'Keep Tahoe Blue' warns about algae advisory signs at Lake Tahoe beaches

LAKE TAHOE, CA — Lake Tahoe is famous for its blueness. The growth and spread of algae is one reason blue lakes around the world can go green in the summer.

Algae forms the base of many food webs, and most algae in Lake Tahoe, though sometimes unattractive, does not pose a health risk to people or animals that enjoy the Lake.

Harmful algal blooms (HABs) are a different story.

HABs are often mistaken for harmless types of algae or cyanobacteria that look like algae, which naturally occur in Tahoe and can be found clinging to rocks and washing up on beaches. HABs, however, pose a risk to public health and safety.

On the California side of Lake Tahoe, the Lahontan Regional Water Quality Control Board (Water Board) tests for, monitors and provides signage to warn the public of risks associated with HABs.

In 2022, HABs were discovered on the south shore along Regan Beach, Barton Beach, Kiva Beach, and in the shallow lagoons of the Tahoe Keys.

What is a harmful algal bloom?

HABs are formed by tiny organisms called cyanobacteria and can make the water a variety of colors such as green, white, red, or brown. HABs may look like thick paint floating on the water or as algal mats in rivers, streams, and along the shallow shorelines of lakes.

In lakes, HABs usually appear as a mix of intense shades of green in a paint-like sheen on the water’s surface. In rivers, HABs look like algal mats that are attached to the channel bottom. Algal mats can also become stranded on shorelines.

Common factors contributing to blooms are warmer temperatures, slow or stagnant water and nutrients in the water that serve as food for algal organisms.

Current research suggests that rising temperatures and changing precipitation events associated with climate change may contribute to the increase in HAB events.

Here’s why:

• Warmer temperatures can favor blooms because many cyanobacteria are adapted to hot conditions where they often outcompete non-toxic forms of algae.

• Intervals of drought can contribute to stagnant or slow-moving water, which helps harmful algae outcompete non-toxic forms of algae.

• If extreme rainfall events become more frequent (as they are projected to) stormwater, surface water, and groundwater discharges will deliver more nutrients to water bodies, fueling the growth of HABs.

When are Lake Tahoe beaches sampled for harmful algal blooms?

When members of the public report possible bloom sightings through this website, Water Board staff respond by collecting and testing water samples whenever possible.

Water Board expert staff also routinely assess Lake Tahoe’s waters at numerous locations before popular holiday weekends, such as Memorial Day, Fourth of July, and Labor Day.

To keep the public safe and informed, Water Board staff recommend that land managers post advisory signs near HAB-impacted waters according to a three-tiered, statewide voluntary guidance system. The advisory levels rise in concern from “Caution” to “Warning” to “Danger,” with precautions to protect people and their pets.

Visual indicators of a HAB, including a scummy or discolored appearance of the water, or a suspected human or animal illness that is under investigation, are enough to justify posting the lowest level Caution advisory sign. Lab testing can also trigger posting the Caution advisory sign.

The next two elevated advisory levels, Warning and Danger, are recommended when lab tests reveal higher levels of toxins in the water, pointing to greater risk for people and animals.

As the risk and advisory levels increase from the low to higher tier, so do the recommended restrictions on interacting with the water and algae.

Where were water samples tested in Lake Tahoe this past summer?

The most visual signs of HABs are normally seen along the shore where water currents push floating blooms to the water’s edge. However, HABs can also be concentrated in open areas throughout a waterbody.

This year, samples were collected from multiple areas, including Kings Beach, Skylandia Beach, McKinney Bay, Sunnyside Marina, Taylor Creek Outlet, Kiva Beach, Pope Beach, Cove East, Barton Beach, Regan Beach, El Dorado Beach, and Connolly Beach.

During field visits to these areas by Water Board staff, visual indicators of a HAB were observed at all locations, so land managers were encouraged to post advisory signage at the Caution tier. After lab testing, only samples collected from Barton, Regan, and Kiva contained toxins that also met the trigger level of the Caution tier.

The south shore of Lake Tahoe has experienced a noticeable amount of material washing up along the shoreline throughout this summer. The material appears to be a combination of aquatic weeds, non-toxic algae called periphyton and metaphyton, debris, and sometimes small amounts of cyanobacteria.

As this washed-up material decays, it can create unpleasant odors and change colors. In response to complaints and new bloom reports filed for these areas, Water Board staff collected and tested water samples.

Results often did not indicate the presence of toxins, yet the Caution advisory level was still triggered because potential toxin-producing organisms can rapidly change in their toxicity and may not be detected in a single field visit.

For this reason, people and pets are advised to keep away from suspected algal bloom material.

Monitoring for harmful algal blooms in the Tahoe Keys

Visual inspections of the waterways and water sampling have been conducted in the Tahoe Keys since 2017 to inform homeowners of potential health risks.

In each of the past five summers, HABs have been detected in the Keys’ shallow lagoons. This year, for the first time, lab results indicated toxins above the Danger advisory level, signaling the highest level of toxins and risk to the public.

In 2022, monitoring at additional areas throughout the lagoons was increased due to the Control Methods Test (CMT), which is a three-year project to evaluate treatment methods to control target aquatic invasive weeds. Sampling of the CMT area is being conducted by contracted field crews and State and Regional Water Board staff.

During work on the CMT project, visual indicators of a HAB were observed in some areas, so additional sampling was conducted. On July 12, a Caution advisory level for HABs was triggered at several locations within the Tahoe Keys lagoons.

At separate times later in the summer, sampling prompted two areas to move to the elevated advisory Warning level. This occurred near the lagoon between Carson Court and Kokanee Way on August 18, and the area between Kokanee Way and Lido Drive on September 1.

Recent sampling at two different areas indicated toxin levels had reached a Danger level advisory; one in the lagoon located at the corner of Venice Drive and Alpine Drive, and a second in the lagoon between Kokanee Way and Marconi Way on September 8 and 15.

To find the most up to date monitoring results visit the State HAB Incident Report Map.

How to identify and report a harmful algal bloom

Most algae in Lake Tahoe are not organisms that form HABs.

The UC Davis Tahoe Environmental Research Center’s nearshore algae monitoring program has been tracking and measuring these algae since 2017.

In Lake Tahoe, algae that are attached to gravel and rocks and appear as yellow-brown stalks, as well as stringy, filamentous algae, are not HABs and do not need to be reported. However, if the algae looks like the images in the advisory signage, please share your sightings of these potential HABs.

In other areas outside of Lake Tahoe, like in streams, HABs can appear as filamentous algae so these materials should be avoided and reported.

If you think you’ve discovered a harmful algal bloom on the California side of the lake, report it to the Lahontan Water Quality Control Board at this site: bit.ly/report-a-bloom. In Nevada, call 1-888-331-6337 to reach the Nevada Division of Environmental Protection.

All reports are taken seriously by the agencies and can help keep others safe.

In addition to contacting the two agencies, you’re encouraged to submit your sightings through the “Algae Watch” survey on the Citizen Science Tahoe Web App (available at citizensciencetahoe.app), which allows Tahoe scientists to locate where algae is and what it’s doing.