The image shows NASA’s MODIS-Aqua satellite data processed using a global algorithm developed by Prof. Shanmugam of IIT Madras. Red, green and blue colour gradients depict very high, intermediate and very low algal bloom density respectively. – Photo: Shanmugam.

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 coastal and ocean waters surrounding India; algal blooms are particularly seen in the Arbian Sea. 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 average accuracy of the optical system developed in 2015 is over 85 per cent.The optical system provides an array of optical parameters and spatial information regarding algal bloom density and the causative algal species. 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.

A global algorithm developed by Prof. Shanmugam has been incorporated into SeaDAS – a satellite ocean colour data analysis software developed and maintained by NASA’s Goddard Space Flight Center.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 can detect and classify 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 is 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 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 of serious concern because they 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.

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 in abundant supply.