Unusual blooms of green Noctiluca miliaris (Dinophyceae) in the Arabian Sea during the winter monsoon

Abstract

A large-scale, ongoing study conducted by the National Institute of Oceanography, India, from 2003 onward in support of India's ocean color program, has documented for the first time ever the appearance of extensive blooms of Noctiluca miliaris Suriray in late winter and early spring for several consecutive years. Until the late 1990s, N. miliaris Suriray (synonym Noctiluca scintillans Macartney), a large dinoflagellate, was a minor component of phytoplankton populations in the Arabian Sea, appearing sporadically in bloom form in coastal regions during the summer southwest monsoon (June―September). Recently, N. miliaris blooms have begun to appear with increased frequency and intensity following the winter northeast monsoon (November―mid-February). These findings contradict established notions of the predominance of diatoms in winter blooms based on large-scale studies such as the Joint Global Ocean Flux Study (JGOFS) in the early 1990s and the International Indian Ocean Expeditions (IIOEs) of 1959―1965. A field program in the Gulf of Oman since February 2003 has also documented the appearance of blooms of N. miliaris blooms with increased intensity in early winter. The present study investigates the temporal evolution and spatial extent of the N miliaris blooms using phytoplankton taxonomic and pigment data from cruises undertaken in 2003― 2004 and 2007 as well as ocean color satellite data. Our findings indicate that N. miliaris blooms are becoming an annual and widespread feature in the Arabian Sea. Aqua-Moderate Resolution Imaging Spectroradiometer (MODIS) sea surface temperature and altimetry data suggest that mesoscale eddies that populate the western Arabian Sea may play a significant role in the production and dispersal of these blooms from the Gulf of Oman into the northern Arabian Sea.