Nitrogen and phosphorus colimitation of phytoplankton in Lake Baikal: Insights from a spatial survey and nutrient enrichment experiments

Lake Baikal, Siberia, is the most biodiverse freshwater lake on Earth. However, despite decades of painstaking limnological research on Baikal, broad spatial data on nutrient (nitrogen (N), phosphorus (P), silica (Si)) concentrations and temperature are sparse, as is our understanding of the bottom-up factors that limit phytoplankton in the lake. Earlier studies have suggested both N and P as limiting nutrients in Baikal, but the evidence, mostly based on elemental ratios, is limited and somewhat conflicting. We present experimental evidence that N and P co-limit phytoplankton productivity in some areas of Baikal during summer, along with the results of a comprehensive spatial survey of surface temperature, nutrients and chlorophyll a (Chl a) in Lake Baikal that support the experimental finding of colimitation. Surface water incubations from two trophically contrasting locations revealed co-limitation by N and P, as well as a positive effect of temperature (fluorescence after 5 d was ∼10% higher at 15°C than at 10°C). In a linear model of the survey data (26 sampling locations), N, P, and their interaction (N × P) were all significant predictors of Chl a concentration, indicating that either N or P (or both) may limit summer phytoplankton, depending on location. In contrast to the incubation experiments, temperature was not a significant predictor of Chl a concentration across the 26 sites we sampled. Lake Baikal is undergoing rapid warming and increased nutrient loading, which may boost phytoplankton productivity in the lake; however, the magnitude of this response will depend on ratios of soluble N and P inputs

The “Melosira years” of Lake Baikal: Winter environmental conditions at ice onset predict under‐ice algal blooms in spring

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