EOLakeWatch: Frequently asked questions

What is remote sensing?

Remote sensing refers to the use of satellites or aircraft to obtain information about the Earth’s environment (for example: water, land surface, atmosphere) from a distance. Remote sensing allows us to obtain information about lake conditions over large geographical areas on a frequent basis and, importantly, allows monitoring of remote regions not easily accessible for ground based observations.

Learn more from Natural Resources Canada.

What satellites do you use?

Our image processing workflow uses data from the European Space Agency’s (ESA’s) Ocean and Land Colour Instrument (OLCI) sensor on the Sentinel 3A and Sentinel 3B satellites. The Sentinel 3A satellite was launched in February 2016 with Sentinel-3B following in April 2018. Historical bloom conditions are derived from OLCI’s predecessor, ESA’s MEdium Resolution Imaging Spectrometer (MERIS) sensor on the Envisat satellite (2002-2012).

To learn more about ESA’s Copernicus Program and the Sentinel series of satellites, visit the ESA website.

More information on ocean colour can be found on NASA's Ocean Color Web page.

What is the resolution of the satellite imagery?

The spatial resolution (each individual pixel size) from the Sentinel-3 OLCI sensor is 300m x 300m. Each lake-wide image is a combination of thousands of individual pixel observations. Although the satellites provide near-daily observations, cloud cover reduces the frequency of useable imagery. To reduce data gaps, we use a 14-day rolling average of daily images to obtain complete lake-wide coverage.

How do you measure algal blooms from satellites?

Optical sensors mounted on Earth Observation satellites view changes in water colour (measured as water-leaving radiance or remote-sensing reflectance) at multiple wavelengths across the electromagnetic spectrum. Phytoplankton, suspended particles, and dissolved materials determine the colour of the water by absorbing and scattering different wavelengths of light. Algorithms have been developed to interpret the spectral signatures measured by satellite sensors and extract quantitative information on bloom conditions. For each pixel in a satellite image, we can determine:

  1.  whether a bloom has been detected, and
  2.  the amount of chlorophyll-a present, as an indicator of the intensity of the bloom.
How do you define bloom conditions?

For the algal bloom imagery and derived algal bloom indices presented on this site, we define an algal bloom as a detection of Chlorophyll-a concentration greater than 10 μg/L. This threshold follows the World Health Organization guideline levels, for relatively mild and/or low probabilities of adverse health effects, of 20,000 cyanobacterial cells per ml (corresponding to 10 μg/L of chlorophyll-a under conditions of cyanobacterial dominance).

Why is there no algal bloom information for 2012-2015?

The MERIS sensor was in operation from 2002 to early 2012. The next satellite sensor to be launched with the same specifications to allow continuity in our methods and products was ESA’s OLCI in 2016. We are developing algorithms to use other sensors (such as NASA’s Moderate Resolution Imaging Spectrometer, MODIS) to fill that data gap.

Where can I find more information about the methods used to derive the algal bloom indices?

You can learn more about the specific methods used to derive the algal bloom indices by consulting the following journal article:

Binding, C. E., Greenberg, T. A., McCullough, G., Watson, S.B., Paige, E. 2018. Remote sensing indices for enhanced algal bloom monitoring on Lake Winnipeg. Journal of Great Lakes Research, 44(3): 436-446.

doi: https://doi.org/10.1016/j.jglr.2018.04.001

Why do you use remote sensing instead of in-lake measurements?

Using remote sensing allows us to obtain information about the Earth’s environment over large geographical areas, and on a frequent basis to better monitor change over time. As technologies improve, so does the spatial resolution and frequency at which remote sensing data products are available. Importantly, remote sensing allows monitoring of remote regions, otherwise not easily accessible for ground based observations.

Ground observations are still very important for validating remotely sensed data, developing new algorithms, and providing additional insight – such as whether or not the bloom is toxic, what phytoplankton species are present, and providing other non-coloured water quality measures that are not directly measureable from space. In-lake measurements also provide us with valuable information about what’s happening at depth below the water surface, a limitation of satellite/airborne remote sensing which typically detects conditions at or near the surface. Ground observations are, however, often logistically difficult and costly to obtain frequently and on large spatial scales, requiring decisions to prioritize the frequency and location of monitoring.

You can measure algal blooms using remote sensing, but what other types of water quality indicators can you measure?

In addition to algal blooms, satellite remote sensing can be used to monitor many different types of water quality indicators including water clarity, suspended sediments, submerged aquatic vegetation, coloured dissolved organic matter (CDOM), water surface temperatures, and oil slicks.

Further useful material on remote sensing of water quality can be found in a special IOCCG (International Ocean Colour Coordinating Group) report: Earth Observations in support of global water quality monitoring [PDF].

Glossary: Terminology and definitions
Bloom flag

When the chlorophyll-a concentration in a pixel is > 10 μg/L (micrograms per liter), the pixel is considered to be in bloom.

Bloom intensity

Average chlorophyll-a concentration measured as μg/L (micrograms per liter) within the total area flagged as algal bloom.  

Bloom extent

Total area of the pixels flagged as in bloom (in km2, or percentage of lake area).  

Bloom severity

Bloom intensity x Bloom extent (μg/L km2).

Bloom duration

The number of days a pixel is flagged as in bloom between June and October.  



Data availability (% of the lake)

The percentage of lake area covered by useable satellite data for each 14 day period. 100% indicates complete lake coverage of satellite data, and 0% indicates no satellite data coverage. Data may be unavailable due to the satellite orbit, cloud cover, or a number of quality control flags.

I would like to use some of the data and images from this web portal, how do I cite the data and imagery?

EOLakeWatch data can be found under Water Quality Remote Sensing within the government's Open Government portal.

When you use our data, please reference the product citation or DOI when available, and acknowledge ECCC EOLakeWatch.


Environment and Climate Change Canada employs every reasonable effort whenever feasible, to ensure the currency, accuracy and precision of the information provided. However, there are some limitations due to the sources of the data and the technology used in its processing and management. Furthermore, the material or any data derived using the data is subject to interpretation. Users are responsible for verifying that the supplied material is appropriate for the use or application for which they wish to employ it. The data released may include provisional and/or validated data. Provisional data should be considered with greater caution as is has yet to be validated with quality control and quality assurance activities, which may result in significant change made to the data. Environment and Climate Change Canada makes no representation or warranty of any kind, either expressed or implied, as to the information presented, nor as to its fitness for any particular use. Environment and Climate Change Canada does not assume nor accept any liability arising from any use of the information and applications used to display or use the information. The data are not intended for any legal, or navigation purposes. The data may change from time to time as updated information is received. If any information seems to be incorrect, raises concerns, or for more information on this data, please communicate with the Environment and Climate Change Canada contact provided within the application.

The material might contain links to third party sites. Environment and Climate Change Canada provides these links only as a service and convenience. Environment and Climate Change Canada takes no responsibility for the content at third party sites and in no way implies an endorsement or recommendation of the products, services or information found there. A third party site may have a privacy policy different from that of Environment and Climate Change Canada. Environment and Climate Change Canada does not assume nor accept any liability arising from any use of third party sourced content.

Page details

Date modified: