Dear Lady of the Lake,

We have a business on Clear Lake and a recent news article showing a very green image of the lake taken from a NASA satellite. Can you please educate us on the image and lake water quality so we can inform our clients with accurate information? We also want to keep our family safe while recreating on the lake.

Thanks for your columns, the information you provide is always interesting and helpful.

— Betsie in Buckingham  

Dear Betsie,

Thank you for asking this topical and relevant question Betsie! I appreciate you reaching out and wanting to learn more and to share that information with others.

As the Lake County News provided on June 1, the image that was released from NASA on May 15 captured large bright green swirls throughout the lake. The majority of the green being depicted in this image was the chlorophyll-a pigments from green algae or phytoplankton.

While the article associated with the image described cyanobacteria (or blue-green algae, which is not really an algae, see below) as present in the lake waters, the majority of the green being seen from space did not belong to cyanobacteria, at this time the image was captured.

To be clear (pun intended!), lake water can appear green, from space or from a beach, but without specialized tools or specific laboratory analysis, there is no way to determine if that water contains green algae, cyanobacteria, or the toxins produced by cyanobacteria that can pose a public health hazards.

Throughout the late spring to summer, cyanobacteria will exponentially grow in abundance and become a dominant organism in Clear Lake, that fact is not being disputed. However, many spin-off news articles distributed using the NASA image falsely indicated that what was being shown in the image was demonstrating “pollution, pollutants, toxic algae, or a harmful algal bloom of cyanobacteria” in Clear Lake. Which was, scientifically, inaccurate, when referencing that specific image.

For this specific image, captured and visualized at a specific day and time, the green being observed was the pigments belonging to green algae, phytoplankton, which is a natural and expected occurrence for a large, natural, nutrient-rich lake during this time in spring.

First things first, to best understand what aerial, satellite and ground images are showing when capturing images of lakes and natural water bodies, we have to understand the difference between the types of organisms that can be found in the water column. Then we will talk about the types of satellite tools used to capture those images, what type of filters are being used, or how those images can be modified to showcase different things.

Additionally, it's important to understand the relationship between water, color, and sunlight, I won’t dive into that in today’s column. Those important concepts are described in wonderful, colorful scientific details in my column from February 27, 2022, “Brown water got you down?”.

Green algae, phytoplankton, cyanobacteria, cyanotoxins, blue-green algae and HABs

Algae, green algae or phytoplankton are as different to cyanobacteria as a mushroom is to a horse — they are not similar at all! They just happen to inhabit the same space — water.

Green algae are microscopic plants (sometimes called phytoplankton or diatoms) that are the primary fish food in the lake. Clear Lake is very green — the majority is from the green algae that is growing in the lake. We have a very healthy, vibrant food web in Clear Lake, and that is because the base of this food web — made of green algae- is so abundant.

The prevalence of green algae is also why we have a world class fishery and people come from all over to catch record-sized fish living in the lake. It’s also why we have so many species of birds and water fowls, and wildlife unique to this lake.

Now it’s important to recognize that algae, or phytoplankton, is not cyanobacteria, and cyanobacteria are not algae. When we are standing on a dock, in a boat or from the beach, they may both look like green water, but they actually originate in different biological kingdoms.

The most recent biological classification system, proposed and published in 2015, situates cyanobacteria in their own phyla within the Bacteria Kingdom while phytoplankton, or green algae, have been included in the Plantae Kingdom, and most recently in the newly designated Viridiplantae Kingdom. It’s important to note that while some physiological features have been attributed to the procaryotes, green algae were not grouped into the same biological kingdom with cyanobacteria as they are remarkably different at the cellular level.

It’s important to note that green algae does not produce toxins in Clear Lake and in general it's very rare for green algae, or phytoplankton, to produce toxins in freshwaters. Green algae does conduct photosynthesis, meaning it takes sunlight and CO2 from the air and creates oxygen as a by-product.

Green algae cells, like terrestrial plants, contain chloroplasts, which have a pigment called chlorophyll-a, which is used during photosynthesis to protect the cell from damage while it absorbs sun energy and converts it to sugars and oxygen.

