Physical and Chemical Environment
Climate: The SMF is situated in the warm, humid tropical region where mean annual minimum and maximum temperatures are 21 and 30 °C, respectively, mean annual relative humidity varies from 70% to 80% and annual rainfall varies from 1640 and 2000 mm [
3].
Hydrological regimes: Stream flow through Ganges, Bahmaputra and Surma-Kushiara Rivers originating from the Himalayas is the largest component (about 90%) of freshwater sources in Bangladesh [
18]. The rivers flow generally from north to south. Out of 15.5 million km
2 of catchments, only about 7.5% lie within Bangladesh and are distributed over most parts of the country (
Figure 1). The Ganges R. sediment-laden freshwater discharge is the main source of water for the BSMF. The annual peak discharge varies from 31,600 to 76,000 m
3·s
−1 and minimum discharges on many occasions were found to vary from 657 to 858 m
3·s
−1 at Hardinge Bridge, about 185 km from BSMF (
Figure 2B). The perennial freshwater bodies like “Beel Dakatia” in the moribund delta of the Ganges floodplain, to a limited extent, also constitute the hydrological regimes of the forests.
Figure 2.
Yearly freshwater discharge patterns at Gorai Railway Bridge on Gorai River (A) and at Hardinge Bridge on Ganges River (B); the two peak max. discharges at both the stations are due to major floods in 1987 and 1998. Max. discharge means peak discharge of a monsoon month; average and minimum discharges are mean of 12-month values. Source: BWDB.
Figure 2.
Yearly freshwater discharge patterns at Gorai Railway Bridge on Gorai River (A) and at Hardinge Bridge on Ganges River (B); the two peak max. discharges at both the stations are due to major floods in 1987 and 1998. Max. discharge means peak discharge of a monsoon month; average and minimum discharges are mean of 12-month values. Source: BWDB.
The discharge from the Ganges is particularly important for the BSMF. The Gorai River splits from the Ganges and the freshwater carried by it is distributed to Passur and Baleswar Rivers through Nabaganga and Madhumati, respectively (
Figure 1A). About 85% of water passes through Passur system and only about 15% through Baleswar R., but the pattern has changed since diversion of water at Farakka Barage in India, 17 km upstream from the Bangladesh border, which was opened on 21 April 1975. The mean monthly available freshwater flow varies from 0.00 to 170 m
3·s
−1 during the dry period and about 4000 to 8880 m
3·s
−1 during the wet period at Gorai Railway Bridge, situated at about 160 km north of the BSMF (
Figure 2A). The zero discharge occurred in two consecutive years, from January to April in 1995 and during March 1996, and only 0.62 m
3·s
−1 was discharged during January 1997. This is the result of sediment-laden freshwater discharge to the BSMF. The max. discharge over 34 years formed a steep slope, indicating that water discharge through Gorai River is decreasing gradually due to sand bars starting from the mouth of the river.
Effect of Farakka Barrage on Salinity: Salinity increase in the BSMF occurred for two reasons: first, the diversion of freshwater at Farakka Barrage and second, by oceanic currents. The BSMF is now facing the two extremes. One immediate effect of the lower extreme is the increased salinity during the dry period. The salinity in the northern part of BSMF increased from 7.50‰ in 1968 to 12.50‰ in 1976 for March and to 18.50‰ in the month of May after two years of operation of the Farakka Barrage [
19].
Oceanic current in the Bay of Bengal circulates in a clockwise direction during January to June when freshwater discharge from the upstream is zero to less than 174 m
3·s
−1 in the BSMF. As a result, the marine water gets deep inside the forests, increasing the salinity. During July to December, the oceanic current circulates in an anticlockwise direction [
20]. At this time, there is tremendous pressure of floodwater from upstream, possibly about 7000 m
3·s
−1. As a result, there is limited saline water intrusion inside the BSMF; therefore, the forests near the coast remain moderately saline. Flooding for nearly four months every year reduces the salinity level greatly within the BSMF.
The BSMF is divided into three subsystems almost in a north-south direction where salinity varies due to hydrological regimes [
21] (
Figure 3A–B).
- (a)
The eastern subsystem is situated between Passur and Baleswar Rivers and receives freshwater from the Ganges through Gorai-Madhumati (which holds little freshwater during the dry period) and lower Meghna. The subsystem is of low salinity (Oligohaline, <5‰).
- (b)
The central subsystem is located west of Passur and east of Sipsa. The Passur is connected with the Ganges through the Gorai River. However, the connection is blocked in the lean period by sand bars (chars). Due to reduced flow in the Ganges, the catchment area is extensively sedimented resulting in degradation of BSMF mainly due to increasing salinity (Mesohaline, 5‰ to <18‰).
- (c)
Western subsystem is located in the west of Sipsa River to the east of Raimangal-Harinbhanga River along the border. The subsystem originated from several perennial water bodies (moribund delta) (
Figure 1). The Sipsa is connected with Passur which is already with low freshwater flow. Thus, the system does not receive any surface water from upstream during the dry period except local run off [
21]. Seawater intrudes making the subsystem saline (
Polyhaline, 18‰ to 30‰).
In the northern part (eastern and central subsystem) of BSMF, water salinity varies from 4‰ to 28‰ in April and May, while in the post-monsoon it is 1‰ to 9‰ [
22]. In a period of eight months (from September to May), water salinity increased three to eight-fold, while the soil salinity increased two to five-fold.
Case studies since the 1930s to the 1990s revealed that the salinity in the BSMF has increased over time (
Figure 3A,B). The BSMF was divided (curved slashed line in the
Figure 3A) into freshwater (northern and eastern) and moderately saline (western and southern) zones in the early 1930s [
15] but after 50 years and 10 years of operation of the Farakka Barrage, the BSMF has been divided into north-eastern freshwater, middle to southern moderately saline and western saline zones [
14] (
Figure 3A). After about 10 years, a largely different mesohaline zone was observed [
23] (
Figure 3B). However, the boundary is tentative varying with the seasonal variability of freshwater from upstream. Similarly, salinity of an area varies with the time of a year; peak salinity level occurs in April and May due to very low freshwater discharge (
Figure 2A) and drops gradually in the soil and abruptly in water from June [
24] (
Figure 3C) due to huge freshwater discharge during the wet season (
Figure 2A,B). The decrease in salinity is due to 112% freshwater flow in the monsoon and the increase is due to low flow; only 43% in the winter and summer compared to 85% pre-1975 [
21]. During monsoon and early autumn, almost the whole BSMF water remains in the range of mesohaline to lower range of oligohaline conditions, which move westward reducing the polyhaline zone that remains only near the coast. It has been estimated that about 60% of the BSMF in the west and south is polyhaline, while 35% is mesohaline in the middle and 5% oligohaline in the eastern part for the rest of a year [
21].
Figure 3.
(
A) Three saline zones of BSMF in 1983 [
14] but divided in to two zones with approximate Curtis demarcation in 1930’s, by a dotted line [
15]; (
B) A largely different distribution pattern of the three zones was found after about 10 years [
23]; (
C) Seasonal variation of salinity in the Passur River water (at Mongla) and surface soil of Sarankhola, Chandpai and Satkhira ranges in the late 1980s [
24]. Figures used with permission.
Figure 3.
(
A) Three saline zones of BSMF in 1983 [
14] but divided in to two zones with approximate Curtis demarcation in 1930’s, by a dotted line [
15]; (
B) A largely different distribution pattern of the three zones was found after about 10 years [
23]; (
C) Seasonal variation of salinity in the Passur River water (at Mongla) and surface soil of Sarankhola, Chandpai and Satkhira ranges in the late 1980s [
24]. Figures used with permission.
Water: A direct relationship of the changed salinity (10.40‰ to 26.2‰ in March) with the TDS (8.03 to > 20 g/L) and conductivity (16.02 to 34.14 mS/cm) was found in different locations [
25]. Alkalinity ranged from 0.362 to 0.438 meq/L irrespective of salinity and tides. Similar is the case with pH that varied from 7.26 to 7.98. Variation of chemical factors between high and low tides at a set location were insignificant. As the forestlands are surrounded by rivers, canals and creeks, the tides and their chemistry have profound effects on the forest vegetation [
21]. The river water salinity line has been found to be similar to the groundwater salinity line of the BSMF [
26].
Tide: Depth and duration of tidal inundation is an important regulator of mangrove productivity [
19,
27]. Tides help in mixing vertical water columns, thereby providing nutrients and enriching water, providing vertical motion to the groundwater table that may transport nutrients to the root zones of the mangroves, transporting oxygen to the root system,
etc. [
27]. Tidal waves vary in different regions of BSMF according to the different hydrodynamic characteristics of rivers and cross channels. Mean tide height in the monsoon is at its highest (2.00 to 2.86 m) and at its lowest (1.56 to 2.00 m) in the dry period. A comparison of monthly tidal levels during a period of 20 years (1977 to 1997) at Hiron point and Mongla revealed that the mean tidal level at Hiron Point varied from 1.7 m in 1977 to 1.723 m in 1997, while at Mongla it varied from 2.233 m in 1977 to 2.356 m in 1997 (personal comm. Mrs. Shamsun Naher, BIWTA).
Soil: The forestland soil is grey in color, finely textured and the subsoil is stratified, compacted at greater depth. At the eastern part of the soil (having good supply of fresh sediments), the top 15 cm soil layer is soft and fertile, whereas in the western part (with little fresh sediments), the soil is a hard mass [
28]. Na content of soils varies from 5.7 to 29.8 meq/100g, where the lower value is found in the eastern part. Mg varies from 4.1 to 9.9 meq/100g. Chloride is a dominant anion varying from 5.7 to 23.2 meq/100g, and the high concentration is found in the southern and western parts. Potassium content varies from 0.3 to 1.3 meq/100g. The high value of the Na and Mg hampers plant growth. Organic carbon and nitrogen are 0.62 and 0.05%, respectively. Organic matter ranges between 4% and 10%. Soils are neutral to mildly alkaline (6.5 to 8.0 pH). The pH in depressions with high organic matter ranges from 5.3 to 6.4 [
28].
Soil salinity in the mangroves is regulated by inundations, freshwater discharge, rainfall, soil types, topography,
etc. BSMF soil salinity is low compared to other mangroves of the world (exceeding that of sea water) [
19]. The soil salinity (upper 15 cm) is lowest (1.0‰ to 4.5‰) in the monsoon but the peak (3.3‰ to 17.3‰) is found during the dry period [
26]. Lowest salinity (0.4‰) in the Chandpai range (oligohaline) and highest (6.3‰) in the Sarankhola range (at Katka) was observed during the last week of March 2010 [
29]. Subsurface soil salinity is higher due to leaching or recharge of groundwater by the tidal water [
21] and that the salinity of surface water salinity line and groundwater salinity line were similar [
26]. The tap root of trees is thus exposed to high salinity.
Effect of Farakka Barrage on the structure of the BSMF: The very low (sometimes nil) discharge in the dry period results in low sediment deposit around forestland situated about 160 km away. In the flood period, on the other hand, around 4000 m
3·s
−1 to 8880 m
3·s
−1 water is discharged at Gorai River, eroding river banks and margins of forestland causing BSMF to be vulnerable to wave or tidal activity. An increase of 112% peak flow in the monsoon has been recorded [
21]. Erosion was found to be concentrated along the banks of major rivers and at the land−water interface of the Bay of Bengal [
4].
Using Landsat TM and Landsat ETM+ images (
Figure 4A–C), changes in the extreme northern part of Chandpai and Khulna Ranges from 1989 to 2000 and 2010 have been studied [
29]. There were several natural calamities during the study period: (i) severe flood covering over 60% of the country and a cyclone in 1988; (ii) cyclone in 1991; (iii) flood covering 75% of the country in 1998; (iv) Cyclones Sidr in 2007 and Aila in 2009. A large number of small rivers and creeks were formed sometime before the year 2000 (
Figure 4C). One possibility for the changes is that the high discharge of 7020 and 8880 m
3·s
−1 at Gorai in 1991 and 1993, respectively (
Figure 2), following tidal surge of the cyclone in April 1991, forced the huge water-mass to pass across the wetlands forming creeks and small rivers. The forest margins also eroded, increasing the water area (
Table 1) by 18.07% in 2000 and again by 7.67% in 2010, a total increase of 27.12% [
29]. There was no change in small rivers and creeks after 2000 though there were Cyclones Sidr in 2007 and Aila in 2009 (
Figure 4C). The eroded soil formed new marshy areas of 5.85 km
2 (3.87 km
2 by the year 2000 and again 1.98 km
2 by 2010). The photographs in
The Bangladesh Sundarbans represent some of these changes [
1]: a small creek on page 24 with a number of fallen trees along the banks; sediment deposition on riversides on pages 23 and 36. The forest area of 443 km
2 was reduced to 427 km
2, a total loss of 16 km
2 which is 3.61% over 20 years (
Table 1) [
29]. If the percentage of small rivers and creek formation are considered for other parts of the BSMF in addition to large rivers, and assuming that the area plus higher erosion near the coast constitute about 33% (about 2000 km
2), then the estimated total forestland loss would be about 127 km
2.
Figure 4.
Landsat TM and Landsat ETM
+ images, showing changes in species composition of parts of forests of Khulna and Chandpai Ranges in 1989 (
A), 2000 (
B) and 2010 (
C) of BSMF [
29]. Detail data of the images is given in
Table 1.
Figure 4.
Landsat TM and Landsat ETM
+ images, showing changes in species composition of parts of forests of Khulna and Chandpai Ranges in 1989 (
A), 2000 (
B) and 2010 (
C) of BSMF [
29]. Detail data of the images is given in
Table 1.
Table 1.
Changes (in ha) of forest cover, marsh and water area from 1989 to 2000 and 2010, determined by GIS technique at Khulna and Chandpai ranges (one spot from each and together). Values within parentheses is percent [
29].
Table 1.
Changes (in ha) of forest cover, marsh and water area from 1989 to 2000 and 2010, determined by GIS technique at Khulna and Chandpai ranges (one spot from each and together). Values within parentheses is percent [29].
Classes | Plant Cover and Water Area | Changes |
---|
| 1989 | 2000 | 2010 | 1989–2000 | 2000–2010 | Overall |
---|
Heritiera fomes | 23,028 | 19,309 | 16,408 | −3719 (16.15) | −2901 (15.02) | −6620 (28.75) |
Excoecaria agallocha | 15,184 | 15,828 | 16,200 | +644 (4.24) | +372 (02.35) | +1016 (6.7) |
Sonneratia apetala | 44 | 1906 | 109 | +1862 (4231) | −1797 (94.28) | +65 (148) |
Bruguiera sexangula | 191 | 8169 | 3026 | +7978 (4177) | −5143 (63) | +2835 (1484) |
Marshes | - | 387 | 585 | +387 (100) | +198 (33.85) | +585 (100) |
Shrubs | 570 | 463 | 367 | −107 (18.77) | −96 (20.73) | −203 (35.61) |
Water | 4727 | 5581 | 6009 | +854 (18.07) | +428 (7.67) | +1282 (27.12) |
Total area | 44,301 | 44,328 | 42,704 | +27 (0.06) | −1624 (3.66) | −1598 (3.60) |
Changes over 40 years of the total Sundarbans from the 1970s to 2000s have been studied using Landsat satellite data [
4]. From the 1970s to 1990s, forestland increased by 1.4%, but from the 1990s to 2000s, the area decreased by 2.5% and the net loss was 1.1%. The loss equals 110 km
2 for the total Sundarbans and about 66 km
2 for the Bangladesh Sundarbans. However, the BSMF is likely to incur more loss due to a cyclone in 1991 and a severe flood in 1998. Of the total loss, approximately 50% was lost at the Bay of Bengal (the extreme southern edge of the BSMF where almost no compensating aggradation took place) [
4]. Within the same tenure, the water bodies in the BSMF have been reported to increase by nearly 30 km
2 [
30] which, in other words, means a decrease in forestland. The decrease of sediment-laden water from the Ganges every year results in the further erosion of forestland. The situation will be further aggravated by global warming in the future. If reclamation of the BSMF is not initiated immediately, it may be that after about 100 years or so, the BSMF will be washed away into the Bay of Bengal [
20]. Predictions suggest that 100% of the mangrove forests could be lost in the next 100 years if the present rate of loss continues [
11].