Diatoms contribute to a substantial portion of primary production in the oceans and many lakes. Owing to their relatively heavy cell walls and high nutrient requirements, planktonic diatoms are expected to decrease with climate warming because of reduced nutrient redistribution and increasing sinking velocities. Using a historical dataset, this study shows that diatoms were able to maintain their biovolume with increasing stratification in Lake Tahoe over the last decades; however, the diatom community structure changed. Increased stratification and reduced nitrogen to phosphorus ratios selected for smallcelled diatoms, particularly within the Cyclotella genus. An empirical model showed that a shift in phytoplankton species composition and cell size was consistent within different depth strata, indicating that altered nutrient concentrations were not responsible for the change. The increase in small-celled species was sufficient to decrease the average diatom size and thus sinking velocity, which strongly influences energy transfer through the food web and carbon cycling. Our results show that within the diverse group of diatoms, small-sized species with a high surface area to volume ratio were able to adapt to a decrease in mixing intensity, supporting the hypotheses that abiotic drivers affect the size structure of planktonic communities and that warmer climate favours small-sized diatom cells.

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Atmospheric deposition provides most of the dissolved inorganic nitrogen (DIN) and total nitrogen in the annual nutrient load of Lake Tahoe. Deposition also contributes significant amounts of soluble reactive phosphorus (SRP) and total phosphorus loading but plays less of a role than in the case of nitrogen. Most of the DIN probably originates outside of the drainage basin in urban and agricultural areas to the south and west. Spatial patterns of SRP deposition differ from those of DIN and suggest a within‐basin terrestrial source, such as leachate from windblown dust or other particles. Because of atmospheric N deposition, the N∶P (molar) ratio in combined loading is well above the Redfield ratio of 16 and consistent with an observed shift from colimitation by N and P to persistent P limitation in the lake phytoplankton.

Secchi depth has been measured in Lake Tahoe an average of every 12 d since July 1967. Because of the unusual clarity of the lake, Secchi depth measurement is responsive to small changes in light-attenuating particles, and the record exhibits strong variability at the seasonal, interannual, and decadal scales. Using recently developed methods of applied time-series analysis, the mechanisms of change were delineated at each scale. The seasonal pattern is a bimodal one, with two minima at approximately June and December. The June minimum is due mostly to cumulative discharge of suspended sediments following melting of the snowpack. The December minimum is probably a result of mixed-layer deepening as the thermocline passes through layers of phytoplankton and other light-attenuating particles that reach a maximum below the summer mixed layer. The interannual scale exhibits two modes of variability, one during the weakly stratified autumn-winter period and the other during the more stratified springsummer period. The first mode is a result of variable depth of mixing in this unusually deep lake, while the second results from year-to-year changes in spring runoff. A decadal trend also exists (Ϫ0.25 m yr Ϫ1 ), resulting from accumulation of materials in the water column. It is not yet understood, however, how much of this change is due to phytoplankton or recent phytoplankton-derived materials and how much is due to other materials such as mineral suspensoids. Based on the available measurements and physical considerations, both categories may play a significant role.

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