This process can be observed in terrestrial plants when they are actively growing in the spring as they sprout, bright green, new leaves are rich with green pigments like chlorophyll-a. For green algae cells in freshwaters, as soon as the air temperature warms, the day light periods become longer, and spring winds move water and disturb nutrient-rich sediments, the cells reproduce rapidly turning waters bright green.

Cyanobacteria, like green algae, also conducts photosynthesis and therefore responds to warm water, sunlight, and nutrients, by growing rapidly in the spring and summer.

Individual algae and cyanobacteria cells are not visible to the naked eye, however during warm parts of the year when they grow exponentially, large areas of water bodies can become green, or even thick with the abundant numbers of cells.

These large growth patterns are called blooms. For cyanobacteria blooms, when composed of toxin-producing genera(s), they can be a public health hazard and then they are referred to as Harmful Algal Blooms or HABs, even though, as we are learning, they are not algae at all!

Not all cyanobacteria genera will produce toxins, and some that are capable of producing toxins don’t always. For more information about cyanobacteria in Clear Lake, visit the county of Lake cyanobacteria webpage or to my first Lake of the Lake column from July 11, 2021, “Concerned about Cyanobacteria in Soda Bay.”

For Clear Lake, there is a comprehensive cyanobacteria monitoring program managed by Big Valley EPA <https://www.bvrancheria.com/clearlakecyanotoxins> . Big Valley has a website with most recent monitoring data posted. They sample about 20 sites every two weeks in the summer and every month in the winter. Results are also posted on the Facebook page called “Clear Lake Water Quality”.

The most recent monitoring data from Big Valley results, from May 21, 2024, resulted in mostly no detections of cyanobacteria toxin with the exception of one site near Jago bay in the lower arm.

The information gathered from this important monitoring program is used when the County of Lake Health Services makes informational sign posting decisions or issues notices or press releases about safe use and recreation in Clear Lake.

Unfortunately, harmful cyanobacteria blooms are becoming more prevalent in California and across the Country. Monitoring efforts are also increasing, which leads to more reporting of conditions. For example, here in California HABs data can be reported and shared on the My Water Quality HABS portal as part of the California Water Quality Monitoring Council.

On the California HABs portal is a link to a HAB Incident Reports Map, which provides data on voluntarily reported blooms in California. The data may include reports under investigation and/or confirmed incidents of HABs, but it’s a good visual to see what conditions are in water bodies in different parts of the state.

Now that we know the difference between green algae and cyanobacteria and how they interact in the waters of Clear Lake, we will explore how imagery can showcase these organisms and help managers understand what is going on in the lake.

Space, satellites and aerial imagery of Clear Lake

The technology and tools that are available to us as natural resource managers is amazing. Imagine if we had a satellite image of Clear Lake every day and we could see the changes and progressions over time. It would make our jobs easier and of course, more interesting!

Unfortunately satellites orbit the earth and we don’t have one dedicated to Clear Lake at all times, and sometimes a single image that captures a snapshot in time doesn’t always reveal the complexity that exists in large, multi-dimensional natural ecosystems.

In this section I will break down how a NASA satellite composites an image from different light wavelengths and how other satellite tools can be used to further identify conditions in the water column of a specific water body.

According to Kathryn Hansen, the managing editor of the NASA Earth Observatory program, the satellite image of Clear Lake was captured by Wanmei Liang, a Science Data Visualizer for NASA, using Landsat data from the U.S. Geological Survey.

Further, the image is considered “natural color.” Satellite sensors collect light within specific ranges of wavelengths, and NASA data visualizers turn that data into an image. While the process is described in detail here in the tutorial, “How to Make a True-Color Landsat 8 Image,” Hansen described to me the process in brief:

Basically NASA combines data collected by Landsat 9 within three bands (red, green, and blue). The satellite’s image data capture instrument (OLI-2 Relative Spectral Response | Landsat Science) collects data in other bands, too, but these are the three bands that combine to produce a natural-color image closest to what the human eye would see. Data visualizers make other minor adjustments for things like white balance and atmospheric corrections, while trying to stay as true to the landscape as possible.

There are other satellite tools out there that one can view and download images. The image in the heading of today’s column, captured on June 7, 2024, was sourced from the European Space Agency’s Copernicus Sentinel 2-L1C, which is a “European wide-swath, high-resolution, multi-spectral imaging mission [satellite]. Its high-resolution optical images have many applications, including land monitoring, emergency response and security services assistance. The satellite's multispectral imager provides a versatile set of 13 spectral bands spanning from the visible and near infrared to the shortwave infrared.”

