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

Annual estimates of phytoplankton primary productivity from water samples collected at the Lake Tahoe Index Station, 1968 to 2023. 

2023 Evaluation

Status
Considerably Worse Than Target
Trend
Rapid Decline
Confidence
High
View Evaluation

Applicable Standard

WQ2: Maintain annual mean phytoplankton primary productivity at or below 52gmC/m2/yr.

Key Points

  • 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).

About the Threshold

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.
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)

Delivering and Measuring Success

EIP Action Priorities

EIP Indicators

Example EIP Projects

Lake Clarity Indicators

Local and Regional Plans

  • Lake Tahoe TMDL

    Science based plan to restore the historic clarity of Lake Tahoe

Monitoring Programs

Rationale Details

Considerably Worse Than Target. Regular measurements of primary production in Lake Tahoe began in 1970. PPr is measured as the rate of inorganic carbon uptake by phytoplankton using the radioisotope carbon-14 (14C) as a tracer. To determine the PPr, the concentration of DIC in the water, the amount of 14C-DIC added, and the amount of 14C retained in particulate matter (14C-POC) are required. PPr measures the rate of production during an incubation period, but not phytoplankton growth because carbon loss due to respiration during the nighttime is not included.

After a comprehensive review of changes in the methodologies used for this complex measurement over time the dataset has been revised. While the long-term increasing trend has not changed, the magnitude of the annual averages was reduced by an average of 32%.
Rapid Decline. PPr has steadily increased over the period of record and is now nearly five times the level of when measurement began. Rapid decline in this context refers to conditions relative to the desired state.

Confidence Details

High. Monitoring protocols were established in 1967 and have been consistent since that time. Recent status is more than four times the adopted standard.
High. Monitoring protocols were established in 1967 and have been consistent since that time. The long term decline is well documented.
High

Additional Figures and Resources

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