Spatial and Temporal Variations in Phytoplankton Abundance and Species Diversity in the Sundarbans Mangrove Forest of Bangladesh

Rahaman SMB^1*, Golder J², Rahaman MS³, Hasanuzzaman AFM¹, Huq KA¹, Begum S², Islam SS¹ and Bir J¹

¹Fisheries and Marine Research Technology Discipline, Khulna University, Khulna 9208, Bangladesh

²Environmental Science Discipline, Khulna University, Khulna 9208, Bangladesh

³Department of Chemistry, Comilla University, Comilla 3500, Bangladesh

*Corresponding Author:

Dr. SM Bazlur Rahaman
Professor, Fisheries and Marine Resource Technology Discipline
Khulna University, Khulna-9208, Bangladesh
Tel: +8801914325048
Fax: +88-041-731244
E-mail: riti_rahaman@yahoo.com

Received date May 31, 2013; Accepted date June 25, 2013; Published date June 30, 2013

Citation: Rahaman SMB, Golder J, Rahaman MS, Hasanuzzaman AFM, Huq KA, et al. (2013) Spatial and Temporal Variations in Phytoplankton Abundance and Species Diversity in the Sundarbans Mangrove Forest of Bangladesh. J Marine Sci Res Dev 3:126. doi:10.4172/2155-9910.1000126

Copyright: © 2013 Rahaman SMB, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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Abstract

The study examined taxonomic composition, abundance and spatial distribution of phytoplankton, and water quality of three major river systems of the Sundarbans. A total of 134 phytoplankton species were identified, and diversity and abundance were found to fluctuate with time and space. A total of 97 species were enumerated in Rupsha-Pashur while 122 and 110 in Khalpatua-Arpangachia and Bhola-Baleswar river system respectively. Abundance was lowest (3.709×103 ± 4.257×102 cellsL-1) in monsoon and highest (2.174×105 ± 1.723×105cellsL-1) in summer in Bhola-Baleswar. Species composition was dominated by Bacillariophyta over the area except in summer in Bhola-Baleswar, where Cyanophyta become dominated. Species diversity, richness and evenness index varied between 2.03-4.64, 1.2-2.44, 0.77-1.5 in Rupsha-Pashur; 2.47-3.85, 1.8-5.84, 0.78-0.94 in Khalpatua-Arpangachia; and 0.66-4.27, 1.19-5.12, 0.59-1.29 in Bhola-Baleswar. Water temperature, pH, DO, Transparency and Salinity were determined between 19.92°C-31°C; 6.7-7.87; 3.93 mgL-1-7.37 mgL-1; 7.5 cm-60 cm; 2-23 ppt, respectively. Nutrient elements i.e. NO3 -, PO4 3-, NH4 +, SiO4 4- fluctuated seasonally from 0.0062 to 1.633 mgL-1, 0.005 to 0.772 mgL-1, 0.038 to 2.467 mgL-1, 3.124 to 27.234 mgL-1, respectively. Chlorophyll-a concentrations were fluctuated seasonally within 0.24 to 5.94 μgL-1 and highest phytoplankton biomass was observed in Bhola-Baleswar in summer. Chlorophyll concentration was found to be correlated positively with transparency, salinity and nutrients.

Keywords

Water quality; Phytoplankton; Biomass production; Monsoon; River system; Estuary; Sundarbans

Introduction

The Sundarbans; the largest single chunk of tidal halophytic mangrove forest in the world; lies in the vast delta on the Bay of Bengal formed by the greater confluence of the Ganges; Brahmaputra and Meghna rivers. It covers 10,000 sq.km of which about 6000 sq.km area is within the political boundary of Bangladesh; and the remaining in India. The area experiences a subtropical monsoonal climate with an annual rainfall of 1600-1800 mm and severe cyclonic storms. Enormous amounts of sediments carried by the rivers contribute to its expansion and dynamics. The Hydro-geochemical environment of this area is highly dynamic in nature with numerous drainage channels and significant coastal processes [1]. The biodiversity includes about 350 species of vascular plants; 250 fishes and 300 birds; besides numerous species of phytoplankton; fungi; bacteria; zooplankton; benthic invertebrates; molluscs; reptiles; amphibians and mammals. Species composition and community structure vary east to west; and along the hydrological and salinity gradients. The Sundarbans with diverse gene pool for flora and fauna provides livelihoods for about 2.5 million people of Bangladesh [2]. Being in the coastal waters; the abiotic and biotic resources of the Sundarbans are highly variable in response to the coastal dynamic processes. The regulation of river flows by a series of dams; barrages and embankments for diverting water upstream for various human needs and for flood control has caused large reduction in freshwater inflow and seriously affected the biodiversity because of an increase in salinity and changes in sedimentation. Ecological characteristics; particularly intermixing of saline and fresh water; seasonally fluctuating salinity and the silt brought down by the rivers greatly influence the distribution and abundance of algae in the Sundarbans. The observed distribution of a given species is a function of differential adaptations (differences in reproduction and growth) in response to environmental factors such as climate; freshwater inflow; salinity; tidal coverage; sediment type; nutrients; etc.

