Seasonal nutrient distribution in the Rupsha-Passur tidal river system of the Sundarbans mangrove forest, Bangladesh
© Rahaman et al.; licensee Springer 2014
Received: 29 March 2014
Accepted: 3 July 2014
Published: 29 July 2014
The Rupsha-Passur River System (RS) is one of the biggest and important river systems in the Sundarbans estuarine ecosystem. It is the largest fresh water supplier into this mangrove forest. A comprehensive study was undertaken to evaluate the tidal and seasonal variations in nutrient concentrations (nitrate, phosphate, sulphate and ammonia) of the RS during October, 2010 (Post monsoon), February, 2011 (Dry winter) and August 2011 (Monsoon). In-situ measurement and laboratory analysis were conducted to measure nutrients and other physico-chemical parameters at discrete water layers of five study points under different tidal cycles.
Following post monsoon, NH3–N concentration showed seasonal variation with increasing trend (0.0703 to 0.0803 mg/L) in dry winter and then significant dropping (0.013 to 0.019 mg/L) in monsoon period. During field observation, no significant tidal variation in ammonia concentration was identified among the study sites. Post monsoon and winter time observation recorded lower SO4 content (7.301 to 37.508 mg/L) at all the sampling stations while in monsoon period, most of the stations showed higher concentration up to 126.92 mg/L. Contrary to post monsoon season with comparatively higher PO4–P value (0.314 to 1.347 mg/L), winter and monsoon period sampling indicated low phosphate concentrations (0.045 to 0.5 mg/L) in the Rupsha-Passur RS. Most of the study sites showed considerable tidal changes in phosphate value during post monsoon and monsoon season while only two stations went through tidal fluctuation in sulphate content during monsoon period. Concentrations of NO3–N were found to fluctuate between 0.083 and 1.233 mg/L with no distinct seasonal distribution pattern. Tidal variation of NO3–N in the experimental sites was not so prominent during post monsoon period as of winter data.
Present study will serve as a basis for future hydrological and environmental studies in the world’s largest Sundarbans intertidal mangrove forest. Study results indicate how nutrient dynamics of such diversified estuarine system are influenced by varying weather conditions. Daily fluctuations in nutrient concentrations and other physicochemical properties due to semidiurnal tidal activity were also figured out through the study. Information generated from the research works will guide all concerned for any future conservation and management initiatives for the world heritage site.
KeywordsWater quality Monsoon Aquatic Ecosystem Hydrodynamic environment Post monsoon
The Sundarbans forest is the largest continuous productive mangrove wetland ecosystem in the world (Rahaman et al. ). It is located at the extreme end of the southern Ganges delta, and it is about 10,000 km2 in southwest Bangladesh and West Bengal of India. A total area of 62% lies in the Khulna region of the southwestern part of Bangladesh, while the remaining 38% is in India (Siddiqi ). The Bangladesh portion of Sundarbans covers an area of 6,017 km2 of mangrove forests, wildlife sanctuaries, and sand bars, and out of this, 1,874 km2 is made up of rivers, creeks, and canals (Wahid ). It is a region of transition between the freshwater of the rivers originating from the Ganges and the saline water of the Bay of Bengal. The coast of the Sundarbans is crisscrossed by a network of complex estuarine system created by rivers Rupsha, Passur, Shibsha, Baleswar, Bhola, Arpangashia, Kholpetua, and other rivers which open into the Bay of Bengal through the Sundarbans reserved forest and carry large amounts of nutrients that vary with tides and seasons which facilitate the productivity in the area (Rahaman et al. ).
The mangrove forest of Bangladesh is one of the most important coastal features of the country. The existence of the mangrove has increased the values of other coastal and marine resources such as the coastal and marine fisheries by increasing productivity and supporting a wide biological diversity. The artisanal fishery, which is highly influenced by mangroves, has been contributing 85% to 95% of the total coastal and marine catch of Bangladesh. The mangrove also supports offshore and deep sea fisheries by playing a significant role as nursery ground for many deep sea fishes and shrimps including the giant tiger shrimp (Penaeusmonodon) which is the major species of the industrial bottom trawl fishery of Bangladesh. The mangrove also contributes significantly to shrimp farming which has been the most significant export-oriented industry since the 1970s.
Estuaries are an important segment of the biogeochemical cycle as they regulate the amount of river-borne major and minor elements entering the coastal environment and ultimately the deep ocean. Estuarine ecosystems are complex and dynamic due to strong gradients in the chemical composition of water, variable suspended matter concentration, and complex hydrodynamic processes. When river water mixes with seawater, different types of physical and chemical processes take place that may affect the partitioning of trace metals between particulate and dissolved phases and hence the composition of the deposited sediments (Forstner and Gnaiger ).
