Anecdotal reports suggest the distribution and abundance of algae in Lake Tahoe’s nearshore environment has changed over the last fifty years. Perceived increases in both attached algae (periphyton) and floating algae (metaphyton) are primary concerns among resource managers, residents, and visitors.
NRAP Focus Area Key Photo
UC Davis researcher Scott Hackly measures periphyton growth (Brant Allen / UC Davis TERC)
State of Knowledge

The Lahontan Water Board has funded UC Davis to perform attached algae (periphyton) monitoring for close to 30 years. The program includes routine sampling at a series of sites distributed around the lake and an annual lake perimeter (synoptic) survey to evaluate conditions during the spring period of maximum growth (See 2016 Periphyton trend Analysis). The 2016 trend analysis of synoptic and routine UC-Davis sites identified no significant trends in periphyton growth. Approximately 30 years of routine data were analyzed at nine locations, and the analysis identified a slight increase at the Pineland and Incline West sites. Review of twelve years of data from synoptic sites and all other lake-wide and regional analyses indicated declining periphyton abundance. UC-Davis advises caution regarding the synoptic data review, as that survey is conducted only once per year. UC-Davis is currently reviewing sampling methods to ensure they are appropriate to capture status and trends of periphyton.

The 2017 UC-Davis State of the Lake hypothesizes that metaphyton growth is associated with presence of Asian clams (See SOTL Report Section 6.3).

Phytoplankton Monitoring


Phytoplankton PPr measurements at Lake Tahoe have been made following the same standard operating procedure since the first observations were made in 1967 (Winder et al., 2009). Lake water is collected at the TERC west shore index station, which was found to be representative of the lake’s deepwater condition (Charles Remington Goldman, 1974). For each sampling event, water samples are collected from 13 different depths (between 0-105m) spanning the photic zone (i.e., the vertical zone in the water column exposed to sufficient sunlight for photosynthesis to occur), and analysed to determine carbon assimilation rates using very sensitive methods needed for pristine or oligotrophic waters (Charles Remington 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 annual average phytoplankton primary productivity. Between 1967 and 2006, measurements w



UC Davis has monitored periphyton in Lake Tahoe since 2000. Monitoring also occurred between 1982 and 1985 and 1989 to 1993. The primary periphyton monitoring work are regular sampling work referred to “routine” sampling at nine sites annually (the number of locations has varied historically from six to ten). At each location algal biomass (as chlorophyll a) is sampled five times annually from natural rock surfaces at a depth of 0.5 meters below the water level at the time of sampling. A second type of sampling, referred to a “synoptic” monitoring occurs once a year at 40 additional sites. The timing of synoptic monitoring varies annually and is intended to capture biomass at its peak in the spring. The synoptic monitoring includes collection of chlorophyll a at a sub-set of the sites, as well as a rapid assessment method that quantifies a periphyton biomass index (PBI).