The algal flora of the Sundarbans is very poorly known; but the available information suggests that the Sundarbans has a highly diverse algal flora comprised of both benthic and planktonic forms ranging from the freshwater to marine environments. There was no previous record of algal flora of the Sundarbans before studies by Islam [3]. A few works on the phytoplankton community structure and its relation to abiotic variables in the Sundarbans river systems were hitherto studied in Bangladesh [4-8]. Various published reports on the algal flora provide only a patchy picture as they are based on short-term surveys of small isolated areas. Since algal flora play very important role in ecological context; the study of phytoplankton community structure is utmost important. Phytoplankton community structures change be influenced by their surrounding environment; physically; chemically; and biologically; in ways which favor or disfavor their continued persistence. The study of phytoplankton community response to these variables is considered useful for interpreting hydro-chemical variations in coastal areas [9]. So the temporal and spatial composition of the phytoplankton population may act as an indicator of the water quality fluctuation in response to changing environment.

The present study was conducted to understand the seasonal variation of phytoplankton population with water chemistry of the major three river systems (Rupsha-Pashur; Khalpatua-Arpangachia; Bhola-Baleswar) in the Sundarbans. Thus it can provide clues to do further research in order to get the factors responsible for disturbance in ecological imbalance of the Sundarbans; one of the unique aquatic habitats of the globe.

Materials and Methods

Sampling strategy

Fifteen stations were selected for the examination of water quality indices. Rupsha; Passur; Shibsha; Arpangachia; Khalpatua; Malancha; Raymangal; Baleswar; and Bhola are major rivers of the Sundarbans. Sampling stations were designed to cover the three major river systems of the Sundarbans such as Rupsha-Pashur; Khalpatua- Arpangachia; and Bhola-Baleswar (Figure 1). Samples were collected temporally namely in monsoon (June-October); winter (November- February); and summer (March-May) from 5 stations of Rupsha- Pashur river system namely (1) Karamjol (89°35.962"E; 22°25.97’N) (2) Karamjol Canal (89°35.445’E; 22°25.737’N); (3) Joymoni (89°37.757’E; 22°21.053’N); (4) Harbaria (89°36.563’E; 22°17.974’N); and (5) Harbaria Canal (89°36.985’E; 22°17.998’N); and from 6 stations of Khalpatua- Arpangachia namely (1) Pashurtala (89°11´934"E; 22°14’038"N); (2) Pashurtala Canal (89°12´043"E; 22°14’077"N); (3) Kalagashi (89°14´541"E 22°12’685"N); (4) Kalagashi Canal (89°14’638"E; 22°12’392"N); (5) Nildumur (89°14´708"E; 22°14’828"N); and (6) Arpangachia (89°18´581"E; 22°12’406"N); and also 4 stations of Bhola- Baleswar namely (1) Bogi (89°50’21.9"E; 22°12’54.4"N); (2) Sharankhola (89°48’42.4"E; 22°12’35.6"N); (3) Supati (89°49.07’E; 22°03.442’N); and (4) Supati Canal (89°48.93’E 22°3.128’ N ).

Figure 1: Location of the study points in Rupsha-Pashur, Khalpatua-Arpangachia, Bhola-Baleswar river systems of the Sundarbans.

Sample collection

For phytoplankton community structure study; 20 L of water collected from surface water of the study area was passed through a plankton net (20 μm mesh sized; silk bolting cloth or nylon monofilament screen cloth). Then the concentrated samples were preserved with Lugol’s Solution (20 g potassium iodide and 10 g iodine crystals dissolved in 200 ml distilled water containing 20 ml glacial acetic acid). For chlorophyll a estimation 200 m³ volume samples were collected in the same way using plankton net and samples for nutrient analysis were collected using a grab sampler. For nutrient analysis water samples were collected from surface; middle bottom of the particular location which was filtered immediately through pre-cleaned 0.45 μm pore-size cellulose filters. The samples were preserved in deep frozen in the dark before analysis.

