Hydrobiologia 367: 163–174, 1998. 163c 1998Kluwer Academic Publishers. Printed in Belgium.

Oxygen concentration profiles in sediments of two ancient lakes: LakeBaikal (Siberia, Russia) and Lake Malawi (East Africa)

Patrick Martin1, Liba Granina2, Koen Martens1 & Boudewijn Goddeeris11 Royal Belgian Institute of Natural Sciences, Freshwater Biology Section, rue Vautier 29, B-1000 Brussels,Belgium2 Limnological Institute of the Siberian Division of the Russian Academy of Sciences, Ulan-Batorskaya 3, Irkutsk664033, Russia

Received 6 August 1997; in revised form 6 February 1998; accepted 2 March 1998

Key words:oxygen microprofiles, sediment, ecological segregation, Lake Baikal, Lake Malawi

Abstract

Oxygen concentration profiles have been measured with microelectrodes in sediments of two major ancient Riftlakes: Lake Baikal (Eastern Siberia) and Lake Malawi (East Africa), along depth transects in the constitutive basinsof the lakes including the deepest point, 1680 m, in Lake Baikal. Sediment oxygen penetration depths (SOPs)display very different patterns in the two lakes. In Lake Baikal, SOPs are variable, show no significant relationshipwith bathymetricdepth and are surprisingly deep on Akademichesky ridge (>50.0 mm), emphasizing the distinctivefeature of this region in the lake. While the Selenga river is an important source of eutrophication, the similarityof SOP-values in the Selenga shallow with those of most other sites suggests either a dilution of organic materialby allochthonous matter, or a strong south-to-north transport of particles. In Lake Malawi, available oxygen isrestricted to a maximum of three millimetres of the sediment, and there is a negative relationship with bathymetricdepth, as a result of a steady decline of oxygen concentration with depth through the water column. Amongst thefew parameters known to affect SOPs, the oxygen consumption by the sediment seems the most significant in bothlakes. SOP-values furthermore confirm differences in the trophic status of Baikal and Malawi, respectively. Theimportance of oxygen as a factor likely to create ecological segregation for benthic organisms is discussed. LakeMalawi offers possibilities of bathymetric segregation but no vertical segregation in the sediment. In contrast, nobathymetric segregation related to oxygen is possible in Lake Baikal, but vertical segregation in the sediment isvery likely.

Introduction

Lake Baikal, situated in the Great Eastern Siberian Riftand Lake Malawi, lying in the western arm of the EastAfrican Rift Valley, have many features in common asa result of their tectonic origin. Both are very large anddeep (31 500 km2 and 30 800 km2, maximal depth of1 637 m and 785 m, respectively; Kozhov, 1963; Mar-tin, 1994; Ribbink, 1994) and their location in a stillactive graben trough has permitted their preservationover time despite continuous infilling by sediments.Both lakes are truly ‘ancient’ or ‘long-lived’ (Gorthner,1994) (25–30 Ma and> 2 Ma for Baikal and Malawi,

respectively; Martin, 1994; Ribbink, 1994) and theyhold very diverse endemic faunas.

Due to their own respective position, the twolakes are subject to totally different climatic regimeswhich strongly affect their physics and chemistry. LakeMalawi is meromictic; there is a density discontinuityaround 230 m depth, mainly determined by salinity,sufficient to maintain a perennially deep, oxygenlesshypolimnion (Beauchamp, 1953; Patterson & Kachin-jika, 1995). In Lake Baikal, there is deep-water renewalbelow the dimictic upper 250 m layer (Kozhov, 1963;Weiss et al., 1991; Shimaraev et al., 1993; Hohmannet al., 1997a, b, Killworth et al., 1997). Water circu-lation carries oxygen to the deepest point and makes

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Figure 1. A – Map showing the location of Lake Baikal and the stations sampled in 1991 (dots), 1992 (triangles) and in 1994 (stars). B – Mapshowing the location of Lake Malawi in the southern hemisphere, the two transects sampled in 1994 and the approximate extent of anoxic,hypolimnetic water (shaded area).

the abyssal area of the lake habitable for metazoanorganisms.

