Two studies have recently been published in Environmental Science & Technology which explore concentrations and possible effects of PFCs on Baikal’s freshwater seal, Pusa sibirica (nerpa) (Ishibashi et al. 2008a, 2008b). Because nerpa are at the top of the Baikal food-chain, they too bio-accumulate toxins such as POPs (persistent organic pollutants) (Nakata et al. 1995). In this pair of papers, Ishibashi et al. initially provide evidence for bio-accumulation of PFCs in Baikal seals, especially in their livers. Ishibashi et al. (2008a) found that bio-accumulation tends to be higher in seal pups than in adult seals, which led them to conclude that significant transfer of PFCs occurs between the pups and their mothers (Fig 1). Moreover, concentrations of PFCs in Baikal seal livers increased between 1992 and 2005. This highlights a worrying trend that these globally distributed contaminants are increasingly impacting on remote ecosystems.

Fig 1 Concentrations of ΣPFCs in the liver of Baikal seal pup/juvenile (age: <0.3–2 yr) and subadult/adult (age: >2 yr) Baikal seals. Data are presented as the mean and standard deviation of ΣPFC concentrations. The * indicates that the concentrations in the liver of pups/juveniles were significantly higher than concentrations in those of subadults/adults (p < 0.05).

Having demonstrated that since the early 1990s PFCs have increasingly accumulated in the livers of Baikal seals, especially young pups, the focus of the second paper was to determine if these man-made, persistent compounds were having a determinable impact on this unique freshwater mammal (Ishibashi et al. 2008b). In order to do this, they relied to previous research which showed that certain metabolic processes in the Baikal seal can be modulated by PFCs. Although they have not as yet determined any toxic, biological impacts of PFCs on the Baikal seal, they have identified specific metabolic responses to these compounds, which they suggest could be useful biomarkers for monitoring future impacts and trends.

Ishibashi, H., Iwata, H., Kim, E.-Y., Tao, L., Kannan, K., Amano, M., Miyazaki, N., Tanabe, S., Batoev, V.B., Petrov, E.A. 2008a. Contamination and Effects of Perfluorochemicals in Baikal Seal (Pusa sibirica). 1. Residue Level, Tissue Distribution, and Temporal Trend. Environmental Science & Technology, 42, 2295 - 2301. doi: 10.1021/es072054f

Ishibashi, H., Iwata, H., Kim, E.-Y., Tao, L., Kannan, K., Tanabe, S., Batoev, V.B., Petrov, E.A. 2008b. Contamination and Effects of Perfluorochemicals in Baikal Seal (Pusa sibirica). 2. Molecular Characterization, Expression Level, and Transcriptional Activation of Peroxisome Proliferator-Activated Receptor α. Environmental Science & Technology, 42, 2302 - 2308. doi: 10.1021/es072055

Nakata, H. Tanabe, S., Tatsukawa, R., Amano, M., Miyazaki, N. and Petrov, E.A. 1995. Persistent organic chlorine residues and their accumulation kinetics in Baikal seal (Phoca sibirica) from Lake Baikal, Russia. Environmental Science & Technology, 29, 2877-2885.

Previous papers on Holocene climate variability in Lake Baikal tended to use what I would now consider inappropriate climate classification schemes (e.g. Blytt-Sernander) which were based on observations made from northern European peat bogs. Moreover, early radiocarbon chronologies from Lake Baikal proved difficult to calibrate. Both these factors meant that robust reconstructions of Holocene climate variability, together with associated ecosystem impacts, were not as advanced as in other regions of the world.

However, three detailed papers were published last year on Holocene climate impacts in the Lake Baikal region which considerably advance our understanding on the extent of climate variability experienced in this region: Mackay 2007; Tarasov et al. 2007; Prokopenko et al. 2007. Each of the papers has taken a very different approach to the problem.

The review by Mackay 2007 sought to provide a synthesis of all the papers published (up to 2006) on Lake Baikal diatoms (including biogenic silica and oxygen isotope analysis of diatom silica) spanning the Quaternary period. Relevant to this entry is the mounting empirical evidence for millennial-scale cycles and centennial cooling events determined from sites around the world. Mackay 2007 therefore re-interpreted diatom and oxygen isotope records from biogenic silica in the context of centennial-scale cooling events during the early to mid-Holocene from well dated profiles. He then compared these cooling events to other regions around the northern hemisphere by reference to several recent Holocene studies (e.g. Mayewski et al. 2004) (Fig 1).

Fig 1: Synthesis of Holocene paleoclimate records from Lake Baikal, and other records from across central Asia, northwest Europe and the North Atlantic ocean. All profiles are scaled to calendar years BP (Mackay 2007).

Central to this synthesis was a study done by Morley et al. 2005, who undertook the first oxygen isotope analysis of Baikal silica. Despite problems with clay contamination, there were several notable episodes of lower δ18O values, which correspond to prevailing cooler temperatures and shifts in river input into Lake Baikal.