Today, the Sundarbans is entirely surrounded and pressured to the landward side as an island by human communities and their agricultural and commercial activities. Coastal polders have been commissioned since 1968 in the immediate upstream catchments to enhance agricultural productive potential of the land by protecting it from saline intrusion. However, the operation and maintenance cost of the polders are very high and most of them are in very bad condition impeding natural drainage of freshwater into western Sundarbans (PDO-ICZMP ). Industrial activity near the upstream river system (RS) as well as the nearby second largest seaport has expanded. Recently, the importance of estuarine processes in modifying the chemistry of the materials accumulating and passing through this interface has been realized. Several geochemical processes, such as precipitation and flocculation of the dissolved and colloidal substances (Coonley et al. ; Sholkovitz ; Gobeil et al. ) desorption-adsorption phenomenon, chemical diagenesis, and exchange with the bottom sediments (Yeats and Bewers ), have been studied within the mixing zone.
Passur river of the Sundarbans has influenced the Bhairab and is connected with the Garai-Madhumati system through Atarabanki and Nabaganga rivers. The Garai-Madhumati is the major spill river of the Ganges and carries out 12% of flow of the Ganges (Hussain and Acharya ). The Rupsha-Passur RS flows through the middle part of the Sundarbans mangrove forest and serves as the lifeline for millions of people inhabiting the mangrove-dominated Sundarbans and for different types of industries - agricultural land, fish farm, and shrimp farm - which are situated on the banks of the Rupsha river. A considerable quantity of toxic and hazardous substance is released into this important aquatic system through these industrial effluents along with huge organic load emanating from agricultural and shrimp culture activities and several non-point sources (such as discharges from fishing vessels and trawlers and runoff from adjacent landmasses).
The mangrove ecosystems are considered to have high primary productivity due to the nutrient enrichment and thus offer ideal fishing grounds. Nutrients are essential chemicals for the survival and growth of plants. A total of 20 elements are required for phytoplankton growth. These are C, H, O, N, P, S, K, Mg, Ca, Na, Fe, Mn, Cu, Zn, Mo, V, B, Cl, Co, and Si (Moss ). The most common and major nutrients are the compounds of nitrogen and phosphorus (Nybakken ). Dissolved organic and inorganic nutrients are found in aquatic environment at all times, and some of them are recycled naturally by decaying living organisms. Aqueous life depends on the availability of dissolved nitrogen and phosphorus (Skinner and Turekian ).
Harris () reported that in an aquatic ecosystem, dissolved inorganic nitrogen such as ammonium, nitrate, nitrite, molecular N2 and DIN are therefore very much dependent on biological uptake and regeneration. It is estimated that nearly 95% of the N in the coastal environment is present as molecular N2. Dissolved nitrate (NO3−), nitrite (NO2−), and ammonium (NH4+) ions are the three major sources of nitrogen, which are required by phytoplankton (Boney ). On the other hand, dissolved orthophosphate is another important nutrient source for phytoplankton as it is taken rapidly by phosphorus-deficient cells from water bodies with lower concentrations (Boney ). Low availability of phosphate in the freshwater environment could be one of the several factors which may cause a decline in primary productivity (Abel ; Law et al. a), while the productivity in coastal waters is normally limited by the concentration of nitrogen compounds (Skinner and Turekian ).
Excess concentrations of nutrients that enter the aquatic ecosystem in the form of sewage and industrial wastes or runoff from urban areas can dramatically increase the primary productivity of the ecosystem, and this consequently may affect the marine water quality (Abel ). The source of additional organic and inorganic phosphate is mainly through discharges of domestic sewage, partly supplied by human waste and mainly from the use of phosphate-rich detergents and fertilizers (Abel ). Inorganic compounds such as ammonium are used preferentially by plants and produced by bacterial breakdown of organic matter and animal excretion. At optimal concentration, this nutrient concentration stimulated excess algal growth in unpolluted marine waters: ammonium concentrations are low and variable, nitrate that is an important source of nitrogen is present in large quantities, and nitrite is present in much lower quantities (Boney ). In polluted waters, it is known that high concentrations of nitrogen, particularly in the form of NH4+, NO2−, and NO3−, can be toxic to many varieties of aquatic organisms and can constitute a human health hazard. Despite huge potential of this mangrove forest, very few attempts have been undertaken to investigate its soil and water properties especially the nutrient dynamics of this resourceful water body. The present study has been made to quantify some major nutrients at different locations and discrete water layers of the Rupsha-Passur RS under varying tidal and weather conditions.
