Dissolved reactive manganese at pelagic redoxclines (part II): Hydrodynamic conditions for...

Journal of Marine Systems xxx (2011) xxx–xxx

MARSYS-02110; No of Pages 11

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Journal of Marine Systems

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Dissolved reactive manganese at pelagic redoxclines (part II): Hydrodynamicconditions for accumulation

Olaf Dellwig a,⁎, Bernhard Schnetger b, Hans-Jürgen Brumsack b, Hans-Peter Grossart c, Lars Umlauf a

a Leibniz Institute for Baltic Sea Research, (IOW), Seestrasse 15, 18119 Rostock, Germanyb Institute for Chemistry and Biology of the Marine Environment, (ICBM), University of Oldenburg, 26111 Oldenburg, Germanyc Leibniz Institute of Freshwater Ecology and Inland Fisheries, Dept. 3 Limnology of Stratified Lakes, 16775 Stechlin, Germany

⁎ Corresponding author.E-mail address: [email protected] (O

0924-7963/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.jmarsys.2011.08.007

Please cite this article as: Dellwig, O., et al.,cumulation, J. Mar. Syst. (2011), doi:10.101

a b s t r a c t
a r t i c l e i n f o
Article history:Received 15 April 2011Received in revised form 25 July 2011Accepted 19 August 2011Available online xxxx

Keywords:Dissolved reactive manganeseManganese(III)RedoxclineLateral currentsIntrusionsAnoxic basinsBaltic SeaBlack Sea

Dissolved reactiveMn (dMnreact) has been determined at the redoxclines of two anoxic deeps from the Baltic Sea(Landsort Deep and Gotland Basin) and two seasonally anoxic freshwater lakes (Lake Dagow and Fuchskuhle,Germany). This dMnreact fraction is rapidly oxidised under oxygen atmosphere and is assumed to consistpredominantly of Mn(III). There is a distinct increase of dMnreact from the outer regions towards the centralpart of the Landsort Deep. Although the presence of MnOx particles in the Gotland Basin is evidence of ongoingoxidation of reduced Mn species, almost no dMnreact was detected. Since completely different processes of Mnoxidation appear rather unlikely, we suggest oceanographic properties are responsible. Lateral currents andintrusions in the Gotland Basin seem to prevent the formation of a stable suboxic zone, a prerequisite necessaryfor accumulation of dMnreact. Such perturbations supply trace amounts of O2 and H2S, causing either immediateoxidation/reduction of dMnreact or deterioration of its stabilising ligands. dMnreact has also been determined inLake Dagow with values significantly exceeding the level of the Landsort Deep due to stable stratification ofthis lake. In contrast, H2S appearance throughout the entire water column and a pH b5 prevent accumulationof dMnreact in Lake Fuchskuhle.

. Dellwig).

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Dissolved reactive manganese at pelagic red6/j.jmarsys.2011.08.007

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Pelagic redoxclines separating oxygenated surface waters fromsulfidic bottomwaters are a prominent feature of stratified anoxic basins.In the Black Sea, the redoxcline typically coincides with the pycnocline,which prevents unrestricted turnover of water masses and at the sametime favours pronouncedO2 consumption and the formation of a suboxictransition zone. This suboxic transition is defined as the zone where O2

and H2S fall below the micromolar level (Murray et al., 1989, 1995)without overlap of both constituents (Glazer et al., 2006). Such zonesrange from a few to some tens of meters in thickness (Glazer et al.,2006; Percy et al., 2008; Yakushev et al., 2007). In accordance withTrouwborst et al. (2006), the suboxic zone will be defined in the follow-ing as the zone where O2 and H2S drop below 3 μM and 0.2 μM,respectively.

Intense microbial activity within this transition (e.g. Detmer et al.,1993; Jost et al., 2008; Labrenz et al., 2007; Taylor et al., 2006; Teboand Emerson, 1986) results in pronounced nutrient and redox-sensitivetrace metal cycles, which are characterised by steep concentrationgradients (e.g. Neretin et al., 2003; Percy et al., 2008; Tebo, 1991; Trefryet al., 1984; Yakushev et al., 2007, 2008).

In numerous biogeochemical investigations (e.g. Boström andIngri, 1988; Clement et al., 2009; Dellwig et al., 2010; Schippers etal., 2005; Tebo et al., 2004) and modelling approaches (e.g. Konovalovet al., 2004; Yakushev et al., 2007) special attention has been paid tothe behaviour of Mn at pelagic redoxclines. Manganese is highlysensitive to changes in redox conditions, resulting in the formationor dissolution of particulate Mn phases. In this context the term“Mn pump” is based on the bacterial oxidation of upward diffusingMn(II) resulting in prominent enrichments of MnOx particles in theuppermost part of the redoxcline (Tebo and Emerson, 1986). Oncethese particles are formed they gravitationally descend through thewater column and re-dissolve when reaching H2S containing waters.Therefore, Mn acts as an important electron donor and acceptor insuch environments. Despite the transfer of O2 during sinking ofMnOx particles, the “Mn pump” also transports certain trace metalsinto the anoxic zone, which are scavenged during formation anddescent of MnOx particles. This process presumably plays animportant role for the trace metal transfer into anoxic basins andreflects the trapping function of such systems (e.g. Brumsack, 2006;Dellwig et al., 2010). For this reason sapropels, i.e. sediments deposit-ed in euxinic basins, provide ideal archives for paleo-oceanographicstudies.

