The entire water body of the Bay of Bengal (BoB) consists of different types of water masses due to the influence of the upper river systems. For convenience the BoB waters may be grouped into the following zones: i. Estuarine Zone, ii. Coastal cleaner water zone, iii. Coral and rocky zone and iv. Off-shore and open sea.
Estuarine zone: In this zone the mixing of the freshwater and seawater takes place with some degrees of variation. This zone has a variable salinity and richer oxygen and organic contents. This zone may again be tentatively sub-divided into: (a) Upper estuaries and (b) Lower estuaries
|(a)||The upper estuaries have a very low salinity usually not exceeding 10 ppt and consist of the river mouths, the rivers and channels around deltaic islands. The bottom materials in this area are silt to soft mud. Usual depths are up to 10 meters. Typical example of important commercial fishes of this area is: Lates calcarifer, Lobotes surinamensis, Pangasius pangasius, Harpodon nehereus etc.|
|(b)||The lower estuaries have a higher salinity up to 20-25 ppt and consist of waters off shoreline up to a depth of 30-35 m. The color of this water is muddy to turbid and the bottom is muddy, occasionally sand and mud also. Typical examples of commercial fishes of this area are: Polynemus indicus, Pristis cuspidatus, Johnius bleekeri, Trachysurus gagoroides, Muraenesox talabonoides, H. nehereus etc.|
Coastal cleaner water zone: This water is rather clean but not typically saline. Typical examples of commercial fishes of this zone are: Chorinemus lysan and other Carangids, Cybium guttatum, Stromateidae spp., Hilsa/Tenualosa spp., Trachysurus thalassinus, Trichiurus spp. etc.
Coral and rocky zone: This zone consists of rocky and weeds grounds along the coast from Cox's Bazar up to the border with Myanmar, and the coral reefs of St. Martin's Island. Typical commercial fishes of this zone are: Lutianus johni, Lutianus rivulatu and other Lutianids, Ephinephalus spp., sharks and many other beautifully colored fishes of families belonging to Labridae, Scaridae, Chaetodontidae, Scorpaenidae etc.
Off-shore and open sea: Blue and typical saline waters far off the coast. Typical example of fishes of this zone is: Mackerel, Rastrelliger kanagurata, Sardine, Sardinella melanura, flying fishes and many pelagic sharks.
Hydro - morphology
The coastal zone of Bangladesh is categorized as tropical maritime climate. Four distinct seasonal weather patterns are governed by the monsoon, the south-west monsoon and the north-west monsoon. These are: dry winter (December-February), transition or pre-monsoon period (March-May), rainy season (June-September) and the second transition or post-monsoon period (October-November). Precipitation continues from late May up to mid October while the peak (around 70%) period is July-September. The BoB is located in the tropical monsoon belt and is strongly affected by monsoons storm surges, and cyclones. The monsoon wind changes direction with the change of seasons, and prevails mainly in the Indian Ocean. It blows from the south-west, generally from April to October, and from the northeast from October to April. The south-west, or summer, monsoon occurs when warm, moist air from the Indian Ocean flows on to the land, and is usually accompanied by heavy rain in areas of South and South-East Asia, constituting the dominant climatic event of the area. The north-east monsoon occurs when cold, dry winter air flows out of the interior of Asia from the north-east and brings the cool, dry winter season.
The climate in the Indian Ocean including BoB is characterized by its monsoon seasons. The wind in this area undergoes a complete shift between January and July resulting in the northeast monsoon in the former and the southeast monsoon in the later. This annual cycle is dominated by the heating and cooling of the Asiatic Continent. During the summer monsoon season strong winds, above 7 on the Beaufort scale are dominant over the BoB and the sea appears very much rough. The winter (December-February) is calm. During pre-monsoon (April-May) and post-monsoon (October-November) seasons tropical cyclones being generated in the area around 850E-950E, 100N and (the southern limit of the Bay of Bengal) usually approach the coast of Bangladesh with full force. The cyclones are very much destructive to the country's coastal zone being accompanied with tidal surge and torrential rainfall. Cloud cover over the Bay is highest during the southwest monsoon, but it becomes clear when the northeast monsoon sets in.
The total annual precipitation is high and varies from about 1,200 mm in the west to over 2,500 mm in the south eastern and northern region. About two thirds of this rainfall occurs during the south-west monsoon period. The south-west monsoon tends to produce an easterly drift in the open water of the Bay, together with a current setting northward and north-eastward along the east coast of India to the head of the Bay. The northeast monsoon tends to produce a westerly drift in the open water of the Bay and a current setting south-westward and southward.
The alternation of northeast and southeast monsoons brings about a complete reversal of the surface current pattern, which is clockwise from January to July and counter clockwise from August to December, according to the direction of the wind. Velocity of current varies from 4.5 to 5.5 knots during spring tide in the summer monsoon, while in the neap tide it is 2.3 -3.9 knots.
