: Izdar, E., Murray, J.W. (Eds.), Black Sea oceanography, NATO ASI Series, vol. 351, Kluwer Academic Publishers, Series C: Mathematical and Physical Sciences, Dordrecht, pp. 187–204. Luther III., G.W., Sundby, B., Lewis, B.L., Brendel, P.J., Silverberg, N., 1997. The interaction of manganese with the nitrogen cycle in continental margin sediments: alternative pathways for dinitrogen formation. Geochimica et Cosmochimica Acta 61, 4043–4052. Luther III, G.W., Church, T.M., Powell, D., 1991. Sulfur speciation and sulfide oxidation in the water column of the Black Sea. Deep-Sea Research 38 (2a), 1121–1137. Lyubartseva, S.P., Lyubartsev, V.G., 1998. Modeling of the Black Sea anoxic zone processes. In: Ivanov, L., Oguz, T. (Eds.), Ecosystem Modeling as a Management Tool for the Black Sea, vol. 2, NATO ASI Series, 2—Environmental Security, vol. 47. Kluwer Academic Publishers, Dordrecht, pp. 385–396. Mee, L.D., 1992. The Black Sea in a crisis: a need for concentrated international action. Ambio 21, 278–285. Mee, L.D., Friedrich, Gomoiu, M.T., 2005. Restoring the black sea in times of uncertainty. Oceanography 18, 100–111. Millero, F.J., 1991. The oxidation of H2S with O2 in the Black Sea. In: Izdar, E., Murray, J.W. (Eds.), Black Sea Oceanography, NATO ASI Series. Series C: Mathematical and Physical Sciences, vol. 351. Kluwer Academic Publishers, Dordrecht, pp. 205–228. Morgan, J., Ducklow, H., 2000. Bacterial Abundance and Dissolved Organic Carbon. In: Black Sea Ecosystem Processes and Forecasting/Operational Database Management System. Report of the Workshop and project Evaluation Meeting, Istanbul, 15–16 May 2000. METU Institute of Marine Sciences, Erdemli. Murray, J.W., Top, Z., Ozsoy, E., 1991. Hydrographic properties and ventilation of the Black Sea. Deep-Sea Research 38 (2a), S663–S689.

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