The fundamental processes of the “Mn pump”, such as Mn(II)oxidation andMnOx reduction, aswell as specific oxidants and reductantshave been investigated intensively by several authors during the past

oxclines (part II): Hydrodynamic conditions for ac-

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2 O. Dellwig et al. / Journal of Marine Systems xxx (2011) xxx–xxx

decades (e.g. Luther, 2005; Neretin et al., 2003; Parker et al., 2004;Schippers et al., 2005; Tebo and Emerson, 1986). An important stepforward in understanding the Mn cycle was the discovery of Mn(III) asan intermediate during oxidation of Mn(II) (Kostka et al., 1995; Luther,2005; Webb et al., 2005). Trouwborst et al. (2006) found evidence ofthe existence of metastable Mn(III) in the suboxic zone of the Black Sea,which amounts to almost 100% of the total dissolved Mn pool at somedepth intervals. The formation of Mn(III) is supported by the fact thattwo one-electron transfer steps are energetically more advantageousthan a single two-electron transfer when oxidising soluble Mn(II) toparticulate MnOx (Luther, 2005). Although Mn(III) tends to dispropor-tionate at high pH, it can be stabilised in the suboxic zone by a varietyof ligands including pyrophosphate (Klewicki and Morgan, 1998; Kostkaet al., 1995; Webb et al., 2005) or siderophores (Duckworth and Sposito,2005; Faulkner et al., 1994; Heintze andMann, 1947; Parker et al., 2004).

Schnetger and Dellwig (in press) developed a simple method to de-termine the reactive fraction of dissolved Mn (dMnreact) at pelagicredoxclines, which is easily oxidised in an oxygen atmosphere. Accord-ing to this method dMnreact can bemeasured as the difference betweentotal dissolved Mn passing through a 0.2 or 0.4 μm filter and the dis-solved Mn(II) remaining after oxidation of dMnreact within less than48 h. The application of this method during cruise M 72–5 with R/V“Meteor” in the Black Sea in 2007 resulted in dMnreact profiles similarto Mn(III) presented by Trouwborst et al. (2006). This suggests themethod to be a semi-quantitative measure of Mn(III).

Furthermore, Trouwborst et al. (2006) determined Mn(III) in thesuboxic zone of Chesapeake Bay, indicating Mn(III) as an importantintermediate in various suboxic environments. Our method wastherefore applied to two anoxic deeps in the Baltic Sea (LandsortDeep and Gotland Basin) and to two seasonally anoxic freshwaterlakes Dagow and Fuchskuhle (Germany) in order to support thewidespread occurrence of dMnreact at pelagic redoxclines. The maingoal of this contribution is to provide insights into the environmentalconditions favourable for dMnreact accumulation in stratified aquaticecosystems. Although the Baltic Sea is most comparable to the BlackSea, the chosen sites offer the opportunity to investigate the responseof the Mn cycle under different hydrodynamic conditions. Due totheir temporally stable redoxcline, seasonally anoxic lakes are alsooften subject to intense Mn cycling (e.g. Davison, 1993; Sigg et al.,1987 and references therein), providing the opportunity to studythe above processes in a freshwater environment.

2. Geological and hydrographical setting

The Baltic Sea is one of the largest brackish water bodies of theworld. Salinity gradually decreases from the Skagerrak/Kattegat inthe west towards the northern parts in the Bothnian Sea, forming alarge estuary. While lower salinity surface waters leave the BalticSea via the Skagerrak/Kattegat, salty bottom waters enter the BalticSea from the North Sea. The formation of the present Baltic Sea iscomparable to the Black Sea as both environments were formed dur-ing the Holocene sea-level rise. A precise timing of the establishmentof the present brackish conditions in the Baltic Sea (Littorina stage) isstill under debate due to the lack of a robust geochronology. However,it is assumed that first marine/brackish intrusions occurred at 9,800BP (Zillén et al., 2008).

While the Baltic Sea is comparatively shallow (average depth about52 m; Reinheimer, 1995), several deeps form important topographicincisions. Currently, the water bodies of the Landsort Deep and theGotland Basin are well stratified due to a permanent pycnocline locatedbetween 60 and 80 m depth. The Gotland Basin represents the largestanoxic/euxinic setting in the Baltic (max. water depth 249 m). In con-trast, the anoxic Landsort Deep forms thedeepest site in the Baltic Proper(max. water depth 459 m), but in comparison has a small spatial extent.

The most pronounced difference between the Baltic and the BlackSea is the lower areal extent and less stable redoxcline of the Baltic

Please cite this article as: Dellwig, O., et al., Dissolved reactive manganecumulation, J. Mar. Syst. (2011), doi:10.1016/j.jmarsys.2011.08.007

deeps. While anoxic conditions have prevailed in the Black Sea forapproximately 7500 years (e.g. Arthur and Dean, 1998), the deepsof the Baltic Sea are sporadically intruded by oxic salt water pulses,which may lead to complete oxygenation of the deeps (Fonselius,1962; Jakobsen, 1995; Matthäus and Frank, 1992; Schinke andMatthäus, 1998). The last prominent oxygenation event occurred in2003 (Feistel et al., 2003) and sufficient time has elapsed for the re-establishment of a redoxcline, reflecting similar chemical gradientsin the Baltic deeps and the Black Sea (e.g. Yakushev et al., 2007).

Lake Dagow and Lake Grosse Fuchskuhle are located in theMecklenburg-Brandenburg Lake District approx. 80 km north of Berlin(north-eastern Germany). Lake Dagow is a eutrophic lake with anarea of 0.24 km2 and a maximum depth of 9.5 m. The lake has a pH ofup to 9.1 at the surface and each year an anoxic hypolimnion developsafter the onset of thermal stratification (March until September; Casper,1985).

Lake Grosse Fuchskuhle (area: 0.02 km2;maximumdepth: 5.6 m) isa naturally acidic bog lake that was artificially divided into four basinsby large plastic curtains for biomanipulation experiments in 1990. Thesouthwest basin is the most acidic (pH down to 4.2) since it is mostinfluenced by water runoff from the adjacent bog area. Therefore,humic substances account for up to 58% of dissolved organic matter(DOC) in the SW basin (Allgaier and Grossart, 2006; Hutalle-Schmelzeret al., 2010). The lake is thermally stratified fromMarch until Novemberand has an anoxic hypolimnion rich in H2S.