The first episode occurred between 10.2 and 10.4 ka cal yr BP, coincident with the prominent cooling event seen at c. 10.35–10.15 ka cal yr BP in several archives from the North Atlantic and Greenland ice cores (see Björck et al., 2001 for a synthesis). It has been suggested that this cooling event was caused by a weakening of the North Atlantic thermohaline circulation (THC) and the southwards extension of polar waters. Moreover, potassium records from GISP2 (a Greenland ice core) indicate at this time a significant strengthening of the Siberian High (Mayewski et al., 2004). The evidence here suggests therefore that weakening of the THC in the North Atlantic at this time has also had substantial and perhaps almost simultaneous impact on the Lake Baikal ecosystem through intensification of the Siberian High.

A second prominent episode of lower δ18O values in the Lake Baikal isotope record can be observed during the mid-Holocene, between ca. 7 ka – 6.5 ka BP. Corrected values (Brewer et al. 2008) show this minimum to have occurred ca. 6.8 ka BP. Nevertheless, these findings are coincident with increasing GISP2 Na+ (ppb) (indicative of a deepening Icelandic Low and GISP2 K+ (ppb) (indicative of a more intense Siberian High pattern (Mayewski et al., 1997)) (Fig 1). Mayewski et al. (2004) suggest that this episode may have been caused by solar forcing, as there is no evidence of other major forcing factors especially freshwater input into the North Atlantic. Low values for δ18O in diatom silica suggests that maximum cooling was caused by an intensification of the Siberian High, altering fluvial input into the lake (Morley et al. 2005). As well as shifts in fluvial sources in the Lake Baikal region, this cooling phase may also have had an impact on diatom production in the lake, as there is evidence of a decline in BioSi values (Colman et al., 1999).

Two further identified cool periods occurred at approximately 3 – 2.5 ka BP (based on diatom abundances only) and at c. 0.5 ka BP (based on diatoms and oxygen isotope values of diatom silica), coincident with the period know as the Little Ice Age (see Mackay et al. 2005) and Post “Recent climate impacts…” (Fig 1). These cool periods are coincident with an increase in steppe landscape identified by Tarasov et al. 2007 below, and with shifts to possible modelled cooler conditions identified by Bush (2005).

The second recent high-resolution paper to investigate Holocene palaeoclimate in Lake Baikal is based on pollen evidence from a core taken on the Buguldieka Saddle, separating the south and central basins (Tarasov et al. 2007). This study is novel for several reasons. It is the first to provide empirically based quantitative estimates of past temperatures and precipitation from the pollen record. Moreover, for the Holocene period, it makes a serious attempt to relate significant periods of prehistoric settlement close to Lake Baikal, with possible impacts on the landscape and extent of deforestation. Finally, the authors also compare environmental reconstructions for the Holocene period with those from the last interglacial between 128 ka – 117 ka years ago (in southern Siberia, this period is commonly known as the Kazantsevo (see Rioual & Mackay 2005).

Pollen in each Holocene core sample was classified into a particular biome, e.g. taiga forest, steppe, tundra etc so that changing landscape patterns could be determined at high resolution. Quantitative reconstructions of mean July temperature, mean January temperature, mean annual precipitation and a moisture index were also determined using a technique called best modern analogue approach (BMA) (Fig 2). Tarasov et al. were able to determine that in the south of Lake Baikal at least, taiga landscape was significant after 13.3 ka, associated with a relatively warm and wet climate. By 9 ka- 7 ka taiga was dominant with fir and spruce at their highest levels for the whole Holocene period. Interspersed into this record, steppe landscapes expanded at last 4 times between 7.5 ka, 5.5 ka, 3 ka and 1-0.5 ka ago, generally coincident with declines in reconstructed temperatures. The authors were also able to determine that human settlements effected little if any impact on deforestation in the region, although climate may have played a role in the occurrence of a well-define cultural hiatus between c. 6.8 – 6.1 ka BP (Weber 2002).

Fig 2 The charts of the climate variables reconstructed from the Buguldeika (VER93-2 st.24GC) pollen record using the BMA approach: annual precipitation (A); the mean temperature of the warmest month (B); the mean temperature of the coldest month (C); and the moisture index (D) (Tarasov et al. 2007).

Prokopenko et al. 2007 provided a new interpretation of Holocene climate change in the Lake Baikal region, based on diatom and pollen records from Lake Baikal itself, and Lake Hovsgol. Lake Hovsogol is also a tectonic rift lake, but is situated at 1645 m a.s.l. to the south-west of Baikal (Fig 3). Its outlet, the Egiin-Gol river, feeds its way into the Selenga River, Baikal’s largest tributary. Prokopenko et al. 2007 provide the first ever data-model comparison for the Holocene in Lake Baikal. Stemming from this, two important points are raised in this paper, which Prokopenko believes challenges conventional wisdom about palaeoclimate during the Holocene in this region of central Asia.

