Hydrohiologia 278: 15 l- 156, 1994. T.B. Reynoldson & K.A. Coates (eds), Aquatic Oligochaete Biology V. 0 1994 Kluwer Academic Publishers. Printed in Belgium.

1.51

Depth distribution of oligochaetes in Lake Baikal (Siberia - Russia)

Patrick Martin, Boudewijn Goddeeris & Koen Martens Royal Belgian Institute of Natural Sciences, rue Vautier 29, B-1040 Bruxelles, Belgium

Key ruords: aquatic oligochaetes, Lake Baikal, bathymetric distribution, vertical distribution

Abstract

In the course of a preliminary sampling program, oligochaetes were collected along two transects in soft sediments in Lake Baikal. The number of oligochaetes present in the samples was counted, without distinguishing between species. The results suggest an exponential decrease in number of individuals (N) relative to depth (11,165 N m- 2 at 21 m, 265 N m- 2 at 1200 m). Most oligochaetes were found in the top 7 cm of sediment. The orange colour of the sediments suggests a high oxygen availability, even at the greatest water depths.

R&sum6

Au tours d’un programme d’echantillonnage preliminaire, les oligochetes du lac Balkal ont CtC recoltes dans le sediment mou preleve le long de deux transects. Les oligochetes presents dans les Cchantillons ont simplement Cte denombres, saris identification d’especes. Les resultats suggerent une diminution exponentielle du nombre d’individus en fonction de la profondeur du lac (11165 N m - 2 a 2 1 m, 265 N m 2 a 1200 m). La plupart des oligochetes ont CtC trouves dans les 7 premiers cm de la couche superieure du sediment. La couleur orangee du sediment suggere une grande disponibilite en oxygene, mCme aux profondeurs les plus grandes.

Introduction

Lake Baikal, in the Great Baikal Rift of eastern Siberia (53 ’ N, 108’ E), is the largest lake in the northern hemisphere (Kozhov, 1963). The “Pearl of Siberia” is nearly 650 km long and between 30 and 70 km wide. It is also the deepest of all lakes, thegreatest depth being 1,637 m (Stewart, 1990a). With a total volume of fresh water of c. 23,000 km’ (Kozhov, 1963), Lake Baikal contains about one fifth of the total surface freshwater supply in the world (Galazii, 1990). It is one of the most fascinating lakes on earth, mainly because of its age (20-25 million, possibly even 50 million years)

and because of the fact that, unlike most other deep lakes, the water circulation carries oxygen to its deepest point (Weiss et al., 1991; Maddox, 1989). The comparable Lake Tanganyika, for ex- ample, is much younger (2-6 millions years; Coulter, 1991) and virtually anoxic below 200 me- tres. Due to its unique ecological characteristics, the life in Lake Baikal is the richest in terms of both biomass and the number of recorded spe- cies, with numerous taxa being endemic.

For the Oligochaeta alone, the rate of ende- mism is nearly 90% (Kozhov, 1963). For the Tu- bificidae described before 1984, 25 species in 10 genera were reported from Lake Baikal, 20 of

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which were considered endemic (Brinkhurst, 1984). In a more recent publication, Snim- schikova (1987) enumerated a total of 104 oli- gochaete species in 31 genera and seven families (Aeolosomatidae and Branchiobdellidae in- cluded) and this in the northern Baikal only.

Finally, in spite of local pollution from indus- try (mainly cellulose plants) and agriculture, the water of the lake remains extraordinarily pure. To date Lake Baikal has been spared the heavy de- velopment that blights North America’s Great Lakes (Stewart, 1990b). Due to its characteris- tics, Lake Baikal is an exceptional ecosystem. Recently, various initiatives were taken to stim- ulate international research on different aspects of the Lake Baikal environment. As a result, a new research centre was founded, the Baikal In- ternational Centre for Ecological Research (BICER). A scientific collaboration was estab- lished between the Freshwater Biology Section at the Royal Belgian Institute of Natural Sciences and the Limnological Institute in Irkutsk, and research projects were defined. One research topic is focused on the distribution of zoobenthos in the lake (bathymetric, vertical and horizontal). This short note gives the preliminary results for Oligochaeta.

