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Phytoplankton Primary Productivity

Status and Trend

Interpretation and Commentary

Phytoplankton Primary Productivity

Status: Considerably Worse Than Target
Trend: Rapid Decline
Confidence: High

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  • Relevance - Phytoplankton primary productivity (PPr) is a measure of the rate at which solar energy is converted into chemical energy by photosynthetic phytoplankton organisms (free-floating algae) and is measured in grams of carbon per square meter per year. Free-floating algae (i.e., phytoplankton) occur throughout Lake Tahoe, and contribute to the decline in water transparency by absorbing light for photosynthesis and by scattering light. From an ecological perspective, algae are a dominant 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, algal 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. Because Lake Tahoe is an ultraoligotrophic lake, it is desirable to maintain phytoplankton primary productivity at low levels.
  • Adopted Standards  - Achieve the long-term water quality standard of annual mean phytoplankton primary productivity: 52 gC/m2/yr (grams of carbon per square meter per year).
  • Indicator - Depth-integrated annual average phytoplankton primary productivity expressed as grams Carbon per meter squared per year (gC/m2/yr).
  • Status – The phytoplankton PPr indicator is used to determine compliance with TRPA’s Pelagic Lake Tahoe phytoplankton productivity standard of 52 gC/m2/yr. The Threshold Standard is based on measurements collected over four years (1968-1971) (Lahontan and NDEP 2010). Phytoplankton primary productivity has remained well above the standard since it was established in 1982. In 2010, phytoplankton PPr was 194 gC/m2/yr. The status of Lake Tahoe’s phytoplankton primary productivity is considerably worse than the standard because the 2010 value is 3.7 times (373 percent) the TRPA’s Threshold Standard.

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  • Trend The line of best fit was determined statistically using a linear regression model. The data show phytoplankton PPr has steadily increased (worsened) since measurements began at Lake Tahoe in 1968 (TERC 2011a). The maximum annual phytoplankton PPr was recorded in 2008 (~215 gC/m2/year). The slope of the trendline for the entire period of record (1968-2010) yields an estimated rate of increase in phytoplankton PPr of 4.3 gC/m2/year, which equates to an 8.3 percent/yr increase in PPr relative to the TRPA Threshold Standard. Thus, this indicator exhibits a rapidly declining trend relative to the adopted standard. It should be noted however, that phytoplankton PPr declined in 2009 and 2010 from a monitoring record high recorded in 2008.


  • Status – There is high confidence in the status determination. Phytoplankton PPr measurements at Lake Tahoe use Standard Operating Procedures and researchers follow an extensive quality assurance and data analysis plan (Winder and Reuter 2009). Researchers have 43 years of data, generated using the same protocols. This is one of the longest continuous records for primary productivity in the world. The most recent value is from 2010. Early studies by UC Davis show that the sampling location is representative of the Lake’s deepwater condition (Goldman 1974).
  • Trend – Confidence in the long-term trend also is “high.” The linear regression model explains 96 percent of the variability in annual average phytoplankton PPr values collected over the period of record. The slope of the line is significantly different from zero (t = 32.73; n = 43; P < 0.001).
  • Overall Confidence - The overall confidence in this indicator is “high” because there is high confidence in both the status and the trend.
  • Human and Environmental Drivers - Increasing nutrients (nitrogen and phosphorus) inputs are considered a main cause of increasing PPr in temperate lakes (Conley et al. 2009). It is suspected that activities associated with urbanization and watershed disturbance influence Lake Tahoe’s PPr through the generation and subsequent runoff or atmospheric deposition of nutrients. The nutrient source analysis conducted for the Lake Tahoe TMDL indicates that both urban andnon-urban sources of nitrogen and phosphorus are important contributors of nutrients to Lake Tahoe (Lahontan and NDEP 2010). Meteorological conditions (e.g., wet vs. dry years) also affect PPr, presumably due to changes in tributary loads of nutrients and differences in the magnitude of physical processes within the Lake. However, the trend analysis suggests these effects have not substantially influenced the overall trend.
  • Monitoring Approach – Phytoplankton PPr measurements at Lake Tahoe have been made following the same Standard Operating Procedure since the first observations were made in 1967 (Winder and Reuter 2009). Lake water is collected at the UC Davis – Tahoe Environmental Research Center (TERC) West Shore Index station, which was found to be representative of the Lake’s deepwater condition (Goldman 1974). Between 1967 and 2006, measurements were taken, on average, every 10-14 days. In 2007, measurement frequency was reduced to once per month due to budget constraints. This reduction in the frequency of measurements was only made after a careful analysis of consequences to the long-term record confirmed that this change in measurement frequency was appropriate. The monthly measurements produced a very similar plot to the bi-monthly data (Winder and Reuter 2009). For each sampling event, water samples are collected from 13 different depths spanning the photic zone (i.e., the vertical zone in the water column exposed to sufficient sunlight for photosynthesis to occur), and analyzed to determine carbon assimilation rates using very sensitive methods needed for pristine or oligotrophic waters (Goldman 1974). Values from the various samples are aggregated to yield a depth-integrated PPr value. These depth-integrated values are aggregated over the calendar year to generate an estimate of average annual phytoplankton PPr.
  • Monitoring Partners – UC Davis–Tahoe Environmental Research Center (TERC), Tahoe Regional Planning Agency, Lahontan Regional Water Quality Control Board.


  • Monitoring Plan
  • Conceptual Model


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Locations where UC Davis measures primary productivity and other water quality parameters.

Trend Charts

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Annual estimates of phytoplankton primary productivity from water samples collected at the Lake Tahoe index station. Phytoplankton primary productivity has steadily increased (worsened) since 1968, and values have been well above the TRPA standard since it was established in 1982 (TERC 2011a).

Additional Info


Additional Information

Last Updated on Friday, 26 October 2012 14:15