Lake Tahoe is naturally an ultra-oligotrophic lake, meaning that it has extraordinary clarity in part because of extremely low rates of algae growth in its water. In lakes, microscopic free-floating algae plants are the primary producers of food for other lake organisms. Tahoe has a naturally unproductive aquatic ecosystem, compared to the vast majority of lakes in the world. That is one of the reasons its water is among the clearest and most transparent of any large lake in the world.
Does that mean the aquatic ecosystem is barren or uninteresting? Well, not if you like ecology and find out what is actually there. There are many different species of free-floating microscopic algae, called phytoplankton, in Lake Tahoe. One of the most abundant types of algae here is the diatom. Diatoms have a hard glass-like silica shell surrounding their biomass, and when they die and sink to the bottom, they make deposits that have been mined as diatomaceous earth where shallow seas have dried up.
Some microscopic algae plants have a tiny whip-like flagellum, allowing them to swim in search of nutrients. In the summer, throughout the lake, various species of algae stratify into layers from the warm bright surface of the water to the lowest edge of the zone of light penetration, more than 350 feet below the surface. Some algae swim up to the surface for photosynthesis during the daylight and back down to harvest nutrients each night.
The favorite foods of these plants are dissolved Nitrogen and Phosphorous. Scientists have learned that phytoplankton generally thrives and grows fastest when it can find 16 atoms of nitrogen for each atom of Phosphorus. This atomic ratio of 16:1 forms the balanced diet for algae.
Since Tahoe has relatively low concentrations of plant nutrients in its water, the growth of algae throughout the lake is limited. As more nitrogen and phosphorus are added to the lake through erosion and other forms of pollution, growth will increase. Currently, because so much nitrogen is deposited into the lake out of the atmosphere, there is a slight excess of nitrogen with regard to the 16:1 ratio (called the Redfield Ratio). Because of that, many species of algae will not increase their growth rate unless we add more Phosphorus to the water. The lake is currently “phosphorus-limited”.
Scientists reported in the Lake Tahoe Watershed Assessment (2000) that the most effective way to limit the decline in lake clarity is to reduce the amount of phosphorus and fine sediment particles that are delivered into the lake each year. Fine sediment particles not only absorb and scatter light like phytoplankton, but they also carry attached phosphorus that becomes available as food for algae. To reduce phosphorus and fine sediment inflow, scientists recommend best management practices (BMPs), which are land management practices that reduce erosion, promote infiltration of runoff down into the ground, and prevent fertilizers or other sources of plant nutrients from moving from the land to the lake.
Scientists have shown that the rate of algal growth has been steadily growing in Lake Tahoe. While most residents have seen the Tahoe Research Group’s Secchi Depth chart, not as many have looked at the chart showing the rise in the algal growth in the past 40 years. In fact, the rate of algae growth in Lake Tahoe has quadrupled since 1959. This trend continues to increase at a rate of approximately 5 percent to 6 percent per year.
Kendrick Taylor of the Desert Research Institute (DRI) is one of many scientists currently studying Tahoe’s water. He measures near-shore water quality several times each month in order to identify areas with poor water quality and to determine how different weather conditions influence water quality. He has measured the turbidity, or lack of clarity, of the water at the mouths of most of Lake Tahoe’s 63 tributary streams. His near-shore turbidity measurements show the combined effects of the fine suspended soil particles and the microscopic algae that cloud the water in locations around the lake. One of the most significant findings of his research is that the lake exhibits plumes of turbid water near the outlets of streams that drain urbanized land, such as Tahoe City, South Lake Tahoe, Incline Village and Kings Beach. This provides further evidence that it is human land use that contributes the pollutants that are causing the loss of Tahoe’s clarity.
Dr. Charles Goldman and other authors of the Watershed Assessment have stated that current rates of pollution will, if continued, result in a lake that is turning green and of merely “ordinary” clarity by the year 2030. The algae are bent on surviving and reproducing. It is up to us to slow down the rate at which we are providing the food for their growth.
The Lake Tahoe Report 014
Air Date: 2003.05.06
Video Segment: Near Shore Water Quality
Interviewees: Kendrick Taylor (DRI)