Chlorophyll is used as a proxy for measuring the phyto- plankton biomass.

An integrated optical system capable of detecting and monitoring algal (or phytoplankton) blooms both spatially and temporally in coastal and open ocean waters has been developed by a team of researchers at the Indian Institute of Technology (IIT), Madras. Very soon, the Hyderabad-based Indian National Centre for Ocean Information Services (INCOIS) will begin using the optical system for detecting and monitoring algal blooms in ocean waters surrounding India. INCOIS is currently in the process of making the system operational.

Phytoplankton are the base of the aquatic food web, providing food and shelter for different organisms including fish. Along with other parameters, phytoplankton biomass (algal blooms) tends to behave as potential zones of fish aggregation. So identifying such algal blooms in real time using satellite data will greatly benefit the fishing community to zero in on fertile fishing locations.

The optical system provides an array of optical parameters and spatial information regarding algal bloom density (chlorophyll) and their causative algal species that are commonly seen in coastal and oceanic waters around India, particularly in the Arabian Sea. Results of the study were published recently in the Journal of Geophysical Research: Oceans.

“A few field-based techniques are available for studying algal blooms. But those techniques are limited in time and space besides being labour intensive, time-consuming and expensive, and hence they cannot be used for monitoring large water bodies. ISRO’s Oceansat-2 satellite launched in 2009 can cover larger areas and provide global ocean colour observations,” says Prof. Palanisamy Shanmugam, the senior author of the paper from the Department of Ocean Engineering, IIT Madras.

The optical-detection system developed by Prof. Shanmugam and his team uses the ocean colour satellite data, in situ measurements and underwater light field data collected from the field to provide algal species-specific information required for their monitoring and assessment.

Unlike the blooms that are found on the surface of water bodies, observing and monitoring subsurface blooms is particularly challenging. Conventional techniques fail when it comes to monitoring subsurface algal blooms. Though the optical-detection system was tested only to detect blooms from near surface waters, Prof. Shanmugam is confident that the optical system is capable of detecting and classifying blooms present under water. “We have not tested to what depth the optical system can be used. We are planning to carry out this study soon,” he says. “We have tested and validated the results of this optical system with in situ measurements of the three algal blooms collected from the ocean waters. The average accuracy of our optical system which was developed in 2015 is over 85 per cent,” he says. The uncertainty in accurately identifying the blooms was primarily due to lack of distinctive water colour, and absence of unique spectral features (in the backscattering coefficients caused by cases of less photosynthetic organisms), fluorescence and chlorophyll signatures associated with the bloom species.

The water colour is determined by particulate matter and dissolved substances in water, while fluorescence is to do with the light energy that gets absorbed by algae and reemitted as fluorescence at a longer wavelength than the absorbed light.

Chlorophyll is used as a proxy for measuring the phytoplankton biomass. The increase in biomass of phytoplankton due to their increased growth or physical aggregation leads to algal blooms. Typically one dominant or a few phytoplankton species are involved in bloom formation.

Some algal blooms including “red tides” and “blue-green blooms” are a serious concern because they can pose significant threats to water quality and risks to human and animal health.

All the major algal blooms are predominantly found to be associated with the cooler water masses off the western coast in the northern Arabian Sea. These blooms then spread into the central Arabian Sea along with a whirling motion of waters and currents. The blooms reach its peak spatial distribution between November and February and minimum in June to September. Strong upwelling along the Arabian Sea coast triggers initiation and growth of algal blooms, while enhanced cooling, vertical mixing, favourable winds, and atmospheric deposition of the mineral aerosols from surrounding deserts further aid its growth. The Bay of Bengal is relatively free of algal blooms except off the Ganges–Brahmaputra Estuarine Frontal system and estuarine and coastal regions where nutrients are abundant supply.