Fig 3 Lake Baikal watershed and bathymetric maps of Lake Hovsgol and Lake Baikal (Prokopenko et al. 2007)

Firstly, over the Holocene period, the role of global CO2 / water vapour feedback was more important than direct insolation forcing in driving regional mid-Holocene temp maximum. This is based on the observation that insolation peaked at 11 ka in the continental interior of Asia, yet diatom and pollen proxy records for Lakes Baikal and Hovsgol suggest significant temperature increases after ca. 7 ka BP. For example, between 4-2.5 ka BP, there is proxy evidence for elevated summer temperatures in Transbaikalia and increased aridity in northern Mongolia, which agree well with regional GCM predictions undertaken by Bush (2005). Note that direct temperature reconstructions by Tarasov et al. 2007 however suggest that for the early to mid Holocene period, mean July temperatures were highest at c. 7 ka BP, while estimates of mean January temperature were highest between c. 8.5 – 8 ka BP.

Secondly, according to Prokopenko et al. 2007 there is no evidence contained in either the Lake Baikal or Lake Hovsgol records for mid-Holocene humidity or precipitation/evaporation (P/E) maxima that can possibly be associated with peak Asian summer monsoon intensity. Instead, and to the contrary there is a consistent decline in P/E since the early Holocene which accelerated at c. 7.5-8 ka, and ended in a prolonged minimum in P/E balance between 6-4 ka. This increase in aridity at this time can be linked to rising temperatures causing an increase in P/E in northern Mongolia.

Clearly there is still some uncertainty in trying to tie all these records together, especially with regard to discrepancies in radiocarbon dating of bulk sediments in Baikal: for example ‘reservoir’ ages applied by Tarasov and Prokopenko for the same core material are different. Nevertheless, consensus is building that the ecosystem of Lake Baikal is being impacted by e.g. centennial cool events, and that terms such as ‘holocene optimum’ originally derived for northern European climates are not applicable to the records determined here.

References used:

Anisimov, O.A., Velichko, A.A., Demchenko, P.F., Eliseev, A.V., Mokhov, I.I., Nechaev, V.P., 2002. Effect of climate change on permafrost in the past, present, and future. Izvestiya. Atmospheric and Oceanic Physics 38, 25–39.

Björck, S., Muscheler, R., Kromer, B., Andresen, C.S., Heinemeier, J., Johnsen, S.J., Conley, D., Koç, N., Spurk, M., Veski, S., 2001. High-resolution analyses of an early Holocene climate event may imply decreased solar forcing as an important climate trigger. Geology 29, 1107–1110.

Bradbury, J.P., Bezrukova, Ye. V., Chernyaeva, G.P., Colman, S.M., Khursevich, G., King, J.W., Likoshway, Ye. V., 1994. A synthesis of post-glacial diatom records from Lake Baikal. Journal of Paleolimnology 10, 213–252.

Brewer, T., Leng, M., Mackay, A.W., Lamb, A., Tyler, J., Marsh, N. (2008) Unravelling contamination signals in biogenic silica oxygen isotope composition: the role of major and trace element geochemistry. Journal of Quaternary Science 23, 321-330.

Bush, A.B.G., 2005. CO2/H2O and orbitally driven climate variability over central Asia through the Holocene. Quaternary International 136, 15–23.

Colman, S.M., Peck, J.A., Hatton, J., Karabanov, E.B., King, J.W., 1999. Biogenic silica records from the BDP93 drill site and adjacent areas of the Selenga Delta, Lake Baikal, Siberia. Journal of Paleolimnology 21, 9–17.

Karabanov, E.B., Prokopenko, A.A., Williams, D.F., Khursevich, G.K., 2000. A new record of Holocene climate change from the bottom sediments of Lake Baikal. Palaeogeography, Palaeoclimatology, Palaeoecology 156, 211–224.

Mackay, A.W. 2007. The paleoclimatology of Lake Baikal: a diatom synthesis and prospectus. Earth-Science Reviews 82, 181-215. doi: 10.1016/j.earscirev.2007.03.002

Mayewski, P.A., Meeker, L.D., Twickler, M.S., Whitlow, S., Yang, Q., Prentice, M., 1997. Major features and forcing of high latitude Northern Hemisphere atmospheric circulation using a 110,000 year long glaciochemical series. Journal of Geophysical Research 102, 26345–26366.

Mayewski, P.A., Rohling, E.E., Stager, J.C., Karle’n,W., Maasch, K.A., Meeker, L.D.,Meyerson, E.A., Gasse, F., vanKreveld, S.,Holmgren, K., Lee-Thorp, J., Rosqvist, G., Rack, F., Staubwasser, M., Schneider, R.R., Steig, E.J., 2004. Holocene climate variability. Quaternary Research 62, 243–255.