3. Materials and methods

3.1. Sampling sites

Fig. 1a shows the sampling sites in the Baltic Sea during a cruise withR/V “Professor A. Penck” in July 2008 (cruise No. 07PE/08/14). Samplingwas done along transects across the Landsort Deep and the GotlandBasin. The transect in the Landsort Deep consists of four stations andwas carried out in E–W direction: LD-1 58°36.22 N, 18°42.44E, 216 m;LD-2 58°38.33 N, 18°31.36E, 219 m; LD-3 58°40.37 N, 18°19.32E, 440 m;LD-4 58°34.99 N, 18°13.94, 459 m. Sampling in the Gotland Basin wasdone at three stations in SW direction: GB-1 56°52,65 N, 20°00,08E,170 m; GB-2 57°10,08 N, 19°59,99E, 230 m; GB3- 57°18,46 N, 20°03,90E, 241 m. The water samples were obtained using a CTD-bottle-rosette. Station GB 3 was also sampled during a cruise with R/V “Alkor”in April 2009.

Sampling at Lakes Dagow (53°10'N, 13°03'E) and Fuchskuhle(southwest basin; 53°08'N, 13°02'E) was carried out in April 2009using a peristaltic pump onboard a rowboat and on a pontoon,respectively. Water samples were taken in the central parts of thelakes.

3.2. Sample preparation

For determination of dissolved reactive Mn (dMnreact), watersamples were taken in accordance to the method described bySchnetger and Dellwig (submitted for publication). Samples weretaken directly from the CTD-bottle-rosette (Baltic Sea) and peristalticpump (freshwater lakes) using pre-cleaned 50 mL PE syringes. Forthe determination of total dissolved Mn 20 mL of the sample was im-mediately transferred into pre-cleaned PE bottles via 0.45 μm SFCAsyringe filters and acidified to 1 vol.% HNO3 (Merck, ultrapure) forpreservation. These samples were also used for analysing phosphate.The remaining water sample in the syringe was stored dark underambient temperature and atmospheric oxygen. After 48 h a secondaliquot of 20 mL was filtered and acidified for assessment of dissolvedMn(II).

For particle analysis by SEM-EDX, 1 L of water was filtered imme-diately after collection through 0.4 μm Millipore Isopore membranefilters (polycarbonate) at two sites (LD 4 and GB 3) in the Baltic Sea

se at pelagic redoxclines (part II): Hydrodynamic conditions for ac-




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Fig. 1. Map of the study sites in the Baltic Sea showing transects across the Landsort Deep (LD 1–4) and the Gotland Basin (GB 1–3).

3O. Dellwig et al. / Journal of Marine Systems xxx (2011) xxx–xxx

and at Lake Dagow. Filters were rinsed with 60 mL of purified waterand dried at 60 °C for 48 h.

Samples for ammonia, phosphate, and H2S (100 μL 5% Zn-acetate)were stored in reaction tubes and kept frozen until analysis. Prior to anysampling, water was taken from the CTD-bottles for O2 determinationwith special Winkler flasks in the Baltic Sea, while an oxygen sensorwas used in situ at Lake Dagow and Fuchskuhle.

3.3. Geochemical analysis

Dissolved Mn and Fe from the Baltic Sea were determined directlyfrom 10-fold diluted samples by HR-ICP-MS (Element II, Thermo Fisher)in 2008. Samples from 2009 were analysed using ICP-OES (iCAP 6300Duo; Thermo Fisher). Phosphate was measured by ICP-OES (iCAP 6300Duo; Thermo Fisher). For ICP-OES measurements, the samples werespiked with Sc as an internal standard and measured using a MicroMistnebulizer (Glass Expansion). Accuracy andprecision of allmeasurementswere checked by spiked CASS-4 solutions (National Research CouncilCanada) and are b5%. Determination of ammonia and O2 by Winklertitration was carried out using the procedures described in Grasshoff etal. (1983). At Lake Dagow and Fuchskuhle an oxygen sensor (Oxi 330IWTW) was used. H2S was determined spectrophotometrically usingthe method of Cline (1969).

3.4. SEM-EDX

Scanning electron microscopy (SEM) and energy dispersive X-raymicro-analyses (EDX) were performed on a FEI Quanta 400 microscopeconnected with an EDAX-Genesis system. The X-ray microanalyses foridentification and quantification of the elements after ZAF-correctionwere done by spot analyses on selected particles taking EDX-spectra(EDAX-Econ 4 detector). The problem of peak overlapping, especiallyfor theMn kβ and Fe kα lines at 6.4 to 6.5 keVwas solved by holographic


peak deconvolution (HPD),which takes the calculated ratio to theMnkαand Fe kβ lines into account (Leipe et al., 1999).

3.5. CTD

Temperature and conductivity (salinity) were measured by theSeaBird CTD (SB911). O2 was determined in parallel by a sensorattached to the CTD. Turbidity was measured by using a BackScat IIfluorometer (Model 1302).

4. Results and discussion

4.1. Oceanographic properties

Fig. 2 shows representative water column profiles from the deepestparts of the Landsort Deep (LD 4) and the Gotland Basin (GB 3), fromthe SE Black Sea site BS 23 (Schnetger and Dellwig, submitted forpublication) as well as from the freshwater lakes Dagow andFuchskuhle. Although the profiles from the Baltic and Black Seas displaycomparable patterns with respect to O2, temperature, and salinity,certain differences were observable between the three sites. While asteep gradient is seen for O2 in the Landsort Deep and Gotland Basin,the decrease of O2 covers a larger interval in the water column of theBlack Sea. Additionally, slightly below the pycnocline (ca. 70 m) O2

drops below the detection limit in the Landsort Deep. The distancebetween the redoxcline (ca. 120 m) and the pycnocline (ca. 70 m) isdistinctly higher in the Gotland Basin because small amounts of O2 arestill detectable until approximately 120 m in depth. This specific patternin the Gotland Basin is most likely due to lateral currents, which will bediscussed in detail in Section 4.3. Comparison of sampling sites withinthe Baltic Sea with that of the Black Sea shows that O2 gradients aregenerally steeper in the Baltic Sea.

