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.
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