I chose this satellite source and image for several reasons, but mostly because it was free to access and share, according to European Union Law. It was also able to provide a “true-color” cloud-free image of Clear Lake from a few days ago, which is important when we are talking about how images from space can showcase a waterbody and current conditions.

Pigments from space!

No, this isn’t a Marvel movie, or adventure story, but the subheading of this section reads as such, does it not? Here we will discuss some unique and valuable tools that satellites can provide for lake managers and the public.

As opposed to the NASA image which primarily displayed the visual light spectrum, there are specific satellite tools and filters that can display the specific pigments produced by both green algae and cyanobacteria in water bodies.

The Harmful Algal Blooms Satellite Analysis Tool freshwater HABmap <https://fhab.sfei.org/> developed by the San Francisco Estuary Institute (SFEI) displays estimated amounts of relevant pigments in water bodies around California.

The map includes approximately 250 water bodies that are large enough to be detected by the satellite. While the default display of the tool reflects a 10-day aggregate of lake conditions, one can toggle on the single, or one-day pixel max option under the imagery tab.

This allows one to see what conditions existed in the lake for a single day of interest, relative to pigments, to better understand if any potentially hazardous conditions need to be considered to protect public health.

I was able to speak with SFEI staff earlier this week in regards to the NASA photo and the misconception that the HABs analysis tool was integrated into the NASA photo. The satellites are completely different and the HABs analysis image capture instrument itself is also different.

According to SFEI, “Satellite data from 2016 to present come from the OLCI instruments on board the Copernicus Sentinel-3 missions provided by EUMETSAT. Additional processing was provided by the NOAA National Ocean Service.”

Remember the image in the heading of this column was sourced from a Sentinel 2 satellite and the NASA image was captured from an instrument aboard a Landsat-9.

If you are excited about space imagery, I encourage you to explore all the data out there that has been collected from both the Landsat and Sentinels missions. According to NASAs Data in Harmony Project website from April 22, 2024, “While the Landsat program provides the longest continuous record of Earth’s land surfaces, Sentinel-2 offers more frequent revisits and additional spectral bands, taking a complete picture of the planet every 5 days.”

I spoke with Tony Hale, PhD and Director for the Environmental Informatics Program at SFEI and he confirmed that the NASA imagery and SFEI are completely separate. He also stressed that while the [HABs] tool can provide some useful information, “it’s critically important to note that the tool offers screening-level analysis by default as a 10-day composite to correct for daily omissions caused by cloud cover and other satellite limitations.

“And even if the imagery detects a high cyanobacterial index or high levels of chlorophyll-a, this must be corroborated by direct sampling before one might declare a threat to public health. This is because the concern is driven largely by the specific species of bacteria, only some of which might produce toxins dangerous to people and wildlife.”

Basically, Dr. Hale indicated that while the SFEI HABs satellite tool is useful for generally assessing a waterbody, the pigments that are displayed, particularly for cyanobacteria, do not differentiate between a toxin producing cyanobacteria bloom and non-toxin producing bloom. Only ground truthing, with lab-analyzed samples from the field, can be used as a verification and confirmation for toxin concentrations.

I wanted to see how the SFEI HABs Satellite tool compared to the NASA photo on May 15, 2024. While the default display reflects a 10-day amalgamation of imagery, I selected a single, one-day pixel max, and matched the days to display a single day of green algae pigment, or chlorophyll-a compared to the same day of cyanobacteria pigment, or phycocyanin.

During 2011-2013, the County of Lake funded and employed a company to conduct specific satellite images of Clear Lake to identify bloom activity and sources of external sedimentation and nutrient loadings. This project was called the Blue Water Satellite Project and it included satellite data and imagery from Landsat 5 & 7 (operated by the United State Geological Survey), paired with field sampling results to calibrate the individual filters used to display chlorophyll-a, sediments, total phosphorus, and phycocyanin.

The satellite technologies and data accessibility has certainly come a long way since this project was funded more than 10 years ago, but the project report, as a public project, is provided on the county of Lake Water Resources webpage, within the Clear Lake Water Quality Section. <>

The purpose of this exercise was to better understand what was being displayed in the NASA image, as this is important information to communicate to our community. Because Clear Lake is an essential part of Lake County’s economy and ecology, as lake managers, and science communicators, it’s our job to make sure that the information (boh visual and in text) being distributed is accurate and based in science.