The freshwater lakes Dagow and Fuchskuhle also reveal a redoxclinebelow the pycnocline as indicated by conductivity and temperature. O2






decreases below 3 m and disappears at 6.5 m depth in Lake Dagowresulting in a redoxcline about 2 m above the sediment. A comparablepattern is seen for Lake Fuchskuhle, however, O2 starts to decrease inthe uppermost part of the water column. Furthermore, conductivityand pH (not shown) are distinctly lower in this lake. While the pHlevel decreases from 8.8 to 7.5 with increasing depth at Lake Dagow,the humic-rich waters from the Fuchskuhle show distinctly lowervalues, varying between 4.5 and 4.7.

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4.2. dMnreact in anoxic deeps of the Baltic Sea and anoxic freshwater lakes

4.2.1. The Landsort DeepThe vertical patterns of total dissolved Mn (total Mndiss) and

dissolved reactive Mn (dMnreact) along with O2, H2S, PO4, and NH4 arepresented for four sites from the Landsort Deep in 2008 in Fig. 3. Thesesites are located on an EW transect from the outer area (LD 1) towardsthe deepest part (LD 4), reflected by water depths increasing from216 m to 459 m (Fig. 3). The O2 and H2S profiles reveal distinctdifferences for each sitewith themost pronounced redoxcline appearingat site LD 3. At this site, the occurrence of a suboxic zone with O2

concentrations below 5 μM and without significant H2S appearance isclearly visible. This zone reaches a thickness of approximately 7 m andis indicated by the grey bar in Fig. 3. Such a suboxic zone is hardly

5 10 15

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S [g/kg] S [g/kg] S [g/kg]

Fig. 2. Oceanographic properties of the deepest sites in the Landsort Deep and theGotlandBasin, from the Black Sea (site BS 23; Schnetger and Dellwig, submitted for publication),and from the freshwater lakes Dagow and Fuchskuhle. Grey bars indicate the approximateposition of the redoxclines, i.e. the transition between oxygenated and sulfidic waters.


recognizable at sites LD 2 and 4 where it may span at most 1 or 2 m, re-spectively. AlthoughH2S samples are only available for the deeper part ofthe redoxcline at site LD 4, a certain overlap of O2 and H2S isfeasible when extrapolating the H2S pattern; an assumption most likelytrue for site LD 2 as well. Thus, in both cases the grey bars only servefor orientation as no clear suboxic zone could be identified. The

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Fig. 3. Vertical distribution of O2, H2S, NH4, PO4, total dissolved Mn and dissolved reac-tive Mn (dMnreact) at sites LD 1–4 in the Landsort Deep. The grey bars indicate the sub-oxic zone at site LD 3 (O2 b3 μM and H2S b0.2 μM) and provide an idea about theapproximate position of this zone at sites LD 2 and 4.




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alternating appearance of O2 and H2S at the most marginal site LD 1 alsoprevents identification of a clearly defined redoxcline. Possible analyticalor oxidation artefacts during H2S measurements can be ruled out whenconsidering the similar NH4 profiles especially at sites LD 1 and 2.

The corresponding profiles of dMnreact reveal a pronouncedoccurrence only at site LD 3, reaching a maximum value of about3 μM within the suboxic zone. When comparing the position, shape,and level of the dMnreact distribution, distinct similarities with theprofile from the Black Sea site 23 are evident (Fig. 4; Schnetger andDellwig, submitted for publication). However, the zone of dMnreact

accumulation is much smaller (about 10 m) at site LD 3 whencompared with the Black Sea site where dMnreact is present in aninterval of at least 30 m. In contrast, sites LD 2 and 4 show distinctlylower values of dMnreact within a narrow depth interval in theuppermost part of the redoxcline. At site LD 1 dMnreact behavesirregularly with values close to the detection limit of the methodapplied. Only one sample in the deeper part of the redoxcline showsa significant enrichment in dMnreact, which corresponds to a drop inH2S and a slight increase in O2 concentration.

The good agreement between the dMnreact pattern of Black Sea site23 (Fig. 4) and polarographic measurements of Mn(III) in the BlackSea by Trouwborst et al. (2006) strongly suggests that Mn(III) formsthe major fraction of dMnreact (Schnetger and Dellwig, submitted forpublication). Additionally, the dMnreact data are comparable to someMn-bound profiles determined by Pakhomova et al. (2009) andYakushev et al. (2009). The authors also suggest that this Mn fractionpredominantly consists of Mn(III) and emphasize pyrophosphate asan important stabilising ligand. Similar to our dMnreact data from theLandsort Deep, Trouwborst et al. (2006) also found a high variabilityin Mn(III) appearance for 12 sites in the Black Sea on both a spatialand partly temporal scale. The authors determined highest Mn(III)abundances mostly at south-western sites and postulated a favourableinfluence of mixed waters from the Bosporus and the cold intermediatelayer (CIL) on the Mn redox cycle and Mn(III) production, respectively.This assumption will be discussed in more detail in Section 4.3.

