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 nine 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). Contrary to public and manager perception, a 2016 data analysis identified neither significant lake-wide or site-scale trends in periphyton growth. In fact, the analysis identified only a slight increase at the Pineland and Incline West sites over this time.

To address the issue that monitoring protocols are not robust enough to detect changes and trends in periphyton biomass, UCD proposed revisions to the monitoring program protocols. In January of 2020, the Nearshore Agency Working Group tasked the Tahoe Science Advisory Council (Science Council) to conduct a peer review of the proposed monitoring protocols. The reviewers concluded the proposed protocols are consistent with best practices for other lakes. Revised protocols are currently being piloted at several locations to facilitate results comparison of old and new protocols.  

In subsequent discussions, Science Council members theorized the perception of increased algal growth may be a result of changes in metaphyton (filamentous algae) growth. Unattached to the lakebed, these filamentous algae tend to get washed onshore, fouling the recreation experience of beachgoers. In the summer of 2020, UCD completed a feasibility study of several remote sensing platforms to characterize metaphyton populations. The study concluded that monitoring metaphyton was feasible at a nominal additional cost to the periphyton monitoring program.   

As such, the Nearshore Agency Working Group in coordination with science partners are working to develop an Integrated Algal Monitoring program that measures status and detects trends in periphyton and metaphyton populations at locations that are tied to Lake Tahoe’s recreational beneficial uses.

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).