Sample analysis

Temperature; pH; transparency; DO; and salinity were measured in-situ and water samples collected for analyzing nutrients (NO₃^-; PO₄^3-; NH₄⁺; SiO₄^4-); chlorophyll a and phytoplankton community composition were measured later in the laboratory. Water temperature was measured with a Centigrade Mercury thermometer; pH with a Microprocessor pH meter (HANNA instruments; pH 211); salinity with a TDS meter (HI 9635; portable multirange conductivity/TDS meter; HANNA); transparency with a secchi disc; and DO in Winkler´s method (APHA 1992).

Quantitative estimation of phytoplankton was done by Sedgewick- Rafter counting chamber (S-R cell) method [6] using Labomed Imaging Device (ivu 15000 microscope). Phytoplankton genera and species were identified [10-18]. Chlorophyll a was determined by spectrophotometric method (APHA 1992); and nutrients were measured by colorimetric methods described in Yin et al. [19].

Species richness; diversity and evenness index calculation

Species richness index (d); species diversity index (H); and evenness index were calculated according to following equations

• Species richness index (d) [20]

d=(S–1)/ Log N

Where:

d=Species richness index

S=Number of species in a population

N=Total number of individuals in S species.

• Species diversity index (H) [21]

Hs=Σ P_i 1ogP_i

Where

Hs=Diversity Index

i=Counts denoting the i^th species ranging from 1–n

P_i=Proportion that the i^th species represents in terms of numbers of individuals with respect to the total number of individuals in the sampling space as whole.

• Evenness index (j) [22]

j=H_s / Log S

Where

J=Equitability index

H_s=Shannon and weaver index

S=Number of species in a population

Data analysis

To establish differences of the phytoplankton community descriptors among the fifteen sites; one-way ANOVA was carried out. Two tailed Pearson correlation was performed to identify relation among various physico-chemical and biological parameters. Analyses were performed using the software package SPSS Statistics 17.0.

Results

Water quality

Water quality parameters namely temperature; pH; transparency; dissolved oxygen; salinity; nitrate; phosphate; ammonium; silicate; and chlorophyll a were determined temporally in 15 stations situated in three major river systems of the Sundarbans presented in Table 1.

Table 1: Annual mean values of water quality variables in Rupsha-Pashur, Khalpatua- Arpangachia and Bhola-Baleswar river systems.

Temperature: Mean water temperature fluctuated seasonally. In Rupsha-Pashur river system temperature ranged from 20.02°C–31°C; highest in summer and lowest in winter. In Khalpatua-Arpangachia and Bhola-Baleswar River systems it was within the range 22.17°C–30.45°C; and 19.92°C-30.25°C respectively and also showed the same kind of seasonal variability of higher values in summer and lower values in winter.

pH: In Rupsha-Pashur; Khalpatua-Arpangachia and Bhola- Baleswar river system pH varied from 7.29-7.87; 7.31-7.82; and 6.7-7.53 respectively. In Rupsha-Pashur pH values showed a seasonal trend of variation with higher values in monsoon then showed a slight gradual reduction through winter and summer. But in Khalpatua-Arpangachia and Bhola-Baleswar river system no clear seasonal trend was observed.

Transparency: In all three river systems; transparency values were very low throughout the year; but also showed a seasonal trend of variation. In both Rupsha-Pashur and Bhola-Baleswar river system; transparency was the lowest in monsoon but was found to rise gradually in winter and summer. It ranged from 8cm-36cm in Rupsha- Pashur with highest in Harbaria during summer and lowest in Joymoni during monsoon. The transparency ranged between 18 cm and 36 cm in Bhola-Baleswar highest in Sharankhola during summer and lowest in Bogi during monsoon. But in Khalpatua-Arpangachia River system the transparency was lower in monsoon but rose in winter and again fell in summer. It was found within the range of 16 cm-66 cm; highest in Pashurtala canal during winter and lowest in Kalagashi during monsoon. Annual mean transparency also was highest (34.66 ± 18.84 cm in Pashurtala) in Khalpatua-Arpangachia.