When comparing the dMnreact distributions at the investigatedsites in the Landsort Deep some preliminary conclusions can bedrawn about the behaviour of this reactive Mn fraction at pelagicredoxclines. A temporally and spatially stable redoxcline forms thecrucial prerequisite for accumulation of dMnreact. Furthermore, lowconcentrations of O2 (b10 μM) and H2S (b5 μM) seem to be essentialfor the stabilisation of dMnreact, which is most likely favoured by

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Fig. 4. Vertical distribution of O2, H2S, PO4, total dissolved Mn and dissolved reactiveMn (dMnreact) from the Black Sea site 23 (modified after Schnetger and Dellwig, sub-mitted for publication). The grey bar indicates the suboxic zone (O2 b3 μM and H2Sb0.2 μM).


complexes with organic or inorganic ligands such as microbial sidero-phores, humic acids, and pyrophosphate as also suggested for Mn(III)(e.g. Duckworth and Sposito, 2005; Faulkner et al., 1994; Klewicki andMorgan, 1998; Kostka et al., 1995). While an elevated O2 level causeseffective oxidation of dMnreact, higher H2S concentrations result in re-duction of dMnreact or inhibit microbial oxidation of Mn(II). The abiot-ic reduction of Mn(III) by H2S was investigated by Kostka et al. (1995)

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Fig. 5. Depth profiles of O2, H2S, NH4, PO4, total dissolved Mn and dissolved reactive Mn(dMnreact) at sites GB 1–3 in the Gotland Basin.









Landsort Deep LD480 m

Gotland Basin GB 3120 m

Mn: 61 %Mn: 59 %Fe: 6.9 %P: 1.1 %Si: 2.2 %

Lake Dagow6.5 m

Mn: 42 %Ca: 7.7 %P: 2.5 %Si: 1.3 %

Black Sea site 23132 m

Mn: 54 %Fe: 2.4 %P: 1.1 %Si: 1.2 %

Fig. 6. SEM-photographs and results of elemental analyses by SEM-EDX of MnOx particles from the upper part of the redoxclines from the Landsort Deep, Gotland Basin, Black Sea,and lake Dagow.


who showed that Mn(III) pyrophosphate complexes are reduced byH2S to Mn(II) and S0 within seconds.

4.2.2. The Gotland BasinAll three sites from theGotland Basin (GB) in 2008were located on a

transect from the shallower southern part towards the deepest part ofthe basin, reveal no clear suboxic zone when considering O2 and H2S(Fig. 5). In fact, at the deepest site (GB 3) both parameters overlap at120 m depth. Less pronounced sites GB 1 and 2 show similar trends aswell. Additionally, these sites are subject to O2 intrusions therebypreventing the formation of a clearly defined suboxic zone. The profilesof dMnreact are near the detection limit even at the deepest site GB 3.Only two depth intervals from sites GB 1 (125 m) and GB 3 (120) reveala very slight accumulation of dMnreact reaching 0.3 μM and 0.12 μM,respectively. However, the pattern and comparable concentrationlevel of particulateMn (Mnpart) as well as the occurrence ofmorpholog-ically identicalMnOx particles in the upper part of the redoxcline at sitesLD4 andGB 3 shows evidence of ongoingMnoxidation (Figs. 3, 5and 6).

Different pathways of Mn(II) oxidation that are based on the com-petition between a direct enzymatic oxidation via Fe catalysis and atwo step oxidation via Mn(III)-siderophore intermediates, were sug-gested by Parker et al. (2007). The authors investigated the behaviourof two Mn(II) oxidising bacterial strains of Pseudomonas putida undervarying Fe concentrations, which are frequently observed in naturalenvironments. Their results suggested that under sufficient Fe supplyparticulate MnOx is produced, while under Fe-limiting conditions Mn(III)-siderophore intermediates are dominating. Although higherconcentrations of dissolved Fe are seen at depth intervals of maximumMn(III) occurrence in the Gotland Basin (e.g. GB 3 125 m: dMnreact0.12 μM, Fe(II) 0.25 μM vs. LD 3 88m: dMnreact 3.1 μM, Fe(II) 0.07 μM),this Fe level is generally lower when compared with the laboratory


experiments performed by Parker et al. (2007), thus pointing towardsFe-limitation and preferential formation of Mn(III), respectively.

Another possibility for the varying abundance of dMnreact may be dif-ferences in composition and concentration of stabilising ligands likesiderophores or pyrophosphate (e.g. Faulkner et al., 1994; Klewicki andMorgan, 1998; Parker et al., 2004; Yakushev et al., 2007). However,comparison of the profiles of phosphate and dMnreact from site LD 3and Black Sea site 23 reveals an opposite behaviour (Figs. 3 and 4).While a parallel pattern of both parameters is seen in the Black Sea,site LD 3 shows slightly decreasing phosphate concentrationswith increasing dMnreact values. Nevertheless, independent of theoccurrence of dMnreact, phosphate concentrations are generallyhigh at the investigated redoxclines indicating sufficient potentialfor stabilisation of intermediate Mn species. Additionally, when con-sidering the high variability of dMnreact occurrence in the LandsortDeep and the small spatial extent of the transect, distinct differencesin ligand composition appear rather unlikely. Therefore, we proposethat the individual hydrodynamic properties of each site are respon-sible for the accumulation or absence of dMnreact, as already sug-gested in Section 4.2.1. Such a conclusion is in accordance with theprofile from site GB 3 in 2009 showing a significant drop in dMnreact

appearance at the depth with an intrusion of O2 containing watermasses.

