Description: Here we present a multi-year dataset recording both precipitation and water column geochemistry from Lake Tahoe and regional precipitation between the Pacific coast (Bodega Bay, CA), central valley (Davis, CA) and Lake Tahoe Basin, collected over a three year period between 2010 and 2013. Precipitation data yield a regional meteoric waterline: δ2Hprecip. = 6.4 (±4.2) + 8.0 (±0.4) * δ18Oprecip. The Lake Tahoe water column data are from seasonal aliquots of water collected from 13 discrete depths between 0 and 450m in the northern part of the lake at the Mid-Lake sample station (39o 7.535oN, 120o 0.731oW). Sampling during each quarter year was done 1-2 times. The measured stable isotope values herein were averaged for each three month seasonal sampling period when multiple samples were collected in a quarter in order to provide resolution at the mean seasonal scale for each quarter year sampling interval. Lake Tahoe water is strongly influenced by the evaporation and precipitation balance of the basin. Maximum isotopic variability in the lake is observed in the epilimnion (shallower than 25m), with minor isotopic variability through the hypolimnion to near lake bottom at 450m. We also present the first analyses of Lake Tahoe δ18Odatiom from middle (~5500ybp) to late Holocene diatom frustules collected from sediment cores. Our δ18Odiatom data suggest mid-Holocene lake water was enriched in 18O by ~ 2-3‰ relative to lake water after 2,000 ybp. Lastly, we also archive the calculations for our water balance equations. Calculated values use the magnitudes of modern Lake Tahoe water mass balance components coupled with measured and estimated δ18O levels in these water balance components. We use estimates of E (evaporation)= 95.8 cm/yr and PRG (Precipitation, surface Runoff, and Groundwater) = 142.1 cm/yr (over the mean surface area of Lake Tahoe), which have previously been shown to be highly accurate over the recent long term water year (WY) average (mean value calculated from WY 1958 to 2004).

Description: Lake Tahoe is an ultra-oligotrophic subalpine lake that is renowned for its clarity. The region experiences little cloud cover and is one of the most UV transparent lakes in the world. As such, it is an ideal environment to study the role of UV radiation in aquatic ecosystems. Long-term trends in Secchi depths showed that water transparency to visible light has decreased in recent decades, but limited data are available on the UV transparency of the lake. Here we examine how ultraviolet radiation varies relative to longer-wavelength photosynthetically active radiation (PAR, 400-700 nm, visible wavelengths) horizontally along inshore-offshore transects in the lake and vertically within the water column as well as temporally throughout 2007. UV transparency was more variable than PAR transparency horizontally across the lake and throughout the year. Seasonal patterns of Secchi transparency differed from both UV and PAR, indicating that different substances may be responsible for controlling transparency to UV, PAR, and Secchi. In surface waters, UVA (380 nm) often attenuated more slowly than PAR, a pattern visible in only exceptionally transparent waters with very low dissolved organic carbon. On many sampling dates, UV transparency decreased progressively with depth suggesting surface photobleaching, reductions in particulate matter, increasing chlorophyll a, or some combination of these increased during summer months. Combining these patterns of UV transparency with data on visible light provides a more comprehensive understanding of ecosystem structure, function, and effects of environmental change in highly transparent alpine and subalpine lakes such as Tahoe.

Description: 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 small-celled 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.