Nearshore Attached Algae
Attached Algae
Periphyton biomass index (PBI)
1) Reduce the loading of dissolved inorganic nitrogen, dissolved phosphorus, iron, and other algal nutrients from all sources to meet the 1967-71 mean values for phytoplankton primary productivity and periphyton biomass in the littoral zone. 2)Support actions to reduce the extent and distribution of excessive periphyton (attached) algae in the nearshore (littoral zone) of Lake Tahoe
Indicators measured include: 1) total yearly suspended sediment load (expressed in million metric tonnes/year); and 2) total annual stream flow (expressed in million cubic meters of water). The load for each day at each stream was estimated from multiple regression of measured values.
This Indicator is reported in the following LT Info areas:

No Subcategories for this Indicator.

Program Periphyton


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).


Associated Programs data not provided.

View Threshold Evaluation

Nearshore Attached Algae is include in the Threshold Dashboard. Threshold Indicators are evaluated against Threshold Standards every 4 years. Thresholds are environmental goals and standards for the Lake Tahoe Basin that indirectly define the capacity of the Region to accommodate additional land development.


Attached algae or periphyton refers to a suite of organisms that grow attached to submerged surfaces (e.g. rocks, boats, buoys, piers). In Lake Tahoe these include stalked diatoms, filamentous green algae, and cyanophytes. The communities occupy different portions of the nearshore and exhibit different growth patterns. Stalked diatoms species and filamentous green algae dominate the shallow area between the low and high lake level (eulittoral zone) and grow rapidly in the spring in die off in the summer. Cyanophytes are more stable communities that dominate the deeper portions of the nearshore (Hackley et al., 2016b). Excessive periphyton growth impacts the aesthetic qualities, and impairs beneficial use of the shorezone. When periphyton dies off and breaks free each year, beaches can be fouled and water contact recreation affected. Periphyton growth can also be a safety concern for people attempting to navigate slippery algal covered surfaces.

Human and Environmental Drivers

Nitrogen and phosphorus together support the growth of algae in Lake Tahoe (Hackley et al., 2013). Phosphorus is a nutrient important to the growth and reproduction of plants, and is considered a pollutant of concern in the Lake Tahoe Region (Lahontan and NDEP, 2010b). It has also been hypothesized the excrement from crayfish in the lake augments periphyton growth (Heyvaert et al., 2013a). Stalked diatoms and green filamentous algae may be the most responsive to fluctuations in nutrient input (Hackley et al., 2016b). The stalked diatoms and filamentous green algae that inhabit the shallow waters grow rapidly in the spring with the influx of nutrients and die back rapidly during summer when nutrients are less abundant and waters warm (Hackley et al., 2016b). Biomass is generally higher on the north and west beaches and lower at the lower in the east and south, a pattern that has remained relatively stable over time (Hackley et al., 2016b). Lake level influences periphyton community composition, at lake elevations below 6225 feet blue-green algae contribute substantially to the periphyton levels, while at higher lake levels stalked diatoms and filamentous green algae dominate.


Management with Numeric

Support actions to reduce the extent and distribution of excessive periphyton (attached) algae in the nearshore (littoral zone) of Lake Tahoe.


Indicator Status, Trend, and Confidence
View Evaluation Legend
Status: Insufficient Data to Determine Status or No Target Established
Trend: Little or No Change
Confidence: Low

Status, Trend, and Confidence Details


Insufficient data to determine status. Neither the management standard nor the numerical standard have a defined numeric target, so it is not possible to assess attainment. The current standard references periphyton biomass as measured between 1967 and 1971 as the goal for biomass in the nearshore. This was the California state standard at the time the thresholds were established in 1982, but at the time of establishment, it was noted that, “there were no measurements of periphyton biomass between 1967-1971 (TRPA, 1982).”


Little or no change. Stalked diatoms and filamentous green algae are more likely to rapidly respond to changes in nutrients and are primary subject of this trend assessment (Hackley et al., 2016b). Lake wide there has been no significant change in periphyton biomass between 1982 and 2015 (considering all routine sites together). Two routine sites, Pineland and Incline West, exhibited statistically significant, but small increases in periphyton between 2000 and 2015. A significant decline in periphyton was observed at the Sugar Pine Point site between1982 and 2015. A significant negative trend was observed in both Chlorophyll A and PBI measured at the 50 spring synoptic sites. Because the spring synoptic sampling does not always catch peak biomass and sampling occurs only once a year, caution is urged in interpretation of the trend based on synoptic sampling at the regional level. For full details and additional results see Hackley et al. 2016.


Status: Low. Where insufficient data exists to determine status, confidence in the status determination is low. There is high confidence in the data presented in the assessment. Periphyton monitoring follows established techniques that produce reliable results. Sampling occurs at a single depth and measurements are likely influenced by lake level.

Trend: Low. No trends were identified at the lake wide level using the more reliable dataset for trend analysis (routine sites) and a declining trend was identified lake wide based on the synoptic data. These results are incongruent with the anecdotal reports of visitors and residents.

Overall: Low.