It’s no secret that Clear Lake sometimes has harmful cyanobacterial blooms during hot portions of summer, and sometimes into fall. When these blooms occur, it’s important for people and pets to be safe when recreating and enjoying the water. Public agencies and lake managers have a responsibility, when the monitoring data is available, to provide any health hazard information to the public.

But if lake managers treated every visible green bloom or swirl as a hazard, and it was not one, then the times when blooms were of real and true concern, and contained hazards, then public agencies warnings and notices would have little meaning.

As a lake biologist I have worked in lakes across several states for the past 15 years. I have a strong appreciation and respect for lakes — along the entire spectrum of lake types from oligotrophic (not productive, low nutrients, clear), mesotrophic (moderately productive, mid range nutrients, light in color) to eutrophic (highly productive, very green, rich in nutrients).

By definition, eutrophic lakes produce and sustain abundant life, and mostly in the color green. Rich green algae blooms and lush, thick aquatic plant beds that grow throughout spring and summer are the defining feature of eutrophic lakes, and they serve a purpose to sustain the web of life that has evolved to depend on those features within those types of lakes.

To expect a eutrophic lake to serve the form and function of an oligotrophic lake is not realistic. Clear Lake is naturally eutrophic, even before post-colonial settlements, However, the lake was surrounded by more wetlands and natural shorelines, which helped compete with the excess nutrients that are now available for more frequent cyanobacterial blooms. Even with 100% restored shorelines, Clear Lake will never be blue and clear, it will always have a rich, abundant, green algae population.

Basically when I saw that NASA image, my first thought as a lake biologist was, “look at all that fish food!” and I was right. The majority of the pigments representing the cells present in the water column, being displayed in that image (at that specific day and time) were mostly derived from green algae cells, which are very much needed to sustain the vibrant fish and wildlife community living in Clear Lake.

Public-funded satellite tools should be used by the public

The use of these important satellite technologies and tools in making informed decisions when recreating on or in freshwater is invaluable. Before planning a water excursion, you can see what current bloom conditions are in a lake or reservoir of interest, and you can use that knowledge to keep your family safe while recreating in the water.

Many people are enjoying Clear Lake, from the majestic views to water sports, even into the summer months. Clear Lake is huge, and while one area of the lake may be impacted by a cyanobacterial bloom, other areas of the lake may not be. There are also other Lake County water bodies that can be recreation destinations if recreating in Clear Lake is a concern for you, your family or pets.

Every day the lake is different, and not every area of the lake is the same on any given day, so you really need to make the right choice for you, based on the conditions you are seeing, both from satellite tools and on the ground, and the type of recreation and water-use you want to participate in on that day. Be aware of your surroundings and look for any posted signage when you are swimming or recreating in any water body.

A good rule of thumb, when recreating in any natural water body, you want to be aware of any weird discolorations, noxious odors, or nuisances, that might make you stop, and consider the conditions. Depending on the season, the presence of “green” water is pretty normal, especially since Clear Lake is eutrophic.

Green water can indicate several things - mostly the presence of green algae. Cyanobacteria can also be present in green water. However, without companion sampling, you really won’t know what is in the water simply by looking at a picture, even if it’s captured from the dock or from space.

— Sincerely, Lady of the Lake

The Lady of the Lake column is written by a limnologist (limnology=study of fresh inland waters) who lives and works in Lake County. Born in northern California, she has a Masters of Science from Michigan State University. She is a Certified Lake Manager from the North American Lake Management Society (NALMS), the current president / chair of the California chapter of the Society for Freshwater Science, and a Lake County Certified Tourism Ambassador since June 2023. She can be reached at This email address is being protected from spambots. You need JavaScript enabled to view it..

Special thanks and citation is given to the following: “NASA Earth Observatory image by Wanmei Liang, using Landsat data from the U.S. Geological Survey”, Kathryn Hansen – Managing Editor, NASA Earth Observatory, Dr. Tony Hale and Randy Turner, San Francisco Estuary Institute & the Aquatic Science Center, https://www.sfei.org/.