The changes in dinoflagellate community structure in both -the water column and sediment in a mesotrophic, tropical port environment were investigated in this study. Since the South West Monsoon (SWM) is the main source of climatic variation, observations were made during two consecutive post-monsoon periods (2001 and 2002) and an intervening pre-monsoon period (2002). The pre-monsoon period supported a more diverse dinoflagellate community in the water column compared to both post-monsoon periods. Heterotrophic dinoflagellates were abundant in the water column as well as sediment. A seasonal cycling between vegetative and resting cysts of autotrophic and heterotrophic dinoflagellates governed by the environmental characteristics of the study area was observed. Temperature, salinity and suspended particulate matter were the main factors affecting dinoflagellate community structure in both the water column and sediment. The dominant dinoflagellates in the water column differed during both post-monsoon periods that followed two dissimilar monsoon events. Prorocentroids and gonyaulacoids dominated the water column subsequent to the 2001 SWM, whereas dinophysoids and unidentified tiny dinoflagellates dominated during the next post-monsoon period. The 2001 SWM started in May, peaked during June-July and reduced gradually to end in October. The 2002 SWM was erratic; it started late (in June) and ended earlier (in September). These observations highlight the potential of the SWM to influence the community structure of dinoflagellates in tropical waters and points to the importance of long-term studies to discern robust variations in dinoflagellate communities in response to fluctuating monsoon regimes.

Trichodesmium, an important marine nitrogen-fixing cyanobacteria, is widespread in tropical waters and contributes substantially to the global chemical cycles by new production. The interesting and intriguing feature of Trichodesmium blooms is the suddenness with which the blooms appear and disappear. A well-defined periodicity and annual rhythm of Trichodesmium blooms in the Indian south central coastal environments of the Arabian Sea between February and May is well documented. However, not much is known for the Bay of Bengal. Analysis of the surface water samples collected from the merchant vessels plying between Chennai and Singapore at regular intervals (February 2000 to September 2002 to study the variations in the micro-phytoplankton community indicated that Trichodesmium persists for a large part of the year in the south-eastern part of the Bay of Bengal. Remotely sensed images from 1997 to 2003 also indicated the prevalence of Trichodesmium in the south-eastern part of the Bay during the winter months and in the west (west Bay of Bengal and east Arabian Sea) during summer. This raises the possible role of physical forcing in the dispersal and seeding of Trichodesmium. The prevalence of stratified conditions in the Bay of Bengal almost throughout the year can be one of the causative factors facilitating the preponderance of Trichodesmium. The inter-annual variations observed indicate that the tracking of Trichodesmium will be useful in understanding the biogeochemistry of the region.KEY WORDS: Indian Ocean · Cyanobacteria · Phytoplankton · Arabian Sea · Remote sensing 356: 93-101, 2008 elements such as iron and phosphorous (Karl et al. 1992, Rueter et al. 1992, Mulholland et al. 2002. Resale or republication not permitted without written consent of the publisherMar Ecol Prog SerIn the Indian Ocean Trichodesmium blooms have been recorded since 1942 (Devassy et al. 1978 and references therein). The first report of the blooms in the Arabian Sea was in 1972 (Ramamurthy et al. 1972), and their occurrence with a well-defined periodicity and annual rhythm between February and May has been described as a phenomenon (Devassy et al. 1978). The regular timing of the blooms suggests that physical factors such as stratification, winds and current patterns influence these blooms. In the Bay of Bengal, blooms have been observed since 1942 (Ramamurthy et al. 1972, Devassy et al. 1978 and references therein, Gomes et al. 2000, Jyothibabu et al. 2003. However, elucidation of spatial and temporal distribution was not attempted in any of the earlier investigations. In the present study, which is the first of its kind, surface samples were collected at regular intervals for a period of 3 yr. It was found that Trichodesmium spp. appear regularly during the fall and winter in the Bay of Bengal and make their presence felt during the summer months along the south west coast of India.Remote sensing techniques provide an alternative solution for mapping Trichodesmium on a global scale. Algorithms to detect Trichodes...

Harmful algal blooms (HABs) have been documented along the coasts of India and the ill effects felt by society at large. Most of these reports are from the Arabian Sea, west coast of India, whereas its counterpart, the Bay of Bengal (BOB), has remained unexplored in this context. The unique characteristic features of the BOB, such as large amount of riverine fresh water discharges, monsoonal clouds, rainfall, and weak surface winds make the area strongly stratified. In this study, 19 potentially harmful species which accounted for approximately 14% of the total identified species (134) of dinoflagellates were encountered in surface waters of the BOB during November 2003 to September 2006. The variations in species abundance could be attributed to the seasonal variations in the stratification observed in the BOB. The presence of frequently occurring HAB species in low abundance (≤ 40 cell L( -1)) in stratified waters of the BOB may not be a growth issue. However, they may play a significant role in the development of pelagic seed banks, which can serve as inocula for blooms if coupled with local physical processes like eddies and cyclones. The predominance of Ceratium furca and Noctiluca scintillans, frequently occurring HAB species during cyclone-prone seasons, point out their candidature for HABs.

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