Studies in biodiversity are overwhelmingly concerned with terrestrial systems and although there are notable exceptions, aquatic diversity of freshwaters is less well researched. Lakes present a special case for diversity studies: they vary greatly in age and type but exist as discrete and discontinuously distributed units that occur in most regions of the earth. Those lakes that are geologically old tend to support endemic biotas, examples being Lake Ohrid and the African Rift Valley lakes. However, of all the world’s lakes, Baikal is perhaps the most remarkable for biodiversity. It is unique in supporting a major deep water (> 1200 m) fauna composed almost entirely of endemic species. So intense have been the speciation processes in Lake Baikal that large species ‘flocks’ have arisen in some groups. Whereas molluscs have radiated strongly in Lake Tanganyika, crustaceans have formed the largest species flocks in Baikal. Particularly intense speciation has occurred in the Gammarididae. A full account of diversity and endemicity can be found in Kozhova, O.M. and Izmest’eva, L.R. (1998) Lake Baikal: evolution and biodiversity. Leiden, Backhuys Publishers. 447 pp..
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.