Dissolved oxygen: Dissolved Oxygen concentration (DO) fluctuated spatially and temporally. In Rupsha-Pashur; Khalpatua- Arpangachia; and Bhola-Baleswar river systems; DO varied from 4.37 mgL^-1 to 7.37 mgL^-1 (highest at Harbaria Canal in monsoon & lowest at Karamjol Canal in summer); 3.93 mgL^-1 to 6.4 mgL^-1 (highest at Kalagashi Canal in monsoon & lowest at Kalagashi in summer); and 4.88 mgL^-1 to 5.65 mgL^-1 (highest at Sharankhola in summer & lowest at Supati Canal in monsoon) respectively. In Bhola-Baleswar DO fluctuated very little and not showed any seasonal trend of variation. But in Rupsha-Pashur and Khalpatua-Arpangachia a little fluctuation of seasonal trend was observed with higher values in monsoon and gradually decreased over the winter and summer period. The highest annual mean value was found 6.25 ± 0.80 mgL^-1 at Harbaria Canal (Rupsha-Pashur).

Salinity: The highest salinity was observed (23‰) at Nildumur in winter. In Rupsha-Pashur; Khalpatua-Arpangachia; and Bhola- Baleswar river systems salinity ranged from 5.17‰ to 16‰ (highest at Harbaria in summer and lowest at Harbaria in monsoon); 9‰ to 23‰ (highest at Nildumur in winter and lowest at Arpangachia in summer); and 2‰ to 16‰ (highest at Supati in summer and lowest at Bogi in monsoon) respectively. In both Rupsha-Pashur and Bhola-Baleswar river system salinity showed similar seasonal variation of lower values in monsoon then rose through the period of winter and summer. But in Khalpatua-Arpangachia salinity was low in monsoon; and then rose in winter and again dropped in summer. Lowest annual mean salinity was 8.29% ± 4.50‰ (Sharankhola) in Bhola-Baleswar river system.

Nitrate: In maximum sampling stations annual mean nitrate (NO₃^- ) concentrations were lower than 1 mgL^-1. The highest concentration (>1 mgL^-1) was observed in Bhola-Baleswar in summer. In Rupsha Pashur river system it ranged from 0.021 mgL^-1 to 1 mgL^-1 (highest at Karamjol Canal in winter & lowest at Karamjol in monsoon); and in Bhola-Baleswar it varied from 0.0062 mgL^-1 to 1.633 mgL^-1 (highest at Bogi in summer & lowest at Supati in monsoon). Both of these river systems showed a seasonal trend of lower NO₃^- values in monsoon and higher values in winter and summer. But in Khalpatua-Arpangachia NO₃^- values were found within the range of 0.05 mgL^-1-0.65mgL^-1 and no clear seasonal trend was observed.

Phosphate: Throughout the study areas; phosphate (PO₄^3-) concentrations were found lower than 1 mgL^-1. In Rupsha-Pashur; Khalpatua-Arpangachia; and Bhola-Baleswar river systems it was observed within the range of 0.04 mgL^-1-0.772 mgL^-1 (highest at Karamjol in summer & lowest at Harbaria in monsoon); 0.063 mgL^-1-0.161 mgL^-1 (highest at Kalagashi Canal in monsoon & lowest at Pashurtala Canal in winter); and 0.005 mgL^-1-0.045 mgL^-1 (highest at Sharankhola in monsoon & lowest at Supati in winter); respectively. In Rupsha-Pashur; and Bhola-Baleswar river systems PO₄^3-concentrations showed no significant seasonal trend of variation. But in Khalpatua- Arpangachia river system a seasonal trend with higher concentrations in monsoon and summer and lower concentrations in winter were observed. Annual mean PO₄^3- value was the lowest (0.011 ± 0.005 mgL^-1 in Supati) in Bhola-Baleswar river system.

Ammonium: Mean Annual ammonium (NH₄⁺) value was found the lowest (0.038 ± 0.018 mgL^-1 in Sharankhola) in Bhola-Baleswar river system. NH₄⁺ values varied from 0.015 mgL^-1-0.087 mgL^-1 (highest at Karamjol Canal in winter & lowest at Karamjol in monsoon) in Rupsha-Pashur river system. But in Khalpatua-Arpangachia and Bhola-Baleswar river systems NH₄⁺ values ranged from 0.008 mgL^-1-0.434 mgL^-1 (highest at Pashurtala in monsoon & lowest at Pashurtala in winter); and 0.008 mgL^-1-0.102 mgL^-1 (highest at Supati Canal in winter & lowest at Supati both in monsoon ans winter) respectively. In Rupsha-Pashur and Bhola-Baleswar NH₄⁺ values were lower in monsoon and higher in winter and summer. But in Khalpatua-Arpangachia trends were found different here NH₄⁺ values were much higher in monsoon and lower in winter and summer season.