Knowledge of the distribution of oxygen in lakesBaikal and Malawi takes on a particular importancegiven the exceptional biological context of these waterbodies. In deep, ancient lakes, oxygen is thought to cre-ate possibilities for ecological segregation, both bathy-metrically and within the sediment. Particularly in thelatter case, a sediment-depth segregation can createpopulations of benthic organisms with a ‘dumb-bell’structure and this may lead to parapatric speciation(Martens et al., 1994).

To date, the distribution of oxygen in the watercolumns of lakes Baikal and Malawi is well document-

ed (Baikal: Tolmachev, 1957; Kozhov, 1963; Votintsev,1990; Weiss et al., 1991; Liebezeit, 1992; Shimaraevet al., 1996; Hohmann et al., 1997a, b; Killworth etal., 1997; Malawi: Beauchamp, 1953; Eccles, 1974;Patterson & Kachinjika, 1995), but data on the distribu-tion of oxygen in the sediment are scarce for both lakes(Martin et al., 1993a, b), because oxygen microelec-trodes were only recently introduced into fresh waterresearch (Sweerts et al., 1986; Sweerts, 1990).

Oxygen measurements in sediments of Lake Baikaland Lake Malawi were first published by Martin et al.(1993a, b). Since then, several new expeditions to theselakes enabled us to add to our data, so that a survey ofthe distribution of oxygen in the sediment can now be

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presented on a lake-wide scale, i.e. from the shallowestto the deepest oxygenated areas (including the deepestpoint in Lake Baikal). The possible influence of oxy-gen on the segregation of organisms in these lakes isdiscussed.

Material and methods

Study sites

Three transects were sampled during the August 1992expedition of the R/V Vereschagin on Lake Baikal,ranging from 23 to 820 m depth, these being repre-sentative of the three constitutive basins of the lake(Martin, 1994; Figure 1A). In 1994, an additional tran-sect was sampled in the widest part of the lake, in thecentral basin, from Barguzin Gulf to the deepest point(1 680 m, as measured by the echosounder), includingfour stations in, or on the slopes of, the subaqueousele-vation called ‘Akademichesky ridge’, which marks theboundarybetween the central and northern basins (Fig-ure 1A). These data supplement the original databasebuilt from sampling three transects in the Selenga shal-low during 1991 (Martin et al., 1993a), a submergedplateau which separates the southern basin from thecentral one (Figure 1A).

In Lake Malawi, two transects, stretching out fromthe Malawi shore to the centre of the basin, were inves-tigated (Figure 1B). Samples were taken from depthsranging from 0.5 m to 140 m (transect 1: a few milessouth of Kambiri Point) and from 0.5 m to 300 m(transect 2: directly off Nkhotakota), hence below theanoxic permanent stratification in the latter case. Ear-lier measurements from the extreme north of the lakehave already been published (Martin et al., 1993b).

Sediment sampling

Sediment samples were taken with a light Reineckbox corer on Lake Malawi (c. 100 kg) while in LakeBaikal, a heavier oceanographic version (c. 1000 kg)was deployed. Subsamples were extracted by meansof a Perspex tube for oxygen measurements (46 mminner diameter, 120 mm long). Tubes were tightlysealed under water in order to avoid trapping air bub-bles inside, and were either immediately immersed intoa thermostatised water bath (Lake Baikal) or kept indark conditions, at ambient temperature, awaiting sub-sequent measurements (1 to 5 hr later; Lake Malawi).

The stopper was removed just before measurements,after a period of stabilisation in the bath.

Oxygen microelectrode measurements

Oxygen microprofiles were measured as described byMartin et al. (1993a). For the second set of measure-ments (1992 and 1994), however, a Clark-style micro-electrode with a guard cathode was used. The absenceof drift, resulting from this improvement (Revsbech,1989), allowed easy identification of the 0% response(representing the oxic-anoxic interface) because thesignal remained at the same stable value when theanoxic sediment was reached, at whatever depth thesensor was sunk.

