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Species of the diatom genus Pseudo-nitzschia, some of which produce the neurotoxin domoic acid (DA), were studied to see how environmental factors affect their distribution in two tropical monsoonal estuaries and how salinity influences the growth and toxicity of P. pungens. Pseudo-nitzschia pungens, P. multistriata, and P. seriata were present in both the Mandovi and Zuari estuaries, whereas P. australis and P. pseudodelicatissima appeared only in the Zuari estuary. Canonical correspondence analysis indicated a significant positive correlation between salinity and the occurrence of P. seriata in the Mandovi estuary and of P. pungens in the Zuari estuary. A strain of P. pungens isolated from the Zuari estuary showed significant variations in specific growth rate between salinities, in support of our finding of a relationship between salinity and its distribution in the field. The lowest growth rate (0.44 day−1) was at a salinity of 5 and it increased to a maximum (1.05–1.19 day−1) at salinities of 15 to 30, declining slightly (0.98 day−1) at a salinity of 35. The isolated strain produced DA but at low levels. DA production rates varied significantly with salinity; they were low and similar at salinities of 5–15 (2.56–3.12 ng ml−1 day−1) and increased with increasing salinity, reaching 5.25 ng ml−1 day−1 at 35. The observed variations in growth rate and DA production by P. pungens indicate the need to focus more on salinity as an environmental factor that affects this, and other Pseudo-nitzschia species, in these monsoonal waters.

Seasonal variations in the composition and abundance of phytoplankton were investigated fortnightly at 3 different regions along the Mandovi estuary from June 2007 to May 2008 in relation to salinity and nutrients. A total of 209 species belonging to 7 divisions were identified during the study period. The highest phytoplankton cell density (5.17 × 104 cells L-1) and biomass (7.68 mg m-3 chlorophyll a) were observed in the upper sections during the nonmonsoon period, while the highest diversity (3.46) was observed in the upper section during the monsoon period. Bacillariophyta was the dominant phytoplankton group at 71% of the total species identified. This group was dominated by Pennales (88) over Centrales (60) at all 3 stations. Contributions of Dinophyta, Chlorophyta, Cyanophyta, Haptophyta, Chromophyta, and Chrysophyta to the total species were 25%, 0.5%, 1%, 0.5%, 1%, and 1%, respectively. Dinophyta did not show significant variations in percentage distribution among the 3 sections and seasons. Twenty-six dominant species of phytoplankton, representing all seasons with cell density of >1000 cells L-1, showed spatial adaptability patterns with respect to salinity. Protoperidinium acbromaticum and Alexandrium ostenfeldii were found to grow only during nonmonsoon periods in upper sections and were reported for first time in the present study. Euryhaline Skeletonema costatum and Thalassiothrix frauenfeldii were present throughout the study period. A total of 36 harmful algal bloom-forming species, with 11 toxin-producing species, have been identified. The presence of Streptotheca thamensis acts as an indicator to evaluate water quality.

Sequence of the images from IRS P4 / OCM satellite and extensive shipboard sampling programme are used to understand the seasonal variation of phytoplankton abundance and types in the Northeastern (NE) Arabian Sea and Lakshadweep Sea. An appreciable degree of spatial and temporal variability is observed in chlorophyll a distribution from November to April months, as well as coastal and offshore stations, indicating marked seasonality in phytoplankton distribution in NE Arabian Sea. During November month (fall intermonsoon) average chlorophyll a (Chl a) by fluorometer was (0.799 mgm^-3) and by OCM it was 0.584 mgm^-3. The higher chlorophyll a observed was due to Trichodesmium (cyanobacteria) blooms. During December the average chlorophyll a was 0.34 mgm^-3 also due to Trichodesmium filaments in water column. During January onwards winter cooling led to increase in nutrients which enhanced chlorophyll a value to 0.64 mgm^-3 due to growth of flagellates (as seen by high chlorophyll b besides chlorophyll a) in water column. February, March and April supported moderately high chlorophyll value (0. 3 to 0.5 mgm^-3) due to growth of prasinophytes (as seen by pigment prasinoxanthin) and blooms of the Noctiluca miliaris. Time series monitoring of Noctiluca bloom was also conducted using OCM based chlorophyll images in NE Arabian Sea. During February chlorophyll a retrieved by OCM was 0.3 to 0.9 mgm^-3. Pigment analysis of water samples indicated the equal important of accessory pigment like zeaxanthin, prasinoxanthin, beta-carotene. The relevance of these pigments estimated by HPLC like zeaxanthin (cyanobacteria), fucoxanthin (diatoms), peridinin (dinoflagellates) is presented and discussed. Similarly, exercise is conducted in Lakshadweep waters where Trichodesmium related peak in chlorophyll a was observed during March onwards in OCM data. The average chlorophyll a in NE Arabian Sea at surface during November was (0.726 mgm^-3), December (0.34 mgm^-3), January (0.723 mgm^-3), February (0.344 mgm^-3), March (0.963 mgm^-3) and April 0.665 mgm^-3. Similar trend was observed in primary productivity estimates. The attempt is made to work out seasonality in the productivity of the Arabian Sea using OCM derived chlorophyll and relation of enhancement in productivity due to development of winter blooms in the Arabian Sea. The environmental conditions (temperature, wind, nutrients and mixed layer depth) affecting these blooms responsible for year to year variation in bloom biomass and productivity is also presented in detail.