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ABSTRACTThe main producer of organic matter in Lake Baikal is diatoms, whose development largely depends on silica concentrations in the lake water. Silica is introduced to the lake from inflowing surface waters. The Selenga River, the main tributary of the lake, delivers ∼50% of the total surface water inflow and more than half of the chemical inputs to Lake Baikal. The average annual input of silica to the lake is ∼114 000 t during years when water level in the river approximates the long term average. The period under study (2001–2017), however, was characterized by exceptionally low water levels in the Selenga River and an annual average reduction of silica input to Lake Baikal up to 56 000 t. Since the beginning of the 21st century, the total deficit of silica input to the lake from the Selenga River is ∼650 000 t. Currently, the silica concentrations in the Selenga River Shoal have declined to values that limit the development of dominant diatom species in Lake Baikal.

Chrysophytes are a characteristic component of phytoplankton in oligotrophic arctic and subarctic lakes. In this study, scanning electron microscopy has been used to analyse the species composition and abundance of silica-scaled chrysophytes in eight small lakes of the Lower Yenisei basin. A total of 38 species and intraspecific taxa have been recorded, including representatives of the generaMallomonas(12 taxa),Spiniferomonas(9),Synura(8),Chrysosphaerella(4), andParaphysomonas(5). Along with 23 widespread taxa, their list includes such rare species asParaphysomonas uniformis hemiradia,P.bandaiensis,Spiniferomonas cornuta,S. crucigera,S. minuta,S. triangularis,S. trioralisf.cuspidata,Mallomonas alataf.hualvensis,M. crassisquamavar.papillosa,M. pseudomatvienkoae,M. floracf.palermii,Synura punctulosa,S. truttae,S. mollispina,S. curtispina, andS. synuroidea. It should be noted thatMallomonas pseudomatvienkoae,M. floracf.palermii,Synura truttae, andS. synuroideahave not been previously described from water bodies of Russia. Statistical analysis revealed that the diversity and content of scaled chrysophyte species in the studied lakes of the Lower Yenisei basin are significantly influenced by the concentration of silicon, water conductivity, and total ions.

AbstractHabitats of organisms in Lake Baikal have been comprehensively monitored for many years. In 2010–2015, we conducted 8 research expeditions to study ionic composition of the water and measure concentrations of dissolved oxygen, nutrients, organic carbon, and persistent organic pollutants in the entire pelagic area of Lake Baikal. Our data showed that the ionic composition was stable at all depths in the pelagic zone of the lake. We also recorded seasonal variability, vertical stratification, and spatial variations in the content of nutrients, organic matter, and gas components. The concentrations of major ions, nutrients, and dissolved oxygen were favourable for the growth of hydrobionts in the offshore pelagic zone. Concentrations of pollutants in Lake Baikal, such as persistent organic pollutants and heavy metals, were low and did not directly affect the biota. The water in specific areas of the littoral zone of Lake Baikal, however, was polluted near large settlements (Listvyanka, Severobaikalsk, Baikalsk, and Slyudyanka) and in bays because of unsatisfactory operations of sewage treatment plants, growing tourist activity, and intense ship traffic. Continuing investigations are needed to elucidate causes of eutrophication in the littoral zone of Lake Baikal.