Geographical location of study areas and their water depth during sampling at different tides
Water depth (m)
89° 35′ 57.86″ E
22° 25′ 54.98″ N
89° 35′ 24.21″ E
22° 25′ 43.22″ N
89° 37′ 49.65″ E
22° 20′ 51.35″ N
89° 36′ 17.67″ E
22° 17′ 57.59″ N
89° 36′ 55.72″ E
22° 17′ 48.28″ N
For in situ measurement and laboratory analysis, water samples were collected from three different depths (surface, middle, and 0.5 m above the bottom) of the study points under three distinct seasons. In order to analyze tidal variations, water samples were collected both during high and low tide conditions. Three samples were taken from each water layer both during high and low tide under each season which makes a total of 270 samples for the whole observation. For each sampling season, 90 water samples were collected and carried to the laboratory for analyzing a variety of water parameters. A Ruttner water sampler and DO sampler were used with marked ropes to collect samples from varying depths of water. After collection of water sample, it was poured into 250-mL plastic bottles and immediately preserved in an icebox and carried to the laboratory for nutrient analysis. During in situ measurement, salinity was recorded by an Atago S/Mill-E refractometer (Atago Co. Ltd., Tokyo, Japan), temperature by digital thermometer with stainless steel sensorprobe, pH by a Hanna microprocessor pH meter (model HI, pH 211; Hanna Instruments, Woonsocket, RI, USA), and dissolved oxygen (DO) by Winkler's method (APHA ).
Collected water samples were analyzed in the laboratory for measuring concentrations of nitrate, ammonia, phosphate, and sulfate. A high-definition spectrophotometer was used for estimating the concentration of major water nutrients. During analysis, phosphate and sulfate were determined by the ascorbic acid method (Ramesh and Anbu ), while ammonia was measured by Nesslerization method (EPA Manual ) and nitrate was measured by phenol disulfonic acid method (Ramesh and Anbu ). Seasonal and tidal variations of nutrient concentrations were evaluated by calculating means and standard errors from the total number of samples taken for each analytic method during high and low tide within each sampling date.
Statistical comparison among means was performed by univariate analysis by SPSS software version 16.0 (SPSS Inc., Chicago, IL, USA).
Results and discussion
Status of physicochemical parameters
Tidal and seasonal fluctuation of mean water temperature at the selected measuring points ranged from 19.23°C to 31.57°C with an average of 27.15°C. Average temperature was found to be 29.37°C to 30.53°C during high tide and 28.8°C to 31.57°C during low tide in post monsoon, whereas in monsoon period, it was 29.8°C to 30.6°C during high tide and 30.9°C to 31.47°C at low tide. Field observation found an average temperature of 20.6°C to 21.67°C during rising tide, while 19.23°C to 21.4°C during dropping tide in dry winter. The tidal variation of temperature is found to be almost negligible but showed significant seasonal variation. Higher temperature was recorded both during monsoon and post monsoon seasons mostly at falling tide condition. Lower water temperature was observed during dry winter. Similar to the present study, Rahaman et al. () and Hoq et al. () found highest and lowest water temperature in the Sundarbans RSs during monsoon and winter seasons, respectively. The uniformity in water temperature values was due to high specific heat of the aquatic phase, which enables water to resist much fluctuation of temperature than the adjacent landmasses. The water temperature has considerable effect on phytoplankton population density by influencing the process of cyst germination (Ishikawa and Taniguchi ; Blanco ). Spatial and tidal uniformity in surface water temperature of the study area, however, has the least probability to affect the plankton community. Nutrient concentrations (nitrate, phosphate, and silicate) of aquatic system may be influenced when temperature rises as it affects the photosynthesis rate (Tait ; Pliński and Jóźwiak ).
The distribution of pH at different water depths of the Sundarbans mangrove RS under varying seasons and tidal cycles was varied. During post monsoon, mean pH ranged from 7.73 to 7.83 at high tide and from 7.77 to 7.97 at low tide. Dry winter showed water pH varying from 7.22 to 7.74 during high tide and from 7.34 to 7.89 at low tide condition. Rupsha-Passur RS indicated water pH to be 7.19 to 7.54 and 7.23 to 7.59 at high and low tide, respectively. During the study period, tidal and seasonal variations of water pH were not so significant. Rahaman et al. () conducted field observations in the Kholpetua-Arpangashia RS and found higher pH values during winter when the tidal level was rising, whereas during post monsoon and monsoon seasons, the recorded pH values of most of the stations were a bit higher at low tide than at high tide. Hoq et al. () reported that river water of the Sundarbans was characterized by slightly alkaline pH and water pH remains neutral to alkaline (7.4 to 8.1) throughout the study period. Generally, fluctuations in pH values can be attributed to factors like removal of CO2 by photosynthesis through bicarbonate degradation, dilution of seawater by freshwater influx, reduction of salinity and temperature, and decomposition of organic matter.
