Globally, lake surface water temperatures have warmed rapidly relative to air temperatures, but changes in deepwater temperatures and vertical thermal structure are still largely unknown. We have compiled the most comprehensive data set to date of long-term (1970–2009) summertime vertical temperature profiles in lakes across the world to examine trends and drivers of whole-lake vertical thermal structure. We found significant increases in surface water temperatures across lakes at an average rate of + 0.37 °C decade−1, comparable to changes reported previously for other lakes, and similarly consistent trends of increasing water column stability (+ 0.08 kg m−3 decade−1). In contrast, however, deepwater temperature trends showed little change on average (+ 0.06 °C decade−1), but had high variability across lakes, with trends in individual lakes ranging from − 0.68 °C decade−1 to + 0.65 °C decade−1. The variability in deepwater temperature trends was not explained by trends in either surface water temperatures or thermal stability within lakes, and only 8.4% was explained by lake thermal region or local lake characteristics in a random forest analysis. These findings suggest that external drivers beyond our tested lake characteristics are important in explaining long-term trends in thermal structure, such as local to regional climate patterns or additional external anthropogenic influences.

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Winter primary production in seasonally ice‐covered lakes historically has not been well studied, but it is increasingly recognized as an important component of lake metabolism. Lake Baikal in Siberia is not only the World's oldest, deepest, and most biologically diverse lake, but also where large under‐ice blooms of the diatom Aulacoseira baicalensis (formerly Melosira) occur in some years. The phenomenon of “Melosira years” is noteworthy both for the intensity of the diatom blooms, in which total under‐ice production can be a majority of total annual production, and for the enigmatic regularity of their occurrence every 3–4 yr. The degree to which these episodic blooms might be controlled by external forcing and endogenous lake processes has been debated for decades. We used a 50‐yr time series of phytoplankton observations to statistically model the occurrence of Aulacoseira blooms as a function of meteorological and climatological predictor variables. The results support the hypothesis that a confluence of meteorological conditions in the preceding fall season, which favor clear ice formation with minimal snow cover, also favor Aulacoseira blooms in the following spring. Further, we observe that this confluence of factors is related to relatively strong states of the Siberian High which, while not strictly periodic, do explain a significant fraction of the interannual bloom pattern. Finally, our analyses show that the timing of the peak abundance of A. baicalensis shifted 1.6 months later across the 50‐yr time series, corresponding with the delay in ice‐on timing that has been associated with climate change.

Plankton sampling data from the period of 1951-2010 for Lake Baikal were used in this study. Data have been collected at least monthly, generally every 7-10 days, in depth profiles from the surface to 250 m at a single offshore sampling site in the southern basin, where the water depth is approximately 800 m. Measurements of abiotic variables and phytoplankton sampling were made at discrete water depths. The data for the top 50 m of the lake were averaged for temperature and biological characteristics, noting this is the water layer in the lake exhibiting most of the lake's photosynthetic production, as well as containing the most plankton organisms. Zooplankton samples were collected with a closing plankton net and enumerated at the species level and also identified by age class. Under-ice phytoplankton demonstrated the presence of some succession in spring phytoplankton, including a decreased proportion of endemic algal species. The openwater complex of the alga species demonstrated that some non-endemic species exhibited positive trends. The zooplankton analyses demonstrated a noticeable increasing trend for total zooplankton, the main zooplankton components being Epischura baicalensis, as well as increased Cladoceran numbers. These trends can be hypothetically explained by the following: (i) global climate change (although Lake Baikal also exhibits long-term temperature oscillations); (ii) regional warming (due to construction of reservoir systems during the 1950s-1970s period); (iii) local chemical pollution attributable to increased industrial and agricultural activity in the lake's watershed and airshed basins; and/or (iv) the natural oscillating behaviour of the lake's plankton components.

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