Material and methods

During a three week visit to Irkutsk in August 1990, a preliminary program was successfully carried out in the southern basin of the lake. Samples were taken with a modified Reineck box corer (surface 1000 cm2), a device normally used for oceanographic work (Eleftheriou & Holme, 1984). Two transects and one isolated station at 1200 m were sampled on soft sediments: south of the Selenga Delta (20-80 m) and on the Posol- skaya Bank (50-400 m) (Fig. 1). Four tubes (di- ameter 6.3 cm, surface 37.7 cm’) were pushed into the sediment raised with the box-corer, and

these are considered replicate samples. As the surface of the box corer was relatively large, it is here assumed that using this type of replicate, rather than taking each core from a separate Rei- neck sample (which was logistically impossible, due to time constraints) will introduce only lim- ited bias in the analyses. One core was used for grain-size analysis, the other three for faunal analysis. These latter cores were subsequently di- vided with an extruder into six vertical slices (O-l cm; l-2 cm; 2-4 cm; 4-7 cm; 7-15 cm and 15-21 cm) and preserved in 10% formalin. The slices were later screened in the laboratory on a mesh size of 250 pm and the organisms then sorted.

In this preliminary work, only whole specimens of Oligochaeta were counted without distinguish- ing between species. The bathymetric distribution was obtained from the combination of the two transects investigated. The densities of the verti- cal distribution were obtained from the mean densities of the different stations.

Results

The bathymetric distribution of oligochaetes (Fig. 2) suggests an exponential decrease of the number of individuals in relation to lake depth (max. -min.: 11,165 N m-* at 21 m, 265 N me2 at 1,200 m). Smallest oligochaetes were found at the deepest station. Incidentally, it is interesting to note that the remarkable freshwater polycha- ete Manayunkia baicalensis Nusbaume, 1901 is occasionally encountered between 21 m and 45 m depth, near the shoreline.

Densities of oligochaetes within the sediment (Fig. 3) seem also to decrease exponentially from the 1-2 cm layer to the 15-21 cm layer. Most oligochaetes (97%) were found in the top 7 cm of the sediment, which appeared to be oxidized as suggested from the sediments orange colour. This indicate high oxygen availability even at the great-

Fig. 1. Map showing the location of Lake Baikal in the northern hemisphere and of the two transects (B) in the southern basin (C) (Posolskaya Bank, Bl; south of the Selenga delta, B2). Depth isopleths are in metres (B).

153

,’ 4” \

‘1,

P

154

. -rransect 1

loo00 0 Transect 2

I ,.,,.._,_._ f, 0 200 400 600 800 1OCO 1200

Depth m

Fig. 2. Bathymetric distribution of oligochaetes. Densities are expressed in N me2 k s.e.

est depths within the lake. The deepest sediment layer (15-2 1 cm) was only inhabited by ?Rhynch- elmis bruchycephala (Michaelsen, 190 1) (Lumbri- culidae), the largest oligochaete of Lake Baikal.

Rhynchelmis brachycephala places the largest part of its body in the deeper anoxic sulfide layer.

Significant interaction in the 2 -way ANOVA (Table 1) indicates that the main factors (bathy- metric depth, sediment depth) are not linearly re- lated and in some way may interact. This means that one cannot simply interpret bathymetric or sediment depth changes.

Discussion

The bathymetric distribution of species is one of the oldest problems in the study of lakes and is an extension of the earliest debates about the ori- gin of organisms found to inhabit lakes (Brinkhurst, 1974). Is there a specially evolved fauna in relation to lake depth or not? With re- gard to Lake Baikal, the question is of great in- terest because of the particularly high rate of en- demism in its waters. On the other hand, the

800

600

400

1 2 3 A 5 6 7 8 9 10 11 12 13 14 15 i6 i7 i8

Fig. 3. Vertical distribution of oligochaetes. Densities are expressed in Nm-* + s.e.

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Table 1. Results of 2-way ANOVA’s on In (N + 1) for 11 depths and 5 segments; for 10 depths (-1200 m left out) and 4 segments (7-l 5 cm left out) and again for 10 depths ( -21 m left out) and 4 segments (7-15 cm left out).

