The phytoplankton species composition and their occurrence and abundance at the mouth of the Moheskhali Channel with the Bay of Bengal, Bangladesh were studied for one year, from August 1998 to July 1999. During the study the occurrence of a number of toxin producing harmful algal species, namely, Dinophysis caudata, D. homunculus, D. mitra, Gonyaulax polyedra, Protogonyaulax catenella and Gymnodinium coeruleum and a bloom of D. caudata resulting in fish kill were observed for the first time in the area. During the bloom in August and September the maximum cell density of D. caudata reached 1.1x 106 cells/L. Some hydrographic parameters e.g., temperature, salinity, pH, local rainfall, nitrate-nitrogen and phosphate-phosphorus were recorded and their relationship with the occurrance and abundance of toxic algal flagellates were studied. No human sickness related to the observed algal blooms was reported. In some other coastal areas of Bangladesh, beyond the studied area, algal blooms and related fish mortality during the same period also occurred. Possibly, cysts and/or living cells of the toxic flagellates were present in the area but it was not reported before because no survey was undertaken during the previous years. In 1998 a historic massive monsoon rainfall occurred in many parts of India and more than 70% of the land of Bangladesh. These flood waters carrying nutrients of agricultural, industrial and sewage origin were discharged as runoff through many rivers including the Ganga, Padma and Brammaputa to the Bay of Bengal, Bangladesh, and that was supposed to have triggered the toxic algal bloom.

Recent evidence suggests that species of copepod zooplankton provide a marine reservoir for the cholera pathogen and facilitate its long-term persistence in certain regions, such as the estuaries of the Ganges and Bramaputra in Bangladesh (Colwell, 1996). The seasonality of cholera epidemics may be linked to the seasonality of plankton (algal blooms) and the marine food chain. Studies using remote-sensing data have shown a correlation between cholera cases and SST in the Bay of Bengal (Lobitz et al., 2000). Interannual variability in cholera incidence in Bangladesh also is linked to ENSO and regional temperature anomalies (Pascual et al., 2000). Epidemiological evidence further suggests a widespread environmental cause of the 1991 epidemic in Peru, rather than point-source contamination (Seas et al., 2001). There is some evidence for a link between warmer sea surfaces and cholera risk in the Bay of Bengal, but it is not possible to extrapolate such findings to cholera incidence inland or in other regions. The potential impact of long-term climate warming on cholera incidence or risk of epidemics remains uncertain.

Climate-related ecological changes may enhance primary and secondary transmission of cholera in developing countries, particularly among populations settled in low-lying coastal areas in the tropics. However, the causal link between sea temperature, plankton blooms, and human disease requires further elucidation and confirmation.