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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.

In May of 2007, a study was initiated by the National Institute of Oceanography (NIO), Goa, India, to investigate the influence of monsoonal rainfall on hydrographic conditions in the Mandovi River of India. The study was undertaken at a location ∼2 km upstream of the mouth of this estuary. During the premonsoon (PreM) in May, when circulation in the estuary was dominated by tidal activity, phytoplankton communities in the high saline (35–37 psu) waters at the study site were largely made up of the coastal neritic species Fragilaria oceanica, Ditylum brightwellii and Trichodesmium erythraeum. During the later part of the intermonsoon (InterM) phase, an abrupt decline in salinity led to a surge in phytoplankton biomass (Chlorophyll a ∼14 mg m − 3), of a population that was dominated by Thalassiosira eccentricus. As the southwest monsoon (SWM) progressed and the estuary freshened salinity and Chlorophyll a (Chl a) concentrations decreased during the MoN, Skeletonema costatum established itself as the dominant form. Despite the low biomass (Chl a