Morley, D.W., Leng, M.J., Mackay, A.W., Sloane, H.J., 2005. Late Glacial and Holocene atmospheric circulation change in the Lake Baikal region documented by oxygen isotopes from diatom biogenic silica. Global and Planetary Change 46, 221–233.

Prokopenko, A.A., Khursevich, G.K., Bezrukova, E.V., Kuzmin, M.I., Boes, X., Williams, D.F., Fedenya, S.A., Kulagina, N.V., Letunova, P.P. & Abzaeva, A.A. 2007. Paleoenvironmental proxy records from Lake Hovsgol, Mongolia, and a synthesis of Holocene climate change in the Lake Baikal watershed. Quaternary Research, 68, 2-17.

Tarasov, P., Bezrukova, E., Karabanov, E., Nakagawa, T., Wagner, M., Kulagina, N., Letunova, P., Abzaeva, A., Granoszewski, W. and Riedel, F. 2007. Vegetation and climate dynamics during the Holocene and Eemian interglacials derived from Lake Baikal pollen records. Palaeogeography, Palaeoclimatology, Palaeoecology 252 (3-4): 440-457.

Weber et al., 2002 A.W. Weber, D.W. Link and M.A. Katzenberg, Hunter–gatherer culture change and continuity in the Middle Holocene of the Cis-Baikal, Siberia, Journal of Anthropological Archaeology 21 (2002), pp. 230–299.

Wolff, E. 2007. When is the “present”? Quaternary Science Reviews 26, 3023-3024

doi:10.1016/j.yqres.2007.03.008 (more…)

(i) the last 150 years: ice cover evidence

Seasonal ice cover is a notable feature of Lake Baikal (Fig 1), a consequence of the extreme continental climate in the region. Ice and snow cover are inextricably linked to the biology and ecology of species in the lake from microscopic algae (e.g. diatoms) to the lake’s largest mammal, the Baikal seal (nerpa). Dates of ice formation and breakup have been recorded at the Lystvyanka station in the south of Lake Baikal continuously since the late 19th century. This record has been the subject of several papers (Livingstone 1999; Magnuson et al. 2000; Shimaraev et al. 2002; Todd & Mackay 2003) investigating ice formation, duration etc on Lake Baikal with respect to changing climate. Ice cover was shown to be a robust indicator of continental-scale winter climate, and patterns of ice formation and break-up are linked to large-scale atmospheric circulation patterns, including the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO), and the position and intensity of the Siberian High. These studies are important as they provide plausible, mechanistic links between regional climate and its impact on biology. Furthermore, all these studies highlight that there is compelling evidence of the link between declining ice duration and thickness in the south basin of Lake Baikal with observed warming trends (Fig 2).

A comprehensive review of ice-cover research on Lake Baikal has recently been undertaken by Kouraev et al. (2007). As well as providing this excellent review, the authors also investigated spatial trends in ice cover across the length of Lake Baikal using remote sensing and satellite data between 1992-2004. They highlight that ice formation across the whole of Lake Baikal is complex, and suggest that since the 1990s, trends in ice duration and thickness have actually increased, in line with observed cooler winters. Although these findings are opposite to previous research, they highlight the periodicity in temperature trends at the sub-decadal level, the high inter-annual variability which exists in these records, and the tight relationship that exists between ice cover on Lake Baikal and prevailing regional temperatures. On longer timescales, it is still predicted that ice cover on Lake Baikal will continue to decrease over the next 100 years, which will undoubtedly have a significant impact on biology within the lake. How do we know? By both looking at the palaeo record for when ice cover would have persisted longer on Lake Baikal (e.g. Edlund et al. 1995; Mackay et al. 2005) and by undertaking a space for time, multivariate approach (Mackay et al. 2006).

(ii) the last 150 years: biological evidence

While there is ample evidence above to show that warming over the last 150 years has significantly impacted on the limnology of Lake Baikal, for example through the reduction in duration and thickness of ice cover, the impacts of recent warming on the ecology of the lake are somewhat less certain. The palaeolimnological record clearly shows that major changes in diatom communities have occurred during the 19th century. Throughout the length of Lake Baikal, sediment cores show that from the middle of the 19th century onwards, there was a lake-wide shift in diatom species which bloom mainly in autumn (e.g. Cyclotella), to spring crops which bloom shortly after ice cover breaks up (e.g. Aulacoseira, Stephanodiscus and Synedra) (Mackay et al. 1998) (Fig 3).