Distinction of the thickness of oxidized layer

In Lake Baikal, there is a clear colour zonation ofundisturbed sediment cores. The depth of the orange-brown to grey change in the sediment was used as thedepth of the oxidized layer. Jørgensen & Revsbech(1989) showed that such a procedure is valid in mod-erately reducing sediments where Eh is of limited use.In Lake Baikal, the two parameters are clearly related(Leybovich, 1983). In Lake Malawi, a visual distinc-tion of an oxidized layer was not possible.

Factors governing penetration depths of oxygen intosediment

The characteristic penetration depth of oxygen intosediments by molecular-scale diffusion is:

h = 2DsCo�:J�1 (1)

where h is the depth of penetration of oxygen intothe sediment (= SOP),Ds the whole diffusion coeffi-cient in the sediment,Co the oxygen concentration atthe sediment surface,� the porosity andJ the oxy-gen consumption per unit area of the sediment surface(Revsbech et al., 1980a; Revsbech & Jørgensen, 1986).

In practice, the depth of the oxic zone dependslargely on the total rate of oxygen uptake of the sedi-ment and thus on the sedimentation of organic matter(Revsbech et al., 1980a; Reimers & Smith, 1986b;Jørgensen & Revsbech, 1989).

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Figure 2.Oxygen concentration profiles measured in various depths

in Lake Baikal. A – northern basin, B – Akademichesky ridge, C –

dimictic zone of central basin (selected profiles), D – abyssal zone of

central basin (selected profiles), E – southern basin (as for Selenga

delta, see Martin et al., 1993a).

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Table 1. Gazetteer and characteristics of the stations investigated in Lake Baikal during the 1991, 1992 and 1994expeditions: sediment oxygen penetration depths (SOP), thickness of oxidized layer (OL) and temperature of thesediment; concentration of oxygen in near-bottom water (2–5 mm above the sediment).

Station Date Co-ordinates Depth SOP OL Tsed O2

(m) (mm) (mm) (˚C) (�mol l�1)