4.2.3. dMnreact in two seasonally anoxic freshwater lakesAn accumulation of dMnreact is also seen in the seasonally anoxic

LakeDagow reachingmaximumvalues of approximately 6 μMbetween7.5 and 8 m depth (Fig. 7). While dMnreact amounts to almost 100% oftotal Mndiss at 7.5 m depth, it represents only one third just half ameter below. This relative decrease of dMnreact percentage ismost likelyrelated to rapidly increasing concentrations of H2S exceeding 30 μM at



0 0.5 1 1.5Mnpart [µM]

0

2

4

6

8

10

dep

th [

m]

O2 [µM]

Dagow

0 10 20 30

0

1

2

3

4

50 1 2

dep

th [

m]

H2S [µM]

Fuchskuhle

0 1 2 3

0 0.5 1.0 1.5

PO4 [µM]

total Mndiss [µM]dMnreact [µM]

0 0.5 1.0 1.5

0 200 400

0 200 4000 25 50

Fig. 7. Vertical distribution of O2, H2S, PO4, total dissolved Mn and dissolved reactiveMn (dMnreact) in the freshwater lakes Dagow and Fuchskuhle. The grey bar indicatesthe suboxic zone in Lake Dagow.


8 m depth. However, the dMnreact fraction of Lake Dagow shows adistinctly higher tolerance against H2S when compared with the Land-sort Deep or Black Sea. In this particular case, such a difference maybe related to a differing composition of stabilising ligands, asmentionedin Section 4.2.2. While the freshwater of Lake Dagow contains highamounts of terrestrial humic acids (ca. 30%) and DOC values up to990 μM (H.-P. Grossart, pers. comm.) the DOC concentration in theGotland Basin only reaches a maximum level of ca. 300 μM in 100 mwater depth (Schneider et al., 2000).

Nevertheless, the general behaviour of Mn appears similar in thefreshwater Lake Dagow and the brackish environments, as indicatedby the depth profile and morphology of formed MnOx particles(Figs. 6 and 7). However, completely different behaviour is seen forLake Fuchskuhle where no dMnreact accumulates throughout theentire water column. Furthermore, the almost constant pattern oftotal Mndiss, which reveals only slightly decreasing values when O2

increases, indicates less pronounced Mn cycling. This difference ismost likely caused by H2S appearing throughout the entire watercolumn and the pH level being generally b5 in Lake Fuchskuhle.Although the H2S level is comparatively low its appearance may causeunfavourable conditions forMn2+oxidising bacteria or at least immedi-ate reduction of formed dMnreact. As was experimentally shown byKawashima et al. (1988), the pH also influences the rate of Mn ox-idation. Even after 72 the authors found no precipitation of MnOx

in unfiltered lake water at pH 5.8, whereas MnOx is formed when


the pH is adjusted to 7.4 and 9.0, which is in the range of Lake Dagow(pH 7.5–8.8).

4.3. Relationship between hydrodynamic conditions and accumulation ofdMnreact

In order to explain low dMnreact in the Gotland Deep, variableconcentrations in the Landsort Deep, and the significant abundancein the Black Sea, fundamental differences between these settingshave to be taken into account. While the redoxcline has beenestablished in the Black Sea for at least 7500 years (e.g. Arthur andDean, 1998) the redoxclines of the Baltic Sea are subject to frequentperturbations. For certain meteorological and hydrographical precon-ditioning, even the deepest parts of the central Baltic Sea are sporad-ically flushed by salty waters intruding from the North Sea (Meier etal., 2006). Depending on their intensity, these major Baltic inflowevents (MBIE) may lead to the complete oxygenation of the deepsor at least to a lowering of the redoxcline (e.g. Ingri et al., 1991;Pohl and Hennings, 1999). Such MBIE will have a significant impacton microbial communities and redox cycles within the deeps. Dueto its geographical position the Gotland Basin is more often affectedby MBIE when compared with the Landsort Deep.

Since the 1970s the rate and intensity of MBIE has decreased,culminating in a stagnation period of about 16 years, which wasterminated between 1993 and 1994 (Nehring et al., 1995a,b). Since2003, the absence of significantMBIE once again led to the developmentof redoxclines in the central Baltic deeps characterised by trace metaland nutrient profiles comparable to the Black Sea (Yakushev et al.,2007). Additionally, the time necessary to re-establish anoxic andeven sulfidic bottom water conditions is relatively short and spansonly several months (Nausch et al., 2005). Therefore, sufficient timeshould be available for microbial communities to adapt to changingconditions. This assumption is supported by the appearance of dMnreactin seasonally anoxic Lake Dagow where the alternation between oxicand anoxic conditions appears on an annual scale. Consequently, speciallocal water column conditions leading to instabilities of the redoxclineshave to be considered, as indicated by our O2 and H2S profiles (Figs. 3and 5).

Besides the rare occurrence of MBIEs described above, it is suggestedhere that the transformation processes in the redoxcline may be stronglymodified on much shorter time scales by small-scale intrusions, laterallyimporting waters with different bio-geochemical properties. Such intru-sions may result from different processes including (i) detachment andsubsequent interleaving of density-driven bottom gravity currents attheir equilibrium levels, (ii) “detrainment” of partially mixed fluid frominflowing bottom gravity currents and subsequent interleaving,and (iii) double-diffusive frontal interleaving, driven by differencesin the molecular diffusion coefficients of salinity and heat (Kuzminaet al., 2005; Ozgokmen et al., 2006; Zhurbas and Paka, 1999). Allthree processes are likely to occur in the Eastern Gotland Basinthat, different from the Landsort Deep, is directly affected by the cas-cading of density-driven inflows originating in the western parts ofBaltic Sea. Intrusions can be easily identified by their temperatureanomalies, and depending on their origin, they may contain eithera surplus or a deficit in H2S or O2 compared to the values found atthe interleaving depth.