Distribution of nutrients
Mean of surface, middle and bottom water layer nutrients measured at the study points under high and low tide conditions
0.533 ± 0.226
7.902 ± 0.733
0.646 ± 0.210
0.038 ± 0.007
0.223 ± 0.047
30.144 ± 0.466
0.303 ± 0.014
0.075 ± 0.002
0.314 ± 0.263
19.724 ± 6.445
0.713 ± 0.116
0.015 ± 0.001
0.492 ± 0.252
7.989 ± 0.242
0.848 ± 0.168
0.050 ± 0
0.112 ± 0.007
21.819 ± 1.039
0.85 ± 0.542
0.075 ± 0.007
0.137 ± 0.012
51.065 ± 53.33
0.856 ± 0.085
0.015 ± 0.003
0.557 ± 0.292
7.674 ± 0.141
0.443 ± 0.306
0.047 ± 0
0.262 ± 0.106
28.454 ± 1.069
0.883 ± 0.259
0.077 ± 0
0.127 ± 0.098
13.697 ± 1.710
0.96 ± 0
0.014 ± 0.00
0.410 ± 0.042
7.819 ± 0.733
0.772 ± 0.159
0.040 ± 0
0.162 ± 0.077
32.451 ± 7.151
0.28 ± 0.264
0.077 ± 0
0.166 ± 0.171
64.911 ± 81.103
0.895 ± 0.091
0.016 ± 0.002
0.892 ± 0.643
8.742 ± 1.854
0.556 ± 0.158
0.050 ± 0.004
0.163 ± 0.127
24.434 ± 2.653
0.24 ± 0.221
0.073 ± 0.004
0.289 ± 0.205
119.75 ± 10.139
0.7 ± 0
0.017 ± 0.002
Relationships among physicochemical parameters and nutrient constituents
The coefficient of correlation of different water quality parameters
Coefficient of correlation
Temperature vs nitrate
Temperature vs ammonia
Temperature vs phosphate
Temperature vs sulfate
pH vs ammonia
pH vs nitrate
pH vs phosphate
pH vs sulfate
DO vs ammonia
DO vs nitrate
DO vs phosphate
DO vs sulfate
Salinity vs ammonia
Salinity vs nitrate
Salinity vs phosphate
Less inversely related
Salinity vs sulfate
The Sundarbans mangrove wetland forest serves as a link between terrestrial and marine ecosystems. The study provides comprehensive seasonal, tidal, and spatial characterization of hydrological regime of the Sundarbans estuarine ecosystem of Bangladesh. With the onset of monsoon, the central part of the Sundarbans experiences the greatest reduction in salinity level. Augmentation of the freshwater inflow from the upstream reduces the salinity in the Rupsha-Passur river system. The major nutrients showed significant seasonal changes in concentration level, and in few cases, considerable tidal variations were also observed. Concentrations of NO3-N were found to vary between 0.083 and 1.233 mg/L with an average figure of 0.663 mg/L and no distinct seasonal distribution pattern. Phosphate concentrations as PO4-P were generally low (0.045 to 0.5 mg/L) in dry winter and monsoon, whereas post monsoon season showed comparatively higher content of phosphate (0.314 to 1.347 mg/L) in the Rupsha-Passur RS. The present study observed lower SO4 content (7.301 to 37.508 mg/L) at all the sampling stations over post monsoon and winter seasons, while most of the stations showed higher concentration (up to 126.92 mg/L) during monsoon. Ammonia distribution showed significant variation with 0.026 to 0.047 mg/L in post monsoon season, then higher content (0.0703 to 0.0803 mg/L) during dry winter and lowest concentration of 0.013 mg/L in monsoon season. Thus, the major nutrients of the Sundarbans mangrove forest RS is regulated by seasonal variations, and in most cases, little impact comes from tidal fluctuation.
The authors would like to acknowledge the financial support of the Ministry of Education, Bangladesh for successful implementation of the research project. The Bangladesh Forest Department offered all the required supports during in situ measurement and sample collection from the river system. We are thankful to the laboratory staffs of Fisheries and Marine Resource Technology Discipline, Khulna University for their efforts and cooperation during sample analysis. Two anonymous reviewers are appreciated for their valuable comments and suggestions.
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