• The phytoplankton primary productivity target established by the threshold standard has been exceeded every year since 1975. Since the late 1960s primary productivity has increased nearly fivefold.
• Annual average primary productivity increased substantially in 2021, 2022, and 2023. In the last three years there has also been greater levels of variance between readings than previously observed (TERC, 2024, 2023). Biovolume (algal biomass) in 2021 and 2022 also found several extremely high peaks, but the peaks were not coincident with peak primary production (TERC, 2023). There were significant shifts in the relative abundance of the major phytoplankton taxonomic groups in the lake in 2021 and 2022, with cyanobacteria abundance increasing dramatically. In 2023 the distribution of algal groups shifted back from the 2021/2022 anomalies to a distribution more like the prior 40 years.
• Work in the nearshore areas of Tahoe found that primary production increased by 40 percent in 2021 when the lake was blanketed in smoke from the Caldor, Tamarack, and Dixie fires (Smits et al., 2024).
• The spatial distribution of chlorophyll-a typically shows a deep chlorophyll maximum (DCM) in the summer, but in 2023, this occurred at shallower depths (TERC, 2024).
• The drivers of the ongoing increase in primary production are not well understood. Potential causes include observed shifts in the dominant algae species and warming (TERC, 2023).

Relevance

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.

Human and Environmental Drivers

Primary production varies with nutrient availability, light conditions, and water temperatures. Nutrient (nitrogen and phosphorus) inputs from anthropogenic sources have long been considered the primary driver of increasing PPr in Lake Tahoe (Goldman, 1988, 1974). 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. Research has also suggested that the climate change-driven increase in Tahoe’s water temperature may also be a contributing factor to the increasing primary production (Coats et al., 2006). 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. Recent research on the nearshore of Lake Tahoe suggests that wildfires and smoke cover also influence primary production. Work by Smits et al. (2023) found that gross primary production increased by 40% in the littoral zone of Lake Tahoe during periods in 2021 when the lake was blanketed in smoke from the Caldor, Tamarack, and Dixie fires (Smits et al., 2024)

EIP Action Priorities

• Acquire Environmentally Sensitive Lands for Restoration and Protection
• Reduce Stormwater Pollution From: Roads and Highways, Forest Roads, Public and Private Parcels

EIP Indicators

• Acres of SEZ Restored or Enhanced
• Miles of Roads Decommissioned or Retrofitted

Example EIP Projects

• 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

• Fine Sediment Load Reduction

  Total Maximum Daily Load implementers collectively prevent roughly 477,000 pounds per year of fine sediment from reaching Lake Tahoe.

• Nitrogen Load Reduction

  Nitrogen load reduction from urban runoff.

• Phosphorous Load Reduction

  Phosphorus load reduction from urban runoff.

Local and Regional Plans

• Lake Tahoe TMDL

  Science based plan to restore the historic clarity of Lake Tahoe

Monitoring Programs

• Pelagic Water Quality (Clarity)

  UC Davis Tahoe Environmental Research Center (TERC) lead water quality monitoring program.

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