Algae are photosynthetic, tiny marine organisms whose cells are eukaryotic. Algae can inhabit the freshwater as well as the marine ecosystem. The different categories of phytoplankton for example, diatoms and dinoflagellates are examples of unicellular, floating algae. Although most types of phytoplankton do not possess any toxicity, a few species of dinoflagellates are able to produce toxins that could be harmful for other marine organisms and even for human beings. During the phenomenon of a plankton-bloom, these toxin-producing dinoflagellates can cause the large-scale deaths of fishes and other marine organisms. Such outbreaks of toxin-producing algae are referred to as harmful algal blooms. A harmful algal bloom in the fresh, salt, or brackish water is found to be generated by the presence of overabundance of the nutrients. This oversupply of nutrients in the coastal water is mainly caused by human activities such as mismanaged wastewater disposal, sewage, and runoff of fertilizer from agriculture. Harmful algal blooms occur naturally; however, the frequency and intensity are found to be increasing due to the human induced environmental pollution. In general, harmful algal blooms could be caused by a diverse organism, including toxic and harmful phytoplankton, cyanobacteria, and benthic algae. The algal bloom could appear as green, red, and brown colors, depending on the species which is forming the bloom. The algal bloom affects the coastal environment as they result in the removal of large amounts of dissolved oxygen from the water, which could lead to the eventual mortality of the fishes and other living organisms in the marine ecosystem. They affect the well-being of the marine ecosystem, health of people as well as the local coastal community and regional economy that depend on fishing and tourism.

Along the coastal regions of India, harmful algal blooms found to be occurring during pre- and post-monsoon seasons. Over this part of the world ocean, the large-scale mortality of marine organisms during the harmful algal bloom event is found to be caused mainly by marine species such as Cochlodiniumpolykrikoides, Kareniabrevis, Kareniamikimotoi, N. scintillans, Trichodesmiumerythraeum, T.thiebautii and Chattonella marina (D’Silva et al., 2012). The occurrences of algal bloom around the Gulf of Mannar coast during 2020 and 2021 can be seen from the Figure 1 (Figure obtained from Rameshkumar. P et al, 2023). The accurate monitoring the harmful algal blooms is greatly benefited by recent advancements in the Satellite remote sensing techniques of ocean color which provides a relatively inexpensive and viable alternative to monitor HABs over a large area and keep a track of their progress. The spatiotemporal variability surface algal blooms can be identified in near real time by utilizing remote sensing techniques which is widely applied as a quick and effective method to monitor the bloom. The algal bloom can be effectively identified using the IRS-P4 OCM during May 2002 along the Saurastra coast, Gujarat as shown in Figure 2 (Figure obtained from R. K. Sarangi et al., 2005).

(Figure 1: Occurrence of Noctiluca bloom (left panel) and Green Noctiluca scintillans bloom (right panel) along the Gulf of Mannar coast.)

(Figure 2: OCM FCC image (channels 8, 6, 5) showing the Trichodesmium bloom patches.)

References:

D’Silva, M. S., Anil, A. C., Naik, R. K., & D’Costa, P. M. (2012). Algal blooms: a perspective from the coasts of India. Natural hazards, 63, 1225-1253.

Rameshkumar. P, Thirumalaiselvan, P. S., Raman, M., Remya, L., Jayakumar, R., Sakthivel, M., Tamilmani, G., … & Gopalakrishnan, A. (2023). Monitoring of Harmful Algal Bloom (HAB) of Noctiluca scintillans (Macartney) along the Gulf of Mannar, India using in-situ and satellite observations and its impact on wild and maricultured finfishes. Marine Pollution Bulletin, 188, 114611.

Sarangi, R. K., Chauhan, P., Nayak, S. R., & Shreedhar, U. (2005). Cover: Remote sensing of Trichodesmium blooms in the coastal waters off Gujarat, India using IRS‐P4 OCM. International Journal of Remote Sensing, 26(9), 1777-1780.