The tidal range at the head of the BoB is strong, ranging from 1.3 m at neap tides to about 4.83 m at spring tide near Sagardip, West Bengal, India. Similarly, 4.27 m neap tides and 6.10 m spring tides are observed in the Sandwip Island area, Bangladesh. This range is reduced toward the south along the eastern shore of the BoB. During the lean period (April and May), the tide penetrates up to 170 km in the north (Khulna-Satkhira area) and 340 km in the northeast region (Sylhet area). Along the east coast, the tide penetrates only from zero to 50 km depending on the topography and presence of channels in the area.
Annual variation of sea level along the Bangladesh coast is single modal with absolute maxima during August. The range of annual variation is maximum at Char Changa is about 5.4 cm and that at Hiron Point in Sunderbans is about 6.5 cm. The lowest MTL is recorded during January. There is a significant difference in sea level along the Bangladesh coast during August September of El-Nino and La-Ninã years. Post-monsoon cyclones and monsoonal floods of La Ninã years have more disastrous potential for coastal regions of Bangladesh due to increased sea level.
The correlation coefficients between September Southern Oscillation Index (SOI) and Mean Tide levels at Hiron Point and Char Changa were +0.39 and +0.33 respectively, which were significant at 95% and 90% levels. It has been observed that, the mean sea level rise at Rangoon was about 16 cm in 1962 compared to the sea level in 1920. In Chittagong mean sea level rose about 25 cm in 1964 in comparison to the sea level of 1944. In Kolkata sea level also rose to about 16 cm and in Diamond Harbour 10 cm. Only in Khidirpur the sea level remain almost constant. In Sagardwip sea level rose almost 30 cm compared to the sea level of 1931. The causes of this change may be due to climate change. The mean tidal level at Hiron Point has shown an increasing trend of about 4.0 mm/year. Similarly at Char Changa near the Meghna estuary and near Cox's Bazar it has registered a positive trend of about 6.0 mm/year and 7.8 mm/year respectively. Thus the increment in the sea level along the Bangladesh is much more pronounced as compared to the Global rate.
Sea surface water temperature : The annual variation of Sea Surface Temperature (SST) is bimodal along Bangladesh coast. The two maxima are observed during May and October. Which coincide with cyclone periods. The annual range of SST along the western and central coasts of Bangladesh is about 5°C. As expected the lowest SST's are observed in January. SST varies from 22.8°C to 32.9°C with maximum in September and the lowest in January-February. Winter lasts for an average period of two months (mid-December to mid-February). BIMSTEC survey during 25 October through 21 December 2007 in the northern Bay of Bengal (latitude 16°N-19°N, longitude 88°E-91°E) reported SST between 27.8°C and 29.7°C. In other seasons, the modal temperature values range between 25°C and 33°C. Depth gradient of temperature showed a sub-surface maximum at about 10-30 m depths due to cooling of surface layer.
Water salinity : Coastal waters are characterized by prolonged low saline regime due to the huge discharge of freshwater from upstream numerous rivers, and a strong semi-diurnal tide mixes the highly turbid coastal waters, which receives sediment from as far as the Himalayas. Inundation by such an enormous amount of freshwater renders the surface waters of the northern part of the Bay almost riverine during the post-monsoon months (September-October), while near estuarine conditions occur during January to June. Surface salinity distribution shows a strong gradient to the north-south direction. Highest salinity of 36 ppt have been reported in March, and 30.3 ppt in February; the lowest salinity of 10.5 ppt was observed in September. BIMSTEC survey reported sea surface salinity (SSS) between 31.5 and 33.6 psu, respectively. High saline water flows into the Bay from the south, then flows northward and eastward by wind driven current. Surface salinity of the Bay is influenced by water circulation and density driven. At the north and the east of the Bay, large rivers supply huge amount of freshwater that leads to lower salinity by 2-3 psu than the rest areas. Sea water salinity in the BoB, Bangladesh is highly seasonal and there is no evidence of long-term changes in the estuarine and coastal salinity regimes. Salinity of offshore water remains low for prolonged period mostly during monsoon and post-monsoon periods due to heavy precipitation and upstream freshwater run-off from the three mighty rivers. Evaporation is high in winter as well as in summer, maintaining raised salinity in coastal water. Winds throughout the year maintain strong evaporation and elevated salinity regime and in summer very high temperature may increase salinity further. Hyper-saline conditions appear from October to July.