11 depths, 5 segments (v2 = 164) Source F (vl, v2) vl P

Depth 17.673 10 *** Segments 39.829 4 *** Depth x segm. 6.283 40 ***

10 depths, 4 segments (~2 = 119) ( - 1200 m) Source F (vl, v2) Vl P

Depth 13.660 9 *** Segments 4.959 3 ** Depth x segm. 5.680 27 ***

10 depths, 4 segments (v2 = 84) (-21 m) Source F (vl, v2) vl P

Depth 10.022 9 Segments 3.647 3 Depth x segm. 2.784 21

Levels of significance: *** P10.001; ** P10.01. vl = degrees of freedom amongst groups v2 = degrees of freedom within groups.

*** ** ***

vertical distribution of species in the sediment, i.e. the depth to which species can penetrate the sedi- ment, is of particular importance for understand- ing their coexistence in a habitat.

Our preliminary results suggest:

(1) an exponential decrease of densities of oli- gochaetes with lake depth; (2) an exponential decrease of densities of oli- gochaetes in relation to the sediment depth; (3) a significant interaction between bathymetric and sediment depths with regard to the densities of oligochaetes; and (4) a correlation between the greatest densities of oligochaetes in the sediment and its oxidized layer (i.e. the oxygen availability).

Due to its depth, Lake Baikal is characterized by a particular deep-water renewal (“ven- tilation”), depending on the decrease in the tem- perature of maximum density of freshwater with increasing pressure, and on wind action at the

surface layer (Weiss etal., 1991). As a result, deeper layers are always oxygenated and nutri- ents in the surface layer are’recycled about four times before being removed to deep waters. The strongly oligotrophic nature of Lake Baikal is therefore a possible explanation of the very low densities of oligochaetes in the deepest zones.

While there are few studies on the macrohabi- tat distribution of oligochaetes (i.e. physical- chemical variables of water or plant associations; e.g. Verdonschot, 1984, 1987), their microdistri- bution in the sediment is somewhat better docu- mented (Brinkhurst & Kennedy, 1965; Sarkka & Paasivirta, 1972; Milbrink, 1973; Kirchner, 1975; Newrkla & Wijegoonawardana, 1987; Healy, 1987). The vertical distribution of oligochaetes in the sediments may be the result of various factors (see Milbrink, 1973 and Newrkla & Wijegoon- awardana, 1987): shelter from predators, nutri- tional preferences, specific rate of maturity, char- acteristics of the bottom substrate, tolerance to anoxia (Fisher & Beeton, 1975; Gnaiger et al., 1987).

The second and the fourth factors, above, are probably related and suggest that, amongst the factors mentioned, oxygen availability is a key factor in the distribution of oligochaetes in the sediments. Kirchner (1975) showed that the ver- tical extent of the oxidized material had a very definite effect on the vertical distribution of the benthic fauna, which was reflected in increased dispersal or spreading out of the biomass in the community (1975: 429).

The burrowing activity of oligochaetes results in oxygen penetration in the sediment far below the oxic layer (oxygen is present in the immediate vicinity of animal burrows; Revsbech & Jor- gensen, 1986). It can be assumed that oligocha- etes penetrate deeper into the sediments not only because the oligotrophic condition of Lake Baikal stimulates this behaviour (search for food), but also because the high oxygen content at the water- sediment interface at all depths makes a deep penetration easier. Finally, it is possible to find oligochaetes such as R. brachycephala far below the oxidized layer, but this is not surprising be- cause of their body length of up to 185 mm (Che-

156

kanovskaya, 1962) which enables them to reach the oxic layer even from such much greater depths in the sediments.

The statistical analyses indicate a significant interaction between bathymetric and sediment depths on the densities of oligochaetes. The direct or indirect nature of this interaction remains un- known. While a direct influence of depth on the densities of oligochaetes in the sediment is un- likely, an indirect influence related to the nature of the sediments (Brinkhurst, 1974) is easier to conceive. It is difficult, however, to substantiate here because of the observed, relative, homoge- neity of the sediment between the stations inves- tigated.

Due to the great depth of Lake Baikal, nutri- ents are recycled several times as they sink to the bottom, resulting in a nutritive depletion of the sediment in relation to depth. The interaction be- tween bathymetric and sediment depths on the densities of oligochaetes could, therefore, be the result of this nutritive depletion if the feeding ac- tivity of these animals has an effect on their ver- tical distribution.

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