Northern basin

1 08/08/92 54˚5903000 N 109˚4000500 E 30 9.5 10 6.6 366

2 08/08/92 54˚5903000 N 109˚3903000 E 70 8.1 43 3.9 390

3 08/08/92 54˚5901800 N 109˚3900300 E 105 4.6 12 3.9 327

4 08/08/92 54˚5902400 N 109˚3800000 E 225 5.5 21 3.7 316

5 07/08/92 54˚5801500 N 109˚3502400 E 485 17.0 29 3.7 376

6 07/08/92 54˚4400000 N 109˚1800000 E 820 >50.0 70 3.6 306

Akademichesky ridge

7 11/08/94 53˚3105600 N 107˚5502900 E 275 >44.5 >340 3.7 421

8 09/08/94 53˚3104300 N 108˚5405800 E 310 >42.5 – 3.6 438

9 11/08/94 53˚2801200 N 107˚5205400 E 415 >38.0 180 3.5 413

10 11/08/94 53˚3504200 N 107˚5202200 E 580 >49.5 130 3.8 391

Central basin

11 11/08/92 52˚4202400 N 107˚3300000 E 33 9.0 33 7.5 309

12 11/08/92 52˚4204500 N 107˚3303000 E 65 5.0 20 6.4 254

13 11/08/92 52˚4400000 N 107˚4000000 E 110 14.5 32 3.6 328

14 10/08/92 52˚4701500 N 107˚4800000 E 240 3.0 23 4.2 295

15 10/08/92 52˚4600000 N 107˚4700000 E 420 13.8 23 3.9 313

16 10/08/92 52˚4604800 N 107˚4203000 E 820 6.1 7 3.5 266

17 07/08/94 53˚2600500 N 108˚4202100 E 200 1.1 1 – 296

18 07/08/94 53˚1803600 N 108˚2405500 E 810 3.6 6 3.6 364

19 08/08/94 53˚2303400 N 108˚3603000 E 1250 1.1 1 4.4 253

20 12/08/94 53˚2504600 N 108˚0504400 E 1365 4.8 19 3.6 385

21 08/08/94 53˚2502200 N 108˚3103400 E 1410 17.3 29 3.6 376

22 06/08/94 53˚0500200 N 107˚2805300 E 1665 4.6 5 3.3 330

23 12/08/94 53˚0901200 N 107˚4801500 E 1680 7.4 46 3.3 394

Selenga delta

24 06/09/91 52˚1800800 N 106˚1205600 E 18 5.5 18 13.7 313

25 05/09/91 52˚1903700 N 106˚0900300 E 79 16.5 45 6.2 331

26 04/09/91 52˚0603500 N 106˚0903600 E 22 6.0 15 13.9 293

27 04/09/91 52˚0600700 N 106˚0505400 E 40 6.5 20 9.7 354

28 04/09/91 52˚0602500 N 106˚0404900 E 60 23.0 50 5.2 423

29 04/09/91 52˚0603600 N 106˚0304800 E 84 10.5 40 5.0 343

30 12/08/92 52˚2401800 N 106˚3003000 E 23 5.0 14 8.5 274

31 05/09/91 52˚2400800 N 106˚3005000 E 25 0.6 1 13.0 65

32 05/09/91 52˚2403200 N 106˚2900500 E 39 4.5 18 8.9 330

33 05/09/91 52˚2501000 N 106˚2604600 E 65 11.0 27 6.9 345

34 05/09/91 52˚2500700 N 106˚2500400 E 75 15.0 30 7.1 346

Southern basin

35 15/08/92 52˚0502900 N 105˚5501500 E 245 4.7 7 5.7 284

36 15/08/92 52˚0605000 N 105˚4903600 E 430 2.0 30 3.7 234

37 13/08/92 52˚1001800 N 105˚5000000 E 810 21.3 28 3.7 335

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Figure 3. Relationship between sediment oxygen penetration depthand thickness of the oxidized layer of the sediment. The open sym-bols refer to stations where the anoxic layer was not reached (seetext) and which were omitted for computing the correlation coeffi-cient (r = 0.669,P�0.05, n = 32; Pearson PMC). The straight lineindicates the significant linear regression (y= 1.54x + 9.01;P>0.8),extrapolated for x-values out of the scatter plot and flanked by its95% confidence intervals.

Results

Lake Baikal

Most oxygen microprofiles in Lake Baikal (Figure 2)have a sigmoid shape, which is typical for sedimentsoutside of photosynthetic influence (Revsbech et al.,1980b; Revsbech & Jørgensen, 1986; Sweerts, 1990),except for profiles measured in Akademichesky ridgestations, which show much variability in oxygen pen-etration depths. SOP-values in this lake (Table 1) varygreatly: they range from 0.6 mm (Selenga delta) tomore than 50.0 mm (Akademichesky ridge and north-ern basin). In addition, oxygen profiles at Akademich-esky ridge sites can have peculiar shapes, as sud-den increases in oxygen concentrations are noticedat depths where values would have been expected todecrease (stations at 415 m and 580 m, near 35 mm;Figure 2).

Overlying waters of Lake Baikal are constantlywell oxygenated: oxygen concentrations measured at2–5 mm above the sediment are never lower than230�mol l�1 (390–440�mol l�1 on Akademicheskyridge) (Figure 2, Table 1), except for one station near

the Selenga delta (65�mol l�1, 25 m). The latteranomaly, caused by local enrichment of sediment withorganic matter, was discussed by Martin et al. (1993a).

As a rule, there is no relationship between SOPand bathymetric depth (r�=0, P�0.05,n= 32, Pear-son Product Moment Correlation). In contrast, SOPis positively correlated with the thickness of the oxi-dized layer (Figure 3). Figure 3 and Figure 4 show aclear distinction between Akademichesky ridge sitesand the station at 820 m in the northern basin, and allother stations. They fully confirm the special natureof these sites, as further suggested by the pattern ofoxygen profiles.

