In estuarine areas of the mangrove forest of Sundarbans, surface water salinity variations through tides, seasons and spaces were studied in a comprehensive way for the first time along with dilution efficiencies and extent of salinity exposure. Since Sundarbans is influenced by several numbers of rivers in a criss-crossed way, the entire area has been divided as western sector (under Hooghly, Saptamukhi and Thakuran rivers respectively), the middle sector under Matla and Bidya river (MB estuary) and eastern areas influenced by Gosaba, Harinbhanga, Jilla and Raimangal rivers (GHJR estuary). Study revealed that unlike western sectors, the tidal variations of salinity in other areas were less with negligible salinity difference (1.4 to 2.0 psu) between high and low tides. In contrast, pronounced seasonal variations of salinity were encountered (9.34 to 30.83 psu) in the region. The most parts of the Sundarban attained almost equal level of salinity in monsoon (12.0 to 14.0 psu) and summer (29.0-30.0 psu) indicating less degree of spatial variations. Significant salinity differences between upstream (US) and downstream (DS) was however, recorded in winter and pre-summer relatively higher in GHJR estuary highlighting more fresh water influx in this region. Higher levels of average dilution factors (DF) were encountered in MB (6.5) compared to GHJR estuary (4.5). The seasonal variation of DF forecast that any pollutant would undergo more dilution during summer and less in monsoon season. The whole Sundarban area could be classified into four distinct zones in respect to salinity variation and its exposure period. This may be useful in finding out the presence of similar type of mangroves species diversity and richness, and may also serve as an important tool in future study to explore the factors, other than salinity responsible for mangrove species extinction from this world famous biological kingdom.
