Optical Detection of Diatom Bloom in the Coastal Waters of Bay of Bengal Using Oceansat-2 OCM

Abstract

Phytoplankton blooms are the indicators of marine ecosystem health status and their monitoring is a key component in the effective management of coastal and oceanic resources. It remained as a challenge in the coastal waters due to the presence of suspended sediments and dissolved organic matter which interferes with the phytoplankton signal. Several methods and techniques were developed to detect, map and monitor the phytoplankton blooms in the coastal waters using in situ and satellite data. To address this issue ISRO took an initiative to monitor and identify the phytoplankton blooms in the North Indian Ocean to be specific in the coastal waters. As part of this program, the blooms are monitored using Oceansat-2 Ocean Colour Monitor (OCM-2) data all along the East coast of India. This paper presents a case study in the bloom dominated coastal waters along Off Godavari delta covering Kakinada and Yanam in the East coast of India. In-situ measurements collected onboard during a bloom event are optical parameters like Remote Sensing reflectance, Rrs(λ), Water leaving radiance Lw(λ) from the Hyperpro-II under water radiometer, absorption due to phytoplankton, aph(λ), from the spectrophotometer, biological parameters of phytoplankton cell counts and HPLC pigments. From the in situ biological measurements, it is confirmed that the bloom was dominated by with diatom species of Skeletonema costatum with a high concentration of Fucoxanthin as the biomarker for diatoms from the HPLC pigment analysis. Based on these results obtained, optical detection of the bloom species using spectral discrimination of Rrs for different cell enumerations was considered and further spectral derivative analysis (first to fifth order) has been carried out to identify the peaks to detect the bloom forming species. The criteria considered as the minima in the second order derivative (dλ2Rrs) for identifying the peak to discriminate the spectra of the bloom forming species. From the analysis, a unique peak at 483 nm which is identified in particular to S. costatum species in the coastal waters which are identified from the chlorophyll product derived from OCM-2. From the results observed, it can ensure that diatom blooms can be detected from OCM-2 using 490 nm spectral band.