Status and trend assessment considered trends in periphyton biomass data for routine sites and spring synoptic sites. Individual trend analysis was done at multiple levels including site-level, regional, and Region and with respect to three explanatory covariates: upland development, lake level and length of time the site was submerged. Trend assessment was further segmented by time period considered in the assessment, first considering the full period of record 1982 to 2015 and second considering only the period of continuous sampling from 2000 to 2015. A Mann-Kendall test was used to asses for trend significance. Full details on the analytic methods are available in Hackley et al. 2016.

Implementation and Effectiveness
Programs and Actions Implemented to Improve Conditions

The heterogeneous nature of the nearshore environment mean that the drivers of periphyton growth are likely to vary importance around the lake. While the importance of drivers is likely to vary, it is generally thought that controlling nutrient inputs will help reduce periphyton growth (Heyvaert et al., 2013a). SEZ restoration and enhancement, urban growth control limits, best management practices (BMPs) to reduce nutrient and sediment discharge from disturbed soils, retrofit regulations for private and commercial property BMPs, reducing private automobile use through improvements to public transit and alternative transportation modes (with the goal of reducing air pollution and the subsequent deposition of nitrogen and fine sediment), and ongoing allocation of water quality mitigation funds to support erosion control and storm water pollution control projects. Projects completed by EIP partners since between 2009 and 2015 that are likely to have resulted in reduced pollutant loads include have:
• Restored or enhanced 27,150 linear feet of stream channel
• Retrofitted 120.55 miles of road and decommissioned an additional 7.4 miles of road
• Restored or enhanced 120 acres of disturbed forested uplands
• Inspected 108.72 miles of unpaved non-urban roads and maintained 98.2 miles
• Issued 18,076 BMP certificates to commercial, multifamily and single family residential properties

The Lahontan Regional Water Quality Control Board is currently funding USGS and University of Nevada- Reno to explore the drivers of the periphyton biomass near Tahoe City. The results of the study will be available in early 2017 and will contribute to our understanding of what causes periphyton growth and what actions can be taken to limit that growth.

Effectiveness of Programs and Actions

Insufficient data exist to quantitatively evaluate the effectiveness of any individual policy, program or action implemented to reducing periphyton abundance or distribution.

Interim Target

Because neither the management standard nor the numerical standard has a specific target, one cannot reasonably establish an interim target.

Target Attainment Date

Because neither the management standard nor the numerical standard have defined numerical targets, one cannot reasonably establish a target attainment date.

Analytic Approach

No changes recommended

Monitoring Approach

Differences in periphyton community composition can confound the detection of trends in periphyton abundance and growth. The trend analysis presented above attempted to control for biomass changes driven by different periphyton communities by including both lake level and time submerged as covariates. The presence of more stable blue-green algae communities at some locations can confound periphyton trend detection, because when lake level drops blue-green algae communities contribute to elevated measures of periphyton. Consideration should be given to a monitoring design that accounts for these differences in data collection. Public perception is that there has been an increase in algae in recent years, yet synoptic sampling suggests that periphyton levels observed in 2015 were some of the lowest recorded. In addition to exploring different ways to monitor periphyton, additional emphasis should also be placed on understanding what is driving public perceptions of nearshore algae.

Modification of the Threshold Standard or Indicator

The management standard directs TRPA to “support actions” to reduce “excessive” periphyton but provides no further guidance or numeric target for assessing progress or attainment. The numeric standard references periphyton biomass as measured between 1967 and 1971 as the goal for biomass in the nearshore. This was the state standard at the time the thresholds were established in 1982, but at the time of establishment, it was noted that, “There were no measurements of periphyton biomass between 1967-1971 (TRPA, 1982).” Investigations of periphyton between 1967 and 1971 focused primarily on periphyton growth rates, but did provide two rough estimates of total biomass in the lake, which ranged from 147 to 2,180 metric tons (Charles R. Goldman, 1974). The appropriateness of this as a reference point has been questioned because the biomass estimates were 1) based on periphyton growth on artificial surfaces, and thus not representative of natural conditions, and 2) based on a limited data (Heyvaert et al., 2013a). The threshold study report suggests that values measured in 1981 off of undeveloped sites may be a close approximate for what values would have been between 1967 and 1971. The 1981 water year reported measure of periphyton biomass off undeveloped areas was generally below 10 grams per square meter (TRPA, 1982). Caution is urged against using a single measure taken in July (a time at which periphyton levels on the like are generally thought to be lower) to establish baseline conditions for periphyton biomass (personal communication Scott Hackley). Ash Free Dry Weight (the unit of measure used to assess periphyton biomass in 1981) has not always been monitoring by UC Davis, and at present only one additional July measure is available, although additional hard copy records maybe available in Davis (personal communication Scott Hackley). UCD-TERC notes “there exists no baseline for periphyton that be considered as pre-disturbance (Hackley et al., 2016b).“ Consideration in standard modification should be given to observed heterogeneity in nearshore environmental conditions.

Attain or Maintain Threshold

No changes recommended


Periphyton monitoring locations

Additional Figures

No figures available.

Historic 2011 Threshold Evaluation
View Evaluation Legend

Status Insufficient Data to Determine Status or No Target Established
Trend Insufficient Data to Determine Trend
Confidence Not available

The 2011 Evaluation Report has not yet be uploaded or is unavailable.