Temperature–salinity (T–S) diagrams are helpful tools for highlight-ing the relationship between intrusions and the variability in dMnreactbecause the signature of intrusions is easily identified by deviationsfrom a straight line (Fig. 8). The most pronounced variations in T and Sacross the entire redoxcline are seen at sites LD 1 and GB 1–3, wheredMnreact is almost completely absent. Enhanced activity of intrusions isalso seen in the deeper part of the redoxcline at site LD-2 (92–127 m)corresponding to the level at which dMnreact disappears. In contrast,sites LD-3 and LD-4 reveal a relatively smooth pattern, which is in ac-cordance with elevated dMnreact abundance. Only small fluctuations



0 5 10 15 206

7

8

9

10

11LD 1 - 2008

5.1 5.3 5.5 5.7

10

11

0 5 10 15 206

7

8

9

10

11LD 2 - 2008

5 5.2 5.4 5.69

10

11

S [

g/k

g]

68-96 m

92-127 m

0 5 10 15 207

9

11

13GB 1 - 2008

5 69

10

11

12103-156 m

0 5 10 15 206

8

10

12LD 4 - 2008

5.2 5.4 5.6 5.8

10

11

78-96 m

70-92 m

T [°C]0 5 10 15 20

6

8

10

12

GB 3 - 2008

5.5 6 6.510

11

12 100-140 m

7

9

11

13

0 5 10 15 20

5.6 6.0 6.4

104-132 m

GB 2 - 2008

0 5 10 15 206

7

8

9

10

11LD 3 - 2008

9.6

10.0

10.4

10.8

5.1 5.3 5.5 5.7

74-90 m

7

9

11

13

3 5 7 9 11

11.5

12

6 7

S [

g/k

g]

S [

g/k

g]

T [°C]

11

12

GB 3 - 2009

120-150 m

17

18

19

20

21

22

6 10 14 18

BS 23

20.3

20.7

21.1

8.2 8.3 8.4 8.5

120-160 m

T [°C]

Fig. 8. Temperature (T) – salinity (S) plots from sites of the Landsort Deep and Gotland Basin and from the Black Sea (site 23; see Schnetger and Dellwig, submitted for publica-tion1). Enlargements provide details of this relationship within the zones where the redoxcline is situated. Numbers in the plots denote water depth ranges.


are seen in the deeper part (N90 m) below the redoxcline at site LD 3.T–S diagrams from the Black Sea site (BS 23) display a linear relationthroughout the redoxcline (Fig. 8), accompanied by a pronouncedpeak of dMnreact in the redoxcline (Fig. 4). In summary, these find-ings suggest a clear negative correlation between the accumulationof dMnreact and the presence of lateral intrusions.

To underline the omnipresence of intrusions at the redoxcline levelin the Gotland Basin during different seasons, we have compiledtemperature and salinity profiles from two additional cruises conductedin January and August 2008. These data have been obtainedwith a free-falling profiler system (MSS90-L from ISW), equipped with precisionCTD sensors (Sea & Sun Tech.), and a fast-response temperature sensor(FP07). The latter provides temperature profiles with a resolution of theorder of 0.01 m, i.e. up to two orders of magnitude higher thanachievable with standard CTD equipment. Moreover, this instrumentwas operated in burst mode with 4 consecutive full-depth profilesobtained in less than 30minwhile the shipwas slowly drifting, yieldingreliable estimates for the vertical and short-term temporal variability ofintrusions.

The two examples from January and August 2008 shown in Fig. 9reveal temperature signals with a remarkable spatial and temporalvariability. In both cases, the signals found in these profiles vary at ascale of less than 1 m in the vertical, and may completely changetheir shape within less than 30 min. Due to the slow drifting of the


ship and the effect of lateral advection, it is more likely that theobserved fluctuations in time represent the horizontal variability ofintrusions, rather than reflecting a local transformation process.Thus, our data indicate the presence of highly distorted (both verti-cally and laterally) patches of intruding water masses, which suggesta large contact surface between ambient and intruding water, and,possibly, enhanced mixing of water with different biogeochemicalcomposition. This picture is rather different from the smooth profilesfound in the central Landsort Deep and in the Black Sea.

The instability of the redoxcline at the investigated sites in theBaltic Sea is in accordance with the O2, H2S, and NH4 profiles(Figs. 3 and 5). The overlapping of O2 and reduced species observedin some of the profiles suggests mixing between oxygenated andeuxinic water masses that are in the vicinity of the strong gradientsbuilt up by the intrusions. Such conditions presumably have animpact on the stability of dMnreact complexes as elevated levels ofO2 and H2S will cause the rapid oxidation or reduction of dMnreact,respectively. Additionally, microbial communities, i.e. Mn(II) andMn(III) oxidising bacteria, have to adapt to an extremely dynamicsystem, which does not provide optimum growth conditions overlonger time periods. Nevertheless, particulate MnOx is present inthe Gotland Basin, indicating ongoing Mn oxidation (Figs. 3, 5 and6). One explanation may be faster dMnreact oxidation rates in theGotland Basin due to intrusions of oxygenated waters, which would





5.8 6 6.2

80

85

90

95

100

105

110

dep

th [

m]

10.5 11 11.5

6 6.2 6.4

110

120

130

140

150

T [ °C]

dep

th [

m]

11 11.5 12 12.5S [g/kg]

23:28 23:38 23:47 23:56

28 Jan 2008

11:16 11:25 11:34 11:43

09 Aug 2008

Fig. 9. High-resolution profiles of water temperature and salinity in the Gotland Basinin January and August 2008. At each cruise, four profiles were carried out within30 min using a free-falling profiler system equipped with precision CTD sensors (Sea& Sun Tech.) and a fast-response temperature sensor (FP07).


significantly shorten the residence time of dMnreact. Higher oxidationrates are also necessary to compensate the loss of dMnreact byreduction via co-existing H2S. This assumption agrees with results

H2S

O2

Mn(II)

dMnreact

MnOx

cixo

bus

stable redoxclinesuboxic zone and dMnreact accumulatio

concentr

dep

th

Fig. 10. Idealised profiles of a stable (left) and a turbulent redoxcline (right). Under stable hydissolved reactive Mn (dMnreact). The turbulent redoxcline is characterised by intense Mnduction of any dMnreact produced. Occurrence of MnOx particles proves ongoing Mn oxidat


from experiments carried out by Schnetger and Dellwig (submittedfor publication), who reported rapidly increasing bacterial oxidationrates under enhanced O2 supply. Additionally, Clement et al. (2009)have shown that Mn(II) oxidation rates increase asymptoticallywith increasing O2 concentration in suboxic waters of the Black Sea.Even O2 levels well below 3 μM stimulated Mn(II) oxidation signifi-cantly. Furthermore, the authors suggested the existence of multipleMn(II) oxidising enzymes.