There is a statistically significant difference in meanvalues among each basin, Akademichesky ridge andSelenga delta (P< 0.0001; ANOVA), and a multiplecomparison procedure (Newman-Keuls test) allows tostatistically isolate the Akademichesky ridge from allother sites (P< 0.05; Figure 4).

Lake Malawi

While most oxygen profiles in Lake Malawi are typ-ically sigmoid (Figure 5A, B), there is a rapid lineardecrease of oxygen concentrations in overlying watersat 200, 260 and 300 m (Figure 5B). As these sta-tions are located below the oxic-anoxic boundary,zero-values of oxygen are expected. It is not to be exclud-ed that the pronounced imbalance of oxygen betweenthe atmosphere and the samples caused contamination.Therefore, unstable values of oxygen concentrationmeasured in overlying waters of these stations werenot reported in Table 2.

SOP values in this lake display little variability andare restricted to the very first millimetres of the sedi-ment (0.0–4.0 mm, Table 2). One station at 1 m depthis an exception (>25.0 mm), but it was measured in acore sampled along the sandy shore, exposed to waveaction. Omitting this station in computations gives nostatistically significant difference between SOP-valuesin both transects (t-test,P= 0.058, 17 df).

In overlying waters, oxygen concentrations con-stantly decrease with increasing depth, as expected,with some variability in the surroundingsof 90 m (tran-sect 1; Figure 5), and vary from 100 to�200�mol l�1

in the upper 140 m (Figure 5).No significant correlation between SOPs and bathy-

metric depth was noted for transect 1, but transect2 shows a negative correlation (Figure 6), probablyresulting from the regular decrease of oxygen with

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Figure 4. Comparison between SOP-values measured in the three constitutive basins of Lake Baikal, including the area around Akademicheskyridge and in front of the Selenga delta.

Table 2. Gazetteer and characteristics of the stations investigated in Lake Malawi duringthe 1994 expedition: penetration depth of oxygen in the sediment (SOP) and temperatureof the sediment; concentration of oxygen in the near-bottom water (2–5 mm above thesediment).

Date Co-ordinates Depth SOP TwaterO2

(m) (mm) (˚C) (�mol l�1)

Transect 1: a few miles south of Kambiri Point

13/03/94 13˚4809000 S 34˚3506600 E 1 >25.0 30.0 209

11/03/94 13˚4809500 S 34˚3506300 E 4 4.0 28.5 194

13/03/94 13˚4809700 S 34˚3600900 E 10 0.8 29.1 145

11/03/94 13˚4900000 S 34˚3705600 E 32 1.8 27.4 86

11/03/94 13˚4900400 S 34˚3708300 E 50 1.5 25.9 135

11/03/94 13˚4900200 S 34˚3803000 E 70 0.4 25.0 109

12/03/94 13˚4900200 S 34˚4000500 E 90 2.6 25.7 176

12/03/94 13˚4901800 S 34˚4109900 E 120 2.2 25.6 141

12/03/94 13˚4607000 S 34˚4406900 E 140 0.7 25.5 99

Transect 2: directly off Nkhotakota

16/03/94 12˚5408600 S 34˚1804500 E 1 2.3 29.4 234

16/03/94 12˚5407800 S 34˚1803000 E 5 1.0 29.0 171

16/03/94 12˚5302700 S 34˚2006700 E 14 1.2 28.2 163

16/03/94 12˚5301600 S 34˚2007100 E 50 1.6 27.4 167

15/03/94 12˚5301100 S 34˚2008200 E 70 0.5 25.9 59

16/03/94 12˚5300100 S 34˚2100400 E 90 0.7 24.8 127

15/03/94 12˚5108300 S 34˚2204200 E 120 0.7 25.5 161

15/03/94 12˚5103100 S 34˚2305600 E 140 0.9 25.5 140

15/03/94 12˚5002600 S 34˚2501700 E 200 0.5 24.8 –

15/03/94 12˚4806600 S 34˚2709900 E 260 0.5 24.3 –

16/03/94 12˚4604000 S 34˚3106100 E 300 0.0 24.0 –

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Figure 5. Oxygen concentration profiles measured in selected sta-tions along two transects in Lake Malawi. A – transect 1: a few milessouth of Kambiri Point, B – transect 2: directly off Nkhotakota.

increasing bathymetric depth, characteristic of LakeMalawi (Patterson & Kachinjika, 1995).