Monitoring of diatom crops in Lake Baikal have only been carried out however since the late 1940s, and these have been largely restricted to the south basin. One study suggests that during the 1950s, productive years in the lake were dominated by large celled, endemic species belonging to the genera Aulacoseria (“Melosira”) and Cyclotella (Popovskaya 2000), although Synedra tends to bloom following years of peak Aulacoseira production. In the 1990s, small-celled, cosmopolitan species belonging to the genus Nitzschia were observed in higher numbers in some years, which has been attributed to both increased nutrient input into the lake from economic development in the catchment (Bondarenko 1999) and to global warming (Popovskaya 2000). However, diatom-monitoring studies over the last decade have shown that increases in Nitzschia cells have not been maintained in recent years (unpublished data). Moreover, these monitoring studies show that trends in the south basin are different from trends in the north basin, and that species’ responses across the whole lake are complex. Considerable uncertainty, therefore, exists about possible drivers influencing planktonic diatom communities in Lake Baikal.

Increasing eutrophication in the south basin of Lake Baikal may account for some of the diatom community changes observed in recent decades, although contamination of the lake water is still relatively low. For example, increases in phytoplankton in shallow water regions of the Selenga Delta since the 1980s may be due to increasing nutrient enrichment entering the lake by its main tributary, the Selenga River. Surface cores taken from the bottom sediments of this region do contain increasing abundances of small centric diatoms commonly associated with more nutrient rich waters (Mackay et al. 1998). However, sediment cores taken from deep-water regions of the lake are impacted heavily by dissolution processes, and from our research it is those very species which are likely to benefit the most from increasing temperatures that are least well preserved in the sedimentary record (Ryves et al. 2003; Battarbee et al. 2005)

(iii) the last 1000 years

The impacts of recent climate variability on the aquatic ecology of Lake Baikal can be put into a longer-term perspective by looking at the palaeolimnological record. The sediments of Lake Baikal are heterogeneous, that is they are composed of material (i) washed in from the catchment via rivers (e.g. clays and silts), (ii) blown onto the lake by wind (e.g. pollen), and (iii) which has grown and developed in the lake itself, including the remains of microscopic plants (algae) such as diatoms. Several teams of scientists working on Lake Baikal have exploited the use of clays, pollen and diatoms to help reconstruct past climates in central Asia. I will review these other records later, as they tend to span longer timescales, i.e. at least 11,500 years.

The Medieval Warm Period

In Lake Baikal, several studies have focused on reconstructing change during the last 1000 years, either semi-quantitatively (e.g. Edlund et al. 1995; Mackay et al. 1998) or quantitatively (Mackay et al. 2005). Even at these relatively short timescales, distinct diatom responses to prevailing climate conditions are apparent. For example, the dominance of the autumnal blooming C. minuta between c. 1640 AD into the early 19th century is coincident with the beginning of the Maunder Minimum, one of the coolest episodes characterising the LIA (Edlund et al. 1995). They suggested that during this period of colder climate, ice and snow cover on the lake will have acted to suppress diatom communities which begin to grow under the ice in spring, resulting in the dominance of autumnal blooming taxa such as C. minuta.

A diatom-inferred model of snow thickness on the frozen Lake Baikal was constructed using transfer function methodologies (Mackay et al. 2003). This model was then applied to a short core extracted from the south basin, spanning the last 1200 years (Mackay et al. 2005). Because diatom dissolution is a significant process in Lake Baikal, an attempt was made to improve bias in the model by using correction factors established for the dominant phytoplankton in the lake (Ryves et al. 2003; Battarbee et al. 2005). Based on diatom composition alone, three significant zones could be recognized, coincident with the broad features of the MWP, the LIA and the period of recent warming (Fig 4).

Ecological and quantitative interpretations demonstrated that between c. A.D. 850 and 1200, S. acus dominated the assemblage, most likely due to prevailing warmer and wetter climate that occurred in Siberia at this time (Fig 4) (Naurzbaev and Vaganov 2000). These conditions will have benefited net growth of this opportunistic species over heavier, larger cells of A. baicalensis. Bradbury et al. (1994) outline how S. acus may indicate climatic conditions that result in earlier ice break-up, coupled with increased fluvial input and concomitant nutrient supply to the lake. A proxy series of Siberian High sea-level pressure (SLP) reconstructions, based on GISP2 K+ concentrations highlights that at this time (Fig 4), the Siberian High was at its weakest during the last 1000 years (Meeker and Mayewski 2002).

