From hydrological angle the Hoogly-Matla
estuarine complex can be subdivided into six zones, i.e.,
Zone (1) Sagor, Mahisini, Ghoramara, Lower Long Sand; Zone
(2) Mahisini Island to the west and the Thakuran river to
the east; Zone (3) The area between rivers Thakuran and Matla;
Zone (4) The core area of the Sundarbans Tiger Reserve; Zone
(5) The buffer zone of the Sundarban Tiger Reserve; Zone (4)
The area north of Zone (5) lying to the east of Matla river
(Chaudhuri and Choudhury, 1994).
The following hydro and hydro biological
characteristics of Zone I were revealed in a survey by Dept.
of Marine Science ,CU in 1984.
1.) Salinity And Inorganic Nutrients
From July to September, the water is slightly brackish as
a result of the influx of fresh water brought about with the
onset of the monsoon. During dry season (November to March)
the fresh water flow reduce to one tenth that of the rainy
season peak. During April, salinity varied from 25.9 to 1.62
ppt. from coastal parts to insides. During the monsoons, salinity
of the coastal water decreased to 6.21 ppt due to the September
influx of freshwater. Seasonal fluctuations in nutrients showed
two peaks in nitrates, phosphate and silicate, one corresponding
to the monsoon season and the other, comparatively smaller
peak, corresponding to the late post monsoon and early pre
2.) Temperature And pH Of Surface Water
Wide fluctuation of surface water temperature has been observed
in different seasons with lowest temperature of 20o C has
been observed in winter months and up to 35o C in summer months.
pH fluctuation although is not very high but a minimum variation
may have deterministic effect on the biological community,
with lowest value detected in monsoon months and highest value
in drier months(February To April).
3.) Extinction Coefficient And Primary
The transparency of water is measured and the extinction coefficient
(Kt) is calculated using the formula Kt =1.44/depth (in metres),
applicable for turbid water. When the extinction coefficient
is compared with gross primary productivity, and diversity
index value of various communities an inverse relationship
is found between them.