Relevance

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.

Excessive periphyton growth impacts the aesthetic qualities and impairs the 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 algae-covered surfaces. The monitoring program has historically been focused on attached algae (periphyton), but in recent years there has been concern that that floating algae (metaphyton) has increased along Lake Tahoe’s shoreline. The monitoring program has been modified to include both attached and free-floating algae to provide a more comprehensive assessment of nearshore algal conditions.

Human and Environmental Drivers

Nitrogen and phosphorus together support the growth of algae in Lake Tahoe. Phosphorus is a nutrient important to the growth and reproduction of plants and is considered a pollutant of concern in the Lake Tahoe Region. It has also been hypothesized the excrement from crayfish and Asian clams in the lake augments periphyton and metaphtyon growth. Stalked diatoms and green filamentous algae may be the most responsive to fluctuations in nutrient input. 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 as water warms. Periphyton biomass generally peaks around April. Biomass is generally higher on the north and west beaches and lower on the east and south shores, a pattern that has remained relatively stable over time. Periphyton often sloughs off later in the season as a result of wave action and can wash up on shore.

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. Recent observations during wildfires in 2021 suggest that smoke cover and ash deposition impact primary production in the nearshore (Smits et al., 2024). Filamentous algal blooms (FABs) in the nearshore of clear lakes with high water quality have been observed around the world. Their increasing frequency is unexplained, but likely the result of complex changes and the interactions between climate change, nutrient transport, lake hydrodynamics, and ecological change (Vadeboncoeur et al., 2021).

EIP Action Priorities

• Conduct Applied Scientific Research
• Operate and Maintain Stormwater Infrastructure
• Reduce Stormwater Pollution From: Roads and Highways, Forest Roads, Public and Private Parcels
• Restore Priority Meadows, Wetlands, and Lake Tahoe Tributaries

EIP Indicators

• Acres of Habitat Restored or Enhanced
• Acres of SEZ Restored or Enhanced
• Acres Treated for Invasive Species
• Fine Sediment Load Reduction
• Miles of Roads Decommissioned or Retrofitted
• Miles of Street Sweeping
• Nitrogen Load Reduction
• Non-Compliant Wood Stoves Removed or Retrofitted
• Parcels with Stormwater BMPs
• Phosphorous Load Reduction

Monitoring Programs

• Periphyton