The Little Ice Age

Between c. A.D. 1200 and 1400, spring diatom crops growing under the ice decline in abundance, due in part to increased winter severity and snow cover on the lake (Edlund et al. 1995; Mackay et al. 2005), which is reflected in cooler early Siberian summers (Fig. 4) (Naurzbaev and Vaganov, 2000). This period is associated with a weakening in the NAO, allowing the expansion of the Siberian High across central Asia (e.g. Krenke and Chernavskaya, 2002) (as indicated by increasing GISP2K+ concentrations; Fig. 4) (Meeker and Mayewski, 2002), resulting in increased anticyclonic activity in the region. The diatom-inferred snow model suggests significantly increased snow cover on the lake between A.D. 1200 and 1775, which mirrors for the large part increases in snow cover in China during AD 1400–1900 (Zhang and Crowley, 1989). Thick, accumulating snow cover inhibits light penetration through the ice, thereby having negative effects on cell division rate and the extent of turbulence underneath the ice (Granin et al., 1999), a factor that helps diatom cells to avoid sinking down through the water column. Consequently, only taxa that whose net growth occurs during autumn overturn (e.g. C. minuta) predominate in the lake at this time.

In recent decades, the impacts of global warming are increasingly being identified in lakes around the world, especially in remote regions such as the arctic (Smol et al. 2005). In this respect Lake Baikal is no exception (Livingstone 1999; Magnuson et al. 2000; Shimaraev et al. 2002; Todd and Mackay 2003). Diatom census data and reconstructions of snow accumulation suggest that changes in the influence of the Siberian High in the Lake Baikal region started in the south basin as early as c. 1750 AD, with a shift from taxa that bloom during autumn overturn to assemblages that exhibit net growth in spring after ice break-up (Mackay 2007) (Fig. 4). The data here mirror instrumental climate records from Fennoscandia for example, which also show over the last 250 years positive temperature trends (e.g. Jones 2001) and increasing early summer Siberian temperature reconstructions (Naurzbaev and Vaganov 2000). Although warming in this southern region of Lake Baikal commenced before rapid increases in greenhouse gases, this does not in way preclude effects of recent climate change further impacting on the limnology and ecology of Lake Baikal. For example, recent increases in S. acus may be related to increasing trends in river input into Lake Baikal (thereby introducing higher concentrations of dissolved silica to the lake) associated with recent warming trends (Shimaraev et al. 2002)

References used

Battarbee, R.W., Mackay, A.W., Jewson, D., Ryves, D.B., Sturm, M., 2005. Differential dissolution of Lake Baikal diatoms: correction factors and implications for palaeoclimatic reconstruction. Global & Planetary Change 46, 75-86.

Bondarenko, N.A., 1999. Floral shift in the phytoplankton of Lake Baikal, Siberia: recent dominance of Nitzschia acicularis. Plankton Biology & Ecology 46, 18-23.

Bradbury, J.P., Bezrukova, Ye.V., Chernyaeva, G.P., Colman, S.M., Khursevich, G., King, J.W., Likoshway, Ye.V., 1994. A synthesis of post-glacial diatom records from Lake Baikal. Journal of Paleolimnology 10, 213-252.

Edlund, M.B., Stoermer, E.F., Pilskaln, C.H., 1995. Siliceous microfossil succession in the recent history of two basins in Lake Baikal, Siberia. Journal of Paleolimnology 14, 165-184.

Granin, N.G., Jewson, D.H., Gnatovsky, R.Yu., Levin, L.A., Zhdanov, A.A., Averin, A.I., Gorbunova, L.A., Tcekhanovsky, V.V., Doroschenko, L.F., Min’ko, N.P., Grachev, M.A., 1999. Turbulent mixing in the water layer just below the ice and its role in development of diatomic algae in Lake Baikal. Doklady Akademii Nauk 366, 835-839.

Kouraev, A.V., Semovski, S.V., Shimaraev, M.N., Mognard, N.M., Legresy, B. & Remy, F. 2007. The ice regime of Lake Baikal from historical and satellite data: relationship to air temperature, dynamical, and other factors. Limnology & Oceanography 52, 1268-1286.

Krenke, A.N., Chernavskaya, M.M., 2002. Climate changes in the preinstrumental period of the last millennium and their manifestations over the Russian Plain. Isvestiya, Atmospheric and Oceanic Physics 38, S59-S79.

Livingstone, D.M., 1999. Ice break-up on southern Lake Baikal and its relationship to local and regional air temperatures in Siberia and to the North Atlantic Oscillation. Limnology & Oceanography 44, 1486-1497.

Mackay, A.W., Flower, R.J., Kuzmina, A.E., Granina, L.Z., Rose, N.L., Appleby, P.G., Boyle, J.F., Battarbee, R.W., 1998. Diatom succession trends in recent sediments from Lake Baikal and their relationship to atmospheric pollution and to climate change. Philosophical Transactions of the Royal Society London 353, 1011-1055.

Mackay, A.W., Battarbee, R.W., Flower, R.J., Granin, N.G., Jewson, D.H., Ryves, D.B., Sturm, M., 2003. Assessing the potential for developing internal diatom-based inference models in Lake Baikal. Limnology & Oceanography 48, 1183-1192.