Lastly, there is a highly statistically significant dif-ference between SOP-values in Lake Baikal and LakeMalawi (t-test,P= 0.0004, 54 df; Figure 7).

Discussion

Reliability of oxygen measurements

Atmospheric contamination, duration of experimentsand unusually thick ‘diffusive boundary layer’ inducedby lack of stirring are the most important artefacts like-ly to affect oxygen measurements (Reimers et al.,1984,1986a; Silverberg et al., 1987; Brotas et al., 1990).Sampling the abyssal of Lake Baikal poses, howev-

Figure 6. Relationship between sediment oxygen penetration depthand bathymetric depth in Lake Malawi. Open circles refer to tran-sect 2, for which a negative significant relationship was observed(r =�0.752, P<0.05, n = 11; Pearson PMC).

er, additional problems due to a dramatic decompres-sion of samples and, to a lesser extent, to temperaturechanges during coring operations. Conceptually, theseare similar to artefacts encountered by Reimers et al.(1984, 1986a) in the deep ocean; these had no signifi-cant effects on oxygen profiles.

In Lake Malawi, field constraints forced us to post-pone measurements of oxygen profiles, resulting inpossible increase of the boundary layer thickness inunstirred samples and decrease of oxygen flux acrossthe sediment-water interface and of SOP-values (Sund-by et al., 1986). However, Reimers et al. (1986a) men-tioned only (and barely) noticeable changes in SOPs inworse situations (after 6 h of exposition to atmospher-ic contamination, at 25 ˚C) than those we encounteredin the field. Atmospheric contamination has probablynegligible effects on SOPs, as long as there is nota too pronounced imbalance of oxygen between theatmosphere and samples, as was suggested above forthe stations at 200 to 300 m depth.

Intralacustrine variability in Lake Baikal

In Lake Baikal, differences in oxygen concentrationsin the water above the sediment (C0) are generallytoo small to significantly affect SOP-values. There isalways a much greater variability in SOPs than in actu-

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al oxygen concentrations, whatever the figure consid-ered (Figure 2). For example, in the northern basinof Lake Baikal, SOPs range from 4.6 to more than50.0 mm, while oxygen concentration values in theoverlying water are between 300 and 400�mol l�1.Moreover, profiles measured in Akademichesky ridgesites suggest exceptionally high SOP-values for oxy-gen concentrations in overlying water in comparisonto other stations.

Similarly, porosity,�, is known to slightly affectSOPs (Sweerts, 1990), but can here be excludedbecause in the upper layer of cores, grain-size and thusporosity (Meade, 1966; Berner, 1971) was very simi-lar between stations. Lastly, while the whole diffusioncoefficient is affected by temperature, the range of vari-ation in temperature is too small to notably influenceSOPs in Lake Baikal (Ds at 3 ˚C�= 0.7 Ds at 14 ˚C,Sweerts, 1990).

Burrowing activity of infauna can theoretically playa role in oxygenation of natural sediments (Revs-bech et al., 1980a; Sweerts, 1990). However, in mostinstances this activity is deemed negligible. Densitiesof Oligochaeta, that compose up to 90% of the biomassof the bottom communities of the lake (Snimschikovaand Akinshina, 1994), decrease in nearly exact inverseproportion to bathymetric depth, resulting in extreme-ly low densities below the dimictic layer (from nearly30 000 ind m�2 at 20 m to less than 1500 ind m�2 from250 m depth onwards; Martin unpubl. data). Never-theless, their possible influence above that boundarycannot be excluded, especially in the sublittoral zone(20–70 m, Kozhov, 1963) and, hence, in the Selengashallow.