Primary productivity and nutrient
No exhaustive study was conducted on the nutrients and productivity aspects of the BoB in the Bangladesh coast. Study from the mouth of Karnafully river in Chittagong to St. Martin's Island in Cox's Bazar showed that concentrations of nitrate, phosphate and silicate vary respectively from 0.16 to 8.98 µg-at/l, 0.08 to 2.33 µg-at/l and 0.67 to 63.31 µg-at/l, respectively with the highest value during monsoon. In the ecosystem study of the BoB concentrations of nitrite, nitrate, silicate and phosphate were found to correlate positively with depths. Average concentration of total alkalinity and total phosphorus in the surface waters of the northern part of the BoB was around 2.20+0.03 meq/l and 1.18+0.71 µM, respectively. Its concentration increased with depths. Concentrations of total alkalinity at 100 m, 200 m, 300 m and 400 m were 2.31+0.01 meq/l, 2.33+0.01 meq/l, 2.34+0.01 meq/l and 2.35+0.01 meq/l, respectively. Concentrations of total phosphorus at 100 m, 200 m, 300 m and 400 m were 2.77+0.64 µM, 2.87+1.17 µM, 2.94+0.73 µM, and 3.34+0.73 µM, respectively. The detectable ranges of nitrite+nitrate, silicate and phosphate in the northern part of the BoB were 0.07-37.38 µM, 0.01-48.56 µM and 0.10-3.13 µM. Concentrations of nitrite+nitrate was almost undetectable at surface and up to 30 m depths, its concentrations increased with depths up to 400 m. Mixed layer depths had very low nutrient concentrations or sometimes even undetectable. Concentrations of silicate as well was almost undetectable at surface and up to 30 m depths, its concentrations also increased with depths up to 400 m. Concentrations of phosphate was almost undetectable at surface and its concentrations increased with depths up to 400 m.
The chlorophyll-a content varies from 0.19 to 12.62 µg/l. The primary productivity of the BoB is comparatively low when compared to the Arabian Sea and shows maximum values of 0.15-1.45 g C/m2/day during north-east monsoon. Integrated primary productivity (up to 120 m) of the BoB varied from 8.9 to 22.1 g C/m2/day. Chlorophyll content in the BoB as determined by Nimbus-7 (CZCS) Satellite Data showed that the BoB has 0.1 to 10 mg/m3 chlorophyll content from coast to deep sea.
The average surface chlorophyll level for the BoB was 0.224 mg/m3. Eddies and circulation patterns as observed in a winter MCSST image of NOAA (AVHRR) data were also observed in a CZCS data. Concentrations of chlorophyll-a at 2-10 m depths in the northern BoB was around 0.0365-1.1162 mg m-3, low latitude stations had higher values than the hig latitude stations and at 100-150 m and 300-400 m depths its concentration was negligible. Spatial distribution of chlorophyll-a content displayed a pattern similar to that of water salinity. Most of the low latitude stations exhibited somewhat higher chlorophyll-a concentrations than in those of high latitude stations.
Two unequal peaks of Chlorophyll-a were observed following start of monsoon (May-June) and post monsoon (November-December) with maximum chlorophyll-a value of 6.14 and 9.63 µg/l respectively.The first peak was found abnormally in early monsoon. The BOBLME is considered a moderately productive (150-300 gC/m2-yr) ecosystem. Benthic phytoplankton and zooplankton production is higher in the coastal areas, which receive nutrient-rich waters.
Ecosystem study of BIMSTEC indicated that a total of 135 phytoplankton species belonging to cyanobacteria, diatom, dinoflagellates and silicoflagellates are present in the Bay. Surface waters of the northern BoB are productive with high phytoplankton densities during the noth-east monsoon, phytoplankton densities were around 261-133,790 cells/l, Oscillatoria erythrare and Proboscia alta were the dominant species and Pseudo-nitzschi pseudodelicatissima along with Chaetoceros messanensis formed plankton bloom. High concentration of zooplankton was found in the northern area of the Bay as well. The zooplankton community consisted of 205 species. The distribution pattern of zooplankton community indicated that the northern part is more productive than other areas of the BoB. Total zooplankton ranged around 154-568 individuals/m3, copepod was the dominant and was followed by sarcodine protozoans.
Average number of fish larvae in the northern part of the BoB was 411 larvae/1,000 m3 and the top five dominant families were Photichthyidae, Bregmacerotidae, Myctophidae, Callionymidae and Carangidae. Their average contribution was Photichthyidae 53.66%, Bregmacerotidae 24.8%, Myctophidae 7.82%, Callionymidae 5.27% and Carangidae 3.74%. A total of about 52 families of fish larvae were identified. As per the category of the adult's habitats six families (Labridae, Callionymidae, Gobiidae, Bothidae, Cynoglossidae and Ostraciidae) were inshore-reef fish and six families (Photichthyidae, Stomiidae, Myctophidae, Gemphylidae, Exocoetidae and Paralepididae) were oceanic fish. The eastern part of the Bay (along Andaman sea) is very rich in ichthyodiversity. In terms of average total fish larvae the Andaman sea area has the highest larve density of 485 larvae/1,000 m3 and richest diversity. Tuna larvae of the family Scombridae were more abundant in the northern BoB (Bangladesh waters) than in the Anadaman sea (eastern part) and Indian waters (western part). In Bangladesh's waters larvae of Katsuwonus pelamis, Thunnus obesus, T. alalunga and T. albacores were found. Their mean abundance ranged around 3 larvae/1,000 m3. The BOBLME is considered a moderately productive (150-300 gC/m2/yr) ecosystem. Benthic phytoplankton and zooplankton production is higher in the coastal areas, which receive nutrient-rich waters.