Mackay, A.W., Ryves, D.B., Battarbee, R.W., Flower, R.J., Jewson, D., Rioual, P.M.J., Sturm, M. 2005. 1000 years of climate variability in central Asia: assessing the evidence using Lake Baikal diatom assemblages and the application of a diatom-inferred model of snow thickness. Global & Planetary Change 46, 281-297.

Mackay, A.W., Ryves, D.B., Morley, D.W., Jewson, D.J., Rioual, P. (2006) Assessing the vulnerability of endemic diatom species in Lake Baikal to predictions of future climate change: a multivariate approach. Global Change Biology 12, 2297-2315.

Magnuson, J.J., Robertson, D.M., Benson, B.J., Wynne, R.H., Livingstone, D.M., Arai, T., Assel, R.A., Barry, R.G., Card, V., Kuusisto, E., Granin, N.G., Prowse, T.D., Stewart, K.M., Vuglinski, V.S., 2000: Historical trends in lake and river ice cover in the northern hemisphere. Science 289, 1743-1747.

Meeker, L.D. & Mayewski, P.A. 2002. A 1400-year high-resolution record of atmospheric circulation over the North Atlantic and Asia. The Holocene, 12, 257-266

Naurzbaev, M.M., Vaganov, E.A., 2000. Variation of early summer and annual temperature in east Taymir and Putoran Siberia. over the last two millennia inferred from tree rings. Journal of Geophysical Research 105, 7317-7326.

Popovskaya, G.I., 2000. Ecological monitoring of phytoplankton in Lake Baikal. Aquatic Ecosystem Health and Management 3, 215-225.

Ryves, D.B., Jewson, D.H., Sturm, M., Battarbee, R.W. Flower, R.J., Mackay, A.W., Granin, N.G., 2003. Quantitative and qualitative relationships between planktonic diatom communities and diatom assemblages in sedimenting material and surface sediments in Lake Baikal, Siberia. Limnology & Oceanography 48, 1643-1661.

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Precise diversity data is sometimes lacking for Baikalian taxa because new species and revised classifications are occurring continuously. Nevertheless, most groups show a high degree of species richness and endemicity. The review below is adapted from Mackay et al. 2002. (Mackay, A.W., Flower, R.J. & Granina, L.Z. (2002) Lake Baikal. In: The Physical Geography of Northern Eurasia: Russia and Neighbouring States. Ed. by M. Shahgedanova & A. Goudie. Published by OUP, Oxford. (Chapter 17) pp 403-421

Vertebrates

The nerpa seal (Phoca sibirica) is Baikal’s only endemic species of mammal, and is thought to have arrived in the lake during the Pleistocene period. Approximately 56 species of fish occur in the lake. The majority of these fish are endemic, although some shallow water species are cosmopolitan (e.g. the perch and the roach) and six species have been introduced. Cottoid fish (‘sculpins’) are all small (usually < 20 cm long) and are particularly diverse, consisting of 29 species, the majority of which are adapted to benthic life. Some species are adapted to shallower waters (e.g. Cottocomephorus and Procotus spp.) whilst others occupy deep water (e.g. Abyssocottus spp.), and may have a marine ancestry. Two exceptional cottoids, Comephorous dybowskii and C. baicalensis (glomyankas) live in open water, optimally around 100 - 300 m depth, and are adapted to deep, open water by possessing large pectoral fins and scaleless, translucent bodies with reduced ossification. These and other cottoid fish and their fry are a key group in the food web structure of Baikal, being the major food source for seals and other fish. The most important fishery in Baikal is that of the omul (Coregonus autumalis migratorius) and, to a much lesser extent, of the Siberian grayling (Thymallus acticus vars.). These open water fish only differ from taxa elsewhere at the subspecies level and C. autumnalis has a circum-global occurrence in northern boreal waters.

Invertebrates

The gammarids are one of the most interesting and diverse group of organisms in Lake Baikal. They are a form of crustacean, but over 99% of species found in Lake Baikal are found nowhere else in the world. Morphologically, the species are very distinct, which is related to wide range of habitats exploited. For example, some species live in the deep, open water, others on the bottom sediments of the lake, while others are more specialised and are associated with grazing on the surfaces of Baikal sponges.

Other Crustacea of particular interest in Lake Baikal include ostracods, which are bivalve molluscs. The ostracods are very diverse with over 90 % of species being endemic, that is are unique to Lake Baikal. The endemic Epischura baicalenisis is often the dominant zooplankton and is a key species in the Baikal food chain, feeding on the algae that grow in the open water.