As mentioned before, Trouwborst et al. (2006) also found astrong spatial dependence of Mn(III) occurrence with highest con-centration levels in the SW part of the Black Sea. The authorsconcluded that intrusions of saline Bosporus waters mixed withO2-rich cold intermediate layer (CIL) waters intensified the Mnredox cycle, thereby favouring the production of Mn(III). This as-sumption is not consistent with our observations in the Baltic Sea,where highest dMnreact levels were observed at the most stableredoxclines. From our data we infer that an additional supply ofO2 via intrusions is not necessary to enhance dMnreact productionsince small amounts of O2 are sufficient for dMnreact accumulation.Moreover, elevated O2 supply seems to cause rapid oxidation ofdMnreact, thus preventing its stabilisation and accumulation. Addi-tionally, the strong variability associated with lateral intrusionsdisturbs redoxcline transformations, and may introduce H2S con-taining waters, which will immediately lead to dMnreact reductionand thus will prevent its accumulation in the lower part of theredoxcline.

Overall, dynamic redoxclines as observed in the Gotland Basin seemto be characterised by intenseMn redox cycling due to the co-existenceof O2 and H2S. Even though Mn(III), which most likely represents themajor constituent of dMnreact determined here, forms an intermediateduring Mn(II) oxidation in such dynamic systems, it is produced in anenvironment unfavourable for significant accumulation. Thus, Mn(III)produced under unstable hydrodynamic conditions is subject to akind of “indirect disproportionation” because O2 injection acceleratesoxidation to MnOx, whereas H2S concurrently will reduce Mn(III) toMn(II). Enhanced turbulence and O2-rich intrusions are not exclusivelyoccurring in the Baltic Sea and are assumed to play an important role forother redox cycles, such as the S cycle (Jørgensen et al., 1991; Konovalov

n

H2S

O2

Mn(II)

negligible dMnreact

MnOx

disturbed redoxclineintense Mn-cycle

without dMnreact accumulation

ation

drodynamic conditions a defined suboxic zone appears which favours accumulation ofcycling due to the co-existence of O2 and H2S which cause immediate oxidation or re-ion in both modes.






et al., 2003; Scranton et al., 2001; Zhang and Millero, 1993; Zopfi et al.,2001).

5. Summary and concluding remarks

A simplemethod for determination of a dissolved reactiveMn fraction(dMnreact) has been applied to the redoxclines of two anoxic Deeps of theBaltic Sea (Landsort Deep and Gotland Basin) and two seasonally anoxicfreshwater lakes (Lake Dagow and Fuchskuhle, northeastern Germany).This dMnreact is rapidly oxidised under an oxygen atmosphere and isassumed to consist mainly of Mn(III). At present, both locations fromthe Baltic Sea are characterised by a stratified water column and sulfidicbottom waters, thus representing an environment comparable to theBlack Sea. However, dMnreact reveals variable concentration levelsincreasing from the outer regions towards the central Landsort Deepwhere the highest values of about 3 μM are observed. In contrast, almostno dMnreact is detected on a transect across the Gotland Basin.

We suggest that the modification of biogeochemical propertiesintroduced by the presence of lateral intrusions offer the most plausibleexplanation for this variability in dMnreact accumulation. The relationshipbetween dMnreact accumulation and hydrodynamic conditions issummarised in the schematic profiles shown in Fig. 10, which presentidealised processes within a stable and a turbulent redoxcline. Stablehydrodynamic conditions provide the opportunity for the formation of adefined suboxic zone where dMnreact that is produced by bacterialoxidation of Mn(II) accumulates. By contrast, in redoxclines subject tointrusions of O2 and H2S containing waters, the accumulation of dMnreactis inhibited. However, both settings are characterised by pronouncedoccurrence of MnOx particles in the uppermost part of the redoxcline,which is evidence of ongoing Mn oxidation. Therefore, we assume thatin this dynamic setting intense Mn cycling occurs because the co-existence of O2 and H2S causes rapid and immediate oxidation orreduction of any dMnreact produced.

High concentrations of dMnreact (max. 6 μM) were also detected inthe seasonally anoxic freshwater Lake Dagow in April 2009, thus indicat-ing a time frame of several weeks necessary for the establishment of themicrobial community and the Mn redox cycle, respectively. AlthoughLake Fuchskuhle is also anoxic in the deeper parts no accumulation ofdMnreact is found. As this lake is characterized by a comparatively stableredoxcline, we suggest that the lack of dMnreact formation is a result oflow pH values (b5) and H2S appearance throughout the entire watercolumn.

Acknowledgements

We thank the captains and crews of R/Vs “Professor Albrecht Penck”,“Alkor”, and “Meteor” for technical support. We are indebted to MichaelGlockzin for taking samples during the cruise with R/V “Alkor” in April2009. Rainer Bahlo, Sebastian Eckert, Eli Gründken, Carola Lehners, andAntje Wegwerth are thanked for analytical assistance and Kathryn Berryfor improving the text. Two anonymous reviewers are thanked for theirconstructive comments. This workwas supported by the German ScienceFoundation DFG through grants BR BR775/12, 15, 23, GR 1540/15-1, theLeibniz Institute for Baltic Sea Research (IOW), and the Leibniz Instituteof Freshwater Ecology and Inland Fisheries (IGB).

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