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Primary productivity is a measure of the rate at which solar energy is converted into chemical energy by photosynthetic organisms. Phytoplankton is used to assess primary productivity in Lake Tahoe. At low levels, primary productivity can become a limiting factor in the population size of organisms that depend directly or indirectly on this source of food. Conversely, extremely high primary productivity can result in nuisance algal blooms, degradation of drinking water taste and odor, low dissolved oxygen, and fish kills. It is suspected that activities associated with urbanization and watershed disturbance influence Lake Tahoe’s primary productivity through the release of nutrients and subsequent transport in runoff, or through the atmospheric deposition of nutrients. Drivers influencing the delivery of fine sediment and nutrients include urban development, anthropogenic and natural disturbance in the undeveloped portions of the watershed and local and regional climate. Many programs throughout the Tahoe Basin are aimed at reducing nutrient inputs into Lake Tahoe such as stormwater reduction and stream restoration projects. Primary productivity has been measured at Lake Tahoe by UC Davis since 1968.
- Status
- Considerably Worse Than Target
- Trend
- Moderate Decline
- Confidence
- High
- Phytoplankton primary productivity has remained above the target established by the threshold standard since 1970.
- Primary productivity at the time the standard was adopted was nearly double the target, and in 2018 primary productivity was four times the standard.
- Primary productivity continues to increase, but the rate of increase has slowed in the last ten years.
Lake Clarity Station
- Primary productivity is a measure of the rate at which solar energy is converted into chemical energy by photosynthetic organisms. Lake Tahoe is an ultraoligotrophic lake and management goals include maintaining this status due to its historic, cultural, economic, and aesthetic value. Monitoring work in Lake Tahoe includes measurements of phytoplankton (free-floating algae) primary productivity (PPr). Phytoplankton occurs naturally throughout Lake Tahoe, and they contribute to reductions in water transparency by absorbing light for photosynthesis, and by scattering light. From an ecological perspective, phytoplankton are a dominant and essential component of the aquatic food web, providing an important source of energy and nutrients that support other organisms in the food web (e.g., zooplankton and herbivorous fish). At low levels, PPr can become a limiting factor in the population size of organisms that depend directly or indirectly on this source of food. Conversely, extremely high PPr can result in nuisance algal blooms, degradation of drinking water taste and odor, low dissolved oxygen, and fish kills.
- Nutrient (nitrogen and phosphorus) inputs from anthropogenic sources are considered the primary driver of increasing PPr in temperate lakes. It is suspected that activities associated with urbanization and watershed disturbance influence Lake Tahoe’s PPr through the release of nutrients and subsequent transport in runoff, or through the atmospheric deposition of nutrients. The nutrient source analysis for the Lake Tahoe TMDL indicates that both urban and non-urban sources of nitrogen and phosphorus are important contributors of nutrients to Lake Tahoe. Meteorological conditions (e.g., wet vs. dry years) also affect PPr, due to changes in tributary loads of nutrients, and differences in the magnitude of physical processes within the Lake (e.g., deep lake mixing). However, the trend suggests these factors have not substantially influenced the overall trend. The source of nutrients that are driving the increase in PPr is currently unknown.
Example EIP Projects
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State Route 89 Water Quality Improvement Project - El Dorado County Line to State Route 28
Caltrans completed this roadway retrofit project in 2017 to improve collection and treatment of stormwater runoff along 8.6 miles of roadway, some portions adjacent to Lake Tahoe.
Lake Clarity Indicators
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Fine Sediment Load Reduction
Total Maximum Daily Load implementers collectively prevent roughly 477,000 pounds per year of fine sediment from reaching Lake Tahoe.
- Considerably worse than target. The current status (204.98 g C m-2 y-1) of Lake Tahoe’s phytoplankton primary productivity is considerably worse than the standard. The 2018 value is four times worse than the TRPA threshold standard.
- Rapid decline. The line of best fit was determined statistically using a linear regression model. The data show phytoplankton PPr has steadily increased (worsened) since annual measurements began at Lake Tahoe in 1968. For the period of record (1968 to 2018) phytoplankton PPr has increased by 3.3 percent annually. The slope of the trend line for the entire period of record (1968 to 2018) yields an estimated rate of increase in phytoplankton PPr of 4.2 g C m-2 y-1, which equates to an eight percent per year increase in PPr relative to the TRPA threshold standard. Thus, this indicator exhibits a rapidly increasing (worsening) trend relative to the adopted standard.
- High. The methodologies for data collection and analysis are well established and broadly accepted. Both the data and analyses have been used in a number of peer reviewed publications.
- High. Monitoring protocols were established in 1967 and have been consistent since that time. The methodologies for data collection and analysis are well established and broadly accepted.
- High.
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