Several groups of insects spend their larval life within Baikal, and these are represented mainly by the Plecoptera (stoneflies), Trichoptera (caddisflies) and the Chironomidae (a family of non-biting midges). Stoneflies and caddisflies are found mainly in shallow waters (not deeper than 20 m), whereas chironomids are can be also be found at much greater depths. On Lake Baikal, the endemic caddisflies are famous for their mass abundances on emergence after ice-break-up in June. Freshly emerged species of Baicalina, and to a lesser extent of Apatania, can form 10 cm thick living caddisfly carpets and these slow moving black and grey insects ascend any structures (such as trees) rising from near the lake shore.

More than 180 species of mollusc occur in the lake of which 125 are endemic (and are predominantly Gastropods). The Baikalian gastropoda do not show nearly as much diversity as found in the African great lakes: c. 15 species with poorly calcified shells occur in deep water (> 200 m depth), while shallow water gastropods are much more common. Segmented worms (the Annelida) are also well represented in Baikal with over 200 species of which about 75 % are endemic. One of the most interesting is the endemic tube-dwelling polychaet Manayunkia baicalensis, because Polychaets are rare in freshwaters and invariably attest to some past connection with marine systems.

Free-living Platyhelminthes (flat worms) or turbellarians were rather less researched, but burgeoning taxonomic work in the late 1980s and 1990s has added some 20 new species per year to this latter group (Timoshkin, 1994). The number of species is now about 80 and are separated mainly on reproductive structures and, more unusually, on colour, with red, yellow, brown, black or variegated taxa occurring in shallower waters. Several species flocks are described (in the Lecithoepitheliata, Tricladida and Prolecithophora; see Timoskin, 1994) and perhaps the most interesting are the wholly endemic deep water species that are relatively extremely large, Baikaloplana valida being up to 30 cm in length. The well known sponges of Lake Baikal are in the main limited by zoochlorellae to the littoral region. Best known is the endemic Lubomirskia baicalensis which forms vivid green branches rising up 70 cm from rocky substrata and species of Baicalospongia which form large crusts over stones.

Other major groups include the Nematoda, Protozoa are Rotifera and these are all rich in species (Kozhov and Izmest’eva, 1998). Much work on the classification and taxonomy of these groups remains to be done and opinions differ on the reported degrees of endemism. Several theories have been suggested for origin of Baikal’s flora and fauna (Kozhov, 1963), and some of todays endemic species are probably derived from salt water ancestors.

Over 80 species of free-living flat worms (Platyhelminthes) have been identified in the lake and perhaps the most interesting are the wholly endemic deep water species, e.g. the extremely large, Baikaloplana valida (being up to 30 cm in length). Freshwater sponges in Lake Baikal are limited to shallow water regions by zoochlorellae, symbiotic green algae. Perhaps the best known include the endemic Lubomirskia baicalensis, which forms vivid green branches rising up 70 cm from rocky substrata, and species of Baicalospongia which form large crusts over stones.

Plants

With the exception of the partially closed shallow bays (termed locally sors) on some eastern shores of Lake Baikal, higher plants are essentially absent from the open shallow water regions. Exceptions are Elodea canadensis which was introduced into the lake in the 1950s and, with cosmopolitan Myriophyllum and Potomogeton spp., it can be locally common in sheltered regions.

Most of littoral Lake Baikal is divided into zones by various species of benthic algae and this zonation is most clearly marked on rocky shores. Ultothrix, Tetraspora and Draparnaldioides species are mainly responsible for the zonation pattern over the upper c. 20 m of the littoral. Benthic macroalgae do however extend to greater depths with Cladophora and Draparnaldioides taxa together with green cushions of the alga Aegagrophila extending to more than 30 m, depending on water clarity and location. With the exception of Ulothrix, all these genera have endemic species in Baikal.

Perhaps the most important and interesting group of algae in Lake Baikal is the Bacillariophyta or diatoms. Diatoms are ubiquitous siliceous microalgae that are very diverse and many species are extremely good indicators of water quality. Furthermore, because they are siliceous, their remains (frustules) are often well preserved in sediments and so can provide an historical record of past environmental conditions e.g. Mackay et al. 2005.

Many diatom taxa are cosmopolitan but Lake Baikal supports a remarkable number of endemic species. Early pioneering studies, especially by Skvortzow described many new endemic benthic and planktonic species. The most common planktonic diatoms in Lake Baikal are endemic and the annual diatoms crops are usually dominated by Aulacoseira baicalensis and Cyclotella minuta. Also abundant are cosmopolitan species including Synedra acus and Nitzschia acicularis. About 30 modern species of diatom plankton and over 400 benthic taxa exist in the lake today. According to Skvortzow, almost 50 % of diatom taxa in Baikal are endemic. However taxonomic revisions and descriptions of new species from utrastructure studies are constantly changing species estimates for Baikal. It is already clear that the proportion of endemic taxa increases towards the deep littoral (20 m and below) and that centres of hyper-endemicity exist around the lake. Although not as extreme as that found in some animal groups, some benthic diatom genera, such as Didymosphenia, provide evidence of recent speciation.