By D. Baccante, Charlie Lake Conservation Society 2023
Monitoring both algae and plants is not only important to assess their role in aquatic productivity, but is also important to detect harmful impacts such as from invasive, non-native species. However, given the very large variations in algal densities throughout the growing season, and extent of aquatic plant densities, it would be prohibitively costly in both financial and human resources to collect enough data to properly describe the range in densities throughout the summer.
Environmental monitoring by remote sensing has increased exponentially and directly proportional to the increased number of satellites, and computing power. Traditionally satellite images were only available to the agencies who sent satellites in space, such as NASA. As the number of agencies launching satellites has increased, it is amazing what is currently available to the public, for free. Satellites are being used not only for their traditional role of gathering intelligence for the military, but they now are a valuable and efficient way for remote sensing and monitoring environmental parameters. The example provided here is one of many similar applications.
One method that appears to be quite useful in quantifying algae and plants on water bodies, and monitoring of many other environmental parameters, is from data gathered by the SENTINEL-2 mission. Because these satellites have a high revisit frequency of sites, it makes them highly suitable to document environmental changes at a fine time scale. For example, locally, people share up-to-date online information such as ice cover conditions on lakes to plan outdoor activities such as ice fishing.
The wide-availability of all the data from the SENTINEL program to users around the globe, encourages sharing of user-developed methodology for a variety of purposes. Users around the world have developed programming scripts that can analyze the spectral information in the images and extract data from the images. Luckily, there’s one called: “Aquatic Plants and Algae Custom Script Detector (APA Script)”. This script, or computer program, analyzes the satellite image and represents algae and aquatic plants on the lake surface in green. An additional advantage of this script is that it is also sensitive to suspended sediments, which can be quite useful to monitor significant sediment inputs into a lake.
This method was tried for both Charlie and Swan lakes. They both are naturally eutrophic lakes and they have significant blooms of algae and aquatic plants during the growing season. The Charlie Lake Conservation Society (CLCS) has carried out many environmental monitoring and stewardship projects in Charlie Lake, and more recently, aquatic plant surveys and algal sampling. As mentioned earlier, these programs can be expensive and resource-intensive beyond the capacity of the CLCS and similar organizations. Thus it is very attractive to explore using free data from satellites to assess this method for long term, large scale, monitoring of algae and aquatic plants in water bodies. A detailed report with all relevant details is available from the BCLSS website here. You can find a couple figures from the report below.
Image of Charlie Lake, BC, taken from a SENTINEL-2 satellite on May 30, 2022 with the application of the APA Script. Algae are just starting to show up, represented by green patches. Of significance in this image is the amounts of sediment coming into the north end of Charlie Lake, as well as in the Peace River (lower left) and the Beatton River (lower right) shown in shades of red and brown. As it turns out, meteorological records from Environment Canada indicate there was a significant rainfall, almost 45 mm, on May 28, 2022.
Plot showing the relative abundance of algae and plants expressed as a percentage of green colour calculated from satellite images of Charlie and Swan lakes.