Sedimentation rates tend to increase from the north-ern to the southern basin in Lake Baikal, with maximalvalues near the Selenga delta, but with extremely slowrates on Akademichesky ridge (Edgington et al., 1991;Colman et al., 1993). As variability in SOPs largelydepends on the sedimentation of organic matter, thisshould explain peculiarities in oxygen microprofiles.

The Akademichesky ridge is a very peculiar regionin Lake Baikal, more or less isolated from turbiditedeposition from tributaries. The ridge was recentlyshown to be the location of ample horizontal mixing ofwater masses of different densities and temperatures,between the central and northern basins, responsiblefor deep-water renewal (Hohmann et al., 1997a). Thismassive exchange of waters can supposedly preventsedimentation of organic particles, and a decrease ofthe boundary layer thickness resulting in an increaseof oxygen flux across the sediment-water interface.

Extremely deep SOP values noticed in this area areconsistent with this scenario.

While the source of organic matter in sedimentsof Lake Baikal is primarily controlled by the primaryproductivity in the euphotic zone (Vykhristyuk, 1980;Williams et al., 1993), the supply of allochthonousmatter by the Selenga river to the lake probably doeshave an appreciable influence on the Selenga shal-low. The Selenga river indeed represents 50% of thetotal drainage and 74% of a total of 2,955.109 kg ofsolids annually discharged into the lake by all tribu-taries (Vlasova, 1983; Agafonov, 1990). Dilution oforganic matter in the sediment (both autochthonousand allochthonous), by the mineral fraction carriedalong by the river, is significant (Vykhristyuk, 1980).Usually, however, this positive effect on the oxygenuptake of the sediment is largely counterbalanced bythe highly degradable organic matter depositing on thesediment. High rates of sedimentation in the near deltaareas indeed prevent organic matter from degradationin the water column (ibid.).

In this respect, the similarity of the SOP-valuesin the Selenga shallow compared to most other sitesis intriguing, the more so because the Selenga riveris probably an important source of organic pollution(Stewart, 1990; Gibbs, 1994). This suggests that thereis a strong south-to-north transport of particles. Thishypothesis was already put forward as an explanationof the presence of the station with an exceptionallylow SOP value in the northern part of the Selenga shal-low (Table 1, Martin et al., 1993a). Counterclockwisecyclonic currents are indeed well known to typicallyoperate in front of main inflows (Verbolov, 1977) andrecently, important flow of cold and relatively salinewater from the Selenga river to the Central basin wasreported by Hohmann et al. (1997a).

Intralacustrine variability in Lake Malawi

Lake Malawi is characterized by a steady decline ofoxygen concentration with depth, through the watercolumn. As a result, oxygen concentrations in waterabove the sediment play a significant role for SOPs.This is evident from the negative relationship observedbetween SOPs and bathymetric depth in transect 2 (Fig-ure 6). In practice, the influence of other factors onSOPs are difficult to notice because SOPs are concen-trated in a very thin layer of the sediment.

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Figure 7. Comparison between SOP-values in Lake Baikal and Lake Malawi for similar significant depth ranges (10–260 m).

Comparison Baikal – Malawi

Temperature probably plays an important role in theimpressive disparity between SOPs-values in the lakesBaikal and Malawi because it affects (1) the dissolutionrate of oxygen in the water column (Mortimer, 1981)and, hence, oxygen concentration near the sedimentsurface (Co), (2) the whole sediment diffusion coef-ficient (Ds) (Sweerts, 1990) and (3) metabolic ratesof organisms (while physiological adaptations are stillpossible; Hardy, 1979; Childress, 1995) and, thus, theapparent consumption of oxygen by the sediment (J).

Sediment porosity, while not affected by tempera-ture, may be also an explanatory factor because sed-iment in Lake Malawi is more sandy than in LakeBaikal, resulting in hypothetical lower porosities (0.36to 0.46 for coarse sand-sized particles – Berner 1971 –compared with 0.50 to 0.70 in Lake Baikal).

Amongst all these factors, the consumption of oxy-gen by the sediment (J) seems to be the most impor-tant one to explain the disparity in SOPs between thelakes. Even if we deliberately take the best estimatesfor Ds, Co and� in Lake Malawi and, conversely, theless favourable values in Lake Baikal, SOPs in LakeMalawi should be shallower than in Lake Baikal onlyby a factor 0.7 (Lake Baikal:� = 0.70;Co = 347�moll�1 in deep waters, Weiss et al. 1991. Lake Malawi:�= 0.36;Co = 234�mol l�1 in surface waters, Patter-son & Kachinjika 1995.Ds at 3 ˚C�=0.5Ds at 28 ˚C,Sweerts, 1990).

Organic matter is probably of great importance aswell. Lake Baikal is indeed oligotrophic (Moskalenko& Votinsev, 1972) while Lake Malawi is mesotrophic(Patterson & Kachinjika, 1995). Accordingly, greateramounts of organic matter available for bacterial degra-dation in sediment of the latter lake are possible, result-ing in higherJ-values and, hence, lower SOPs-values.No measurements of oxygen uptake by the sedimentare available to date, however. They are clearly neededin the future.

Oxygen and ecological segregation

Segregation resulting from clinal preference alongan ecological gradient is one possible factor affect-ing intra-lacustrine speciation (Martens et al., 1994;Martens, 1997). In this respect, oxygen can be respon-sible for both bathymetric and vertical segregations oforganisms. With regard to this, Lake Baikal and LakeMalawi differ in three fundamental issues.

1) Lake Baikal has 1.8 times more potentially oxy-genated, and hence habitable, sediment surface thanLake Malawi (given that the depths of the lakes arevery low compared with their width – 1-3% only –,their surface area can be considered similar to theirbottom area. Accordingly, the area of oxygenated bot-tom is about 31 500 km2 in Lake Baikal (completesurface) versus only 17 105 km2 in Lake Malawi (areacontained between the lake shore and the 230 m iso-bath).

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2) In Lake Malawi, there is a gradient of oxy-gen from the surface to the anoxic hypolimnion,which offers the possibility of bathymetricsegregation.Such a segregation is well-documented for cichlids ofAfrican great lakes, in relation with differential toler-ance to hypoxia within species (Galis & Smit 1979;Hoogerhoud et al., 1983; Chapman et al., 1995). Con-sequently, it is legitimate, a priori, to speculate on theexistence of a similar phenomenon in other groups oforganisms. In contrast, Lake Baikal is oxygenated toabout the same level in the whole water column, whichexcludes oxygen as a factor responsible for a possiblebathymetric segregation within the lake. This does notexclude possibilities for bathymetric segregations inthis lake.

3) As for the depth of oxygen penetration into thesediment, the situation in the two lakes is reversed.Being nearly anoxic, the sediment of Lake Malawioffers no possibility for oxygen-related vertical segre-gation of benthic organisms, this in contrast to LakeBaikal. It should be noted, however, that this addition-al possibility of segregation is only valid for benthicgroups, in contrast to (2) that affects not only the ben-thos but also the pelagial.

Acknowledgements

We are especially grateful to Dr M. Grachev, Directorof the Limnological Institute of Irkutsk for supportinginternational research on Lake Baikal. The research onLake Malawi could not have been carried out withoutthe help Dr A. Menz, Manager of the UK/SADC Pelag-ic Fish Resource Assessment Project, and his team,who is sincerely thanked for allowing K. Martens andP. Martin to use his laboratory facilities and for givingthem ship time. We are indebted to the captain andthe crew of the R/V Vereschagin (Baikal) and of theR/V Usipa (Malawi) for their active assistance dur-ing the fieldwork. We also express our gratitude toCl. Devries-Duchene who kindly inked various figuresand to P. Dumont who made the calculations relativeto oxygen profiles. This work was organized underthe auspices of BICER (Baikal International Centerfor Ecological Research) and with financial support ofthe Belgian Ministry of National Scientific Institutions,and the Siberian Branch of the Academy of Sciencesof Russia, and INTAS (International Association forthe Promotion of Cooperation with Scientists from theIndependent States of the former Soviet Union; projectINTAS-94-4465).

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