Water Chlorophyll & Algae Bloom Mapping
Algae blooms are becoming a type of water-based plant imagery mapping that is becoming more established and well-documented, though it is still much less established than land-based vegetation. Landsat does not have the necessary red edge band for effective algae bloom mapping, but Sentinel-2 does and has a revisit time of about 5 days, so there is a decent chance that there will be clear imagery of an area of interest.
Table 6 from Mishra and Mishra (2012). See more below.
Mishra and Mishra (2012) established the normalized difference chlorophyll index (NDCI) as an effective method to map chlorophyll-a concentration and, therefore, algae blooms (access paper through previous link for more details). Mishra and Mishra used a MERIS (MEdium Resolution Imaging Spectrometer) red band centered at 665 nanometer (nm) and red edge band centered at 708 nm. These bands are the same or very similar to Sentinel-2, which has a red band (band 4) centered at 665 nm and red edge band (band 5) centered at 704 (though bandwidth for Sentinel-2 red is 39 nm compared to 20nm for MERIS; red edge are 20 nm for both).
NDCI based on Sentinel-2 surface reflectance =
([Band 5] - [Band 4]) / ([Band 5] + [Band 4])
You can now independently identify and map algae blooms. Simply follow the steps on this website to download free QGIS software and Sentinel-2 imagery (explained in Free Courses page and Free Satellite Imagery page). Convert Sentinel-2 to surface reflectence based on the tutorial/course available for free on this webisite. Make the imagery calculation for Sentinel-2 band 4 and 5 with the QGIS raster calculator over high-resolution background imagery (both explained in a free Sentinel-2/QGIS course). Click on the calculated raster with the identify feature tools to view values and compared the values to those in Table 6 from Mishra and Mishra above to get a sense of the severity of the bloom. (You can continue to learn about QGIS with the QGIS User Manual available on this website or the QGIS website.)
Reference
Mishra, S. and D. Mishra. 2012. Normalized difference chlorophyll index: A novel model for remote estimation of chlorophyll-a concentration in turbid productive waters. Remote Sensing of Environment: 117; 2012; pp. 394-406. (Can be accessed through previous link.)