There is great debate about the impacts of recent climate change on ecosystems around the world, especially with regard to whether impacts are ‘natural’ or linked to increasing temperatures associated with increasing greenhouse gases (GHGs). Over the last c. 1000 years, three distinct phases of climate are often classified in the literature: the Medieval Warm Period (MWP), the Little Ice Age (LIA) and recent warming since c. 1850 AD. Although these terms still predominate throughout the literature, increasingly calendar ages are being used to precisely date climate changes of interest. Some of our best and unambiguous records of change are derived from monitoring programmes. However, these records do not usually extend very far back in time, and few in central Asia extend back more than a 100 years or so.
(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)
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