This data set includes imnological data used for a report "Lake Tahoe Seasonal and Long-Term Clarity Trend Analysis Report."
The clarity of Lake Tahoe, observed using a Secchi disk, continues to be a sentinel metric. It is influenced by both physical and biological processes and has had a declining trend over decades of monitoring, with differences apparent between summer and winter patterns. The report summarized key findings of an investigation on Lake Tahoe water clarity, its long-term variability, and the relative importance of several influencing variables and processes. This investigation focused on factors relevant to the apparent divergence in clarity between summer and winter periods, on how in-lake physical and ecological drivers influenced both seasonal and historic clarity trends.
The trend analyses confirm that clarity declines during the winter period (Dec-Mar) have plateaued (i.e. there is no longer a statistically significant trend up or down) over the last 20 years. The reasons for this are complex and not caused by a single factor but are the net result of multiple physical and biological processes. In-lake physical and ecological drivers also influenced seasonal and historic trends, with winter mixing having a significant impact on seasonal trends. Winter clarity improves during the winter months because deep winter mixing dilutes light attenuating particles and brings up clearer water from depth to the near surface. Months following deep mixing are influenced by the increase in nitrate that is brought to the surface and contributes to algal productivity during the spring and summer periods. Increases in lake temperatures due to climate change have resulted in the lake becoming stratified earlier and stratification lasting longer. The increase in temperatures at the near surface of the lake, combined with changing stream temperatures, affects the relative buoyancy of the lake and the insertion depths of incoming stream flows. This analysis also provides insight on how the abundance of a small diatom, Cyclotella, influences fall and summer clarity. Counts of Cyclotella greater than 100 cells/mL have direct negative impact on clarity, whereas counts below this threshold do not show much impact.