Issue (Who cares and why?)
Lake Tahoe is world renowned for its striking blue color and amazing clarity. In fact, the EPA has granted Tahoe the status of Outstanding National Resource Water. This designation affords the highest level of protection, strictly forbidding degradation of water quality. However, since 1968, there has been a decline of Lake Tahoe’s clarity at an alarming rate of nearly one foot per year. Total Maximum Daily Loads (TMDLs) are water quality restoration plans required by the federal Clean Water Act to ensure that water quality standards in impaired water bodies are achieved and beneficial uses are restored and protected. The Lake Tahoe Water Clarity Model is being developed to support the TMDL process.
The production of algae in the lake has become limited by the supply of phosphorus, meaning that inputs of phosphorus from the watershed are likely to be causing much of the current loss in clarity of the water. However, not all phosphorus entering the lake can be utilized by algae. Currently, we do not have a reliable estimate for biological available phosphorus loading to Lake Tahoe. The model assumes all chemical forms of phosphorus are contributing to the lakes decline in clarity. This project looks at updating the model by determining the amount of biologically available phosphorus in Lake Tahoe.
What has been done?
Over a two year time period, across all seasons, investigators collected samples from Lake Tahoe’s steams, groundwater, and shorelines. They then concentrated suspended samples onto glass fiber filters, incubated them with phosphorus-starved algal cultures for 21 days and measured the total phosphorus released from the filters to determine what portion of the phosphorus was taken up by algae.
Impact
Phosphorus was identified as a nutrient limiting factor to algal growth in Lake Tahoe. Biologically available phosphorus from the tributary stream suspended sediments were, on average 21% of the total extractable phosphorus, 36% for urban runoff, and 5% for potentially erodable stream bank soils. This information proved pivotal in updating the Water Quality sub-model in the Lake Tahoe Water Clarity Model, which in turn decides how best to implement Total Maximum Daily Loads.