Detection of Trichodesmium bloom patches along the eastern Arabian Sea by IRS-P4/OCM ocean color sensor and by in-situ measurements

Abstract

The detection of blooms of the marine cyanobacterium Trichodesmium from space has been studied using high resolution ocean color imagery in the visible wavebands of the OCM monitor (Ocean Color Monitor) on the IRS-P4 satellite platform. The standard detection protocol developed by Subramaniam et al. [Deep-Sea Res-II, 49 (2002) 107-121], has been used in this paper. Localized bands of Trichodesmium were detected in OCM imageries of 16th,18th, 20th and 22nd March 2002 in decreasing numbers with time. The patches were aligned approximately parallel to the shoreline, and distributed over the shallow waters off the west coast of India. An analogous search of Trichodesmium bloom patches for these days using available SeaWiFS (Sea Viewing Wide field of View Sensor) sensor revealed features of reduced spatial resolution compared to that observed by the higher resolution OCM sensor, and at locations further offshore. In a field study during 19th to 22nd March 2002, we encountered Trichodesmium patches (~ 90 km ) off the coast of Goa. Microscopic analysis confirmed that these patches belonged to the Trichodesmium erythraeum. These field patches were further offshore from the positions occupied by the near-shore patches seen in the OCM imageries during 18th to 22nd March 2002. Radiometric measurements made in the cruise-detected patches show water leaving radiances averaging ~ 1 μW/cm²/nm, below the detection threshold of the present satellite detection scheme. It is clear that further efforts are needed to improve the detection range of present protocols so as to detect the presence of less reflective Trichodesmium patches of the type encountered during our cruise.