Silicate: Silicate (SiO₄^4-) concentration was observed extremely high throughout the study area. Annual mean SiO₄^4- concentration was found within the range of 8.238 ± 5.17 mgL^-1-17.56 ± 9.22 mgL^-1 (highest in Sharankhola; Bhola-Baleswar and lowest in Nildumur; Khalpatua-Arpangachia). Both Rupsha-Pashur and Bhola-Baleswar showed similar seasonal trend with lower values in monsoon and higher values in winter and summer. But in Khalpatua-Arpangachia SiO₄^4- concentrations were recorded lower in monsoon; and it rose in winter and again fell in summer.

Chlorophyll: Concentration of chlorophyll a fluctuated within the range of 0.24 μgL^-1-3.11 μgL^-1 in Rupsha-Pashur; 0.42 μgL^-1-2.43 mgL^-1 in Khalpatua-Arpangachia; and 0.57 μg/L-5.94 μg/L in Bhola-Baleswar. In Rupsha-Pashur and Bhola-Baleswar river systems; phytoplankton biomass showed a seasonal trend with lower values in monsoon and gradually raised levels in winter and summer. But in Khalpatua- Arpangachia it was lower in monsoon; rose in winter and again dropped in summer. Annual mean biomass production was the highest (2.82 ± 2.28 μgL^-1 in Bogi) in Bhola-Baleswar river system.

There were significant differences in water quality data (p<0.05) among river systems and also sampling sites.

Phytoplankton community composition

Throughout the study areas a total of 134 phytoplankton species dominated by diatoms were identified. 99 species from 41 genera of Bacillariophyta; 18 species from 6 genera of Pyrophyta; 12 species from 9 genera of Chlorophyta; 4 species from 4 genera of Cyanobacteria; and 1 Species of Ochrophyta were present. There were significant differences (p<0.05) in abundance and diversity of phytoplankton communities. In Rupsha-Pashur; Khalpatua-Arpangachia; and Bhola-Baleswar river systems; abundance varied in the range of 3.755×10³ cellsL^-1-1.015×10⁵ cellsL^-1 (highest at Harbaria Canal in summer & lowest at Karamjol in monsoon); 2.951×10³ cellsL^-1 to 4.197×10⁴ cellsL^-1 (highest at Kalagashi in winter & lowest at Kalagashi Canal in summer); and 3.246×10³ cellsL^-1-5.031×10⁵ cellsL^-1 (highest at Bogi in summer & lowest at Bogi in monsoon) respectively. In Rupsha-Pashur and Bhola-Baleswar similar seasonal trend was observed with lower density in monsoon and then rose rapidly through winter and summer. But in Khalpatua- Arpangachia phytoplankton density was low in monsoon; rose in winter and again dropped in summer. Annual average density was the highest (8.347×10⁴ cellsL^-1) in Bhola-Baleswar river system (Table 2).

Table 2: Annual mean values of phytoplankton species abundance in three major river systems.

Species diversity: Most diverse genera were Coscinodiscus with 12 species; Protoperidinium with 11 species; Thalassiosira with 10 species; Chaetoceros and Paleurosigma with 7 species. Maximun species diversity was observed in Khalpatua-Arpangachia during winter (Table 3).

Table 3: List of the most abundant phytoplankton genera found in the Sundarbans river-systems at different seasons.

Species diversity index; richness index and evenness index values showed spatial and temporal variation. Annual mean diversity index was highest (2.63 ± 0.49) in Khalpatua-Arpangachia. It ranged from 1.31to 4.64 (highest at Karamjol in winter & lowest at Karamjol in monsoon) in Rupsha-Pashur; 1.58to 3.85 (highest at Arpangachia in winter & lowest at Nildumur in winter) in Khalpatua-Arpangachia; and 0.66 to 4.27 (highest at Bogi in winter & lowest at Supati in summer) in Bhola-Baleswar. Richness values were found in the range of 1.21-4.99 in Rupsha-Pashur; 1.18-11.24 in Khalpatua-Arpangachia; and 1.19-5.12 in Bhola-Baleswar; and evenness index varied from 0.77 to 1.5 in Rupsha-Pashur; 0.82 to 0.97 in Khalpatua-Arpangachia; and 0.59 to1.21 in Bhola-Baleswar. All these index values showed no clear seasonal trend of variation (Table 2).

Throughout the study area over all three river system Bacillariophyta dominated the community composition. It was 81.82%-100% in Rupsha-Pashur; 64.63%-96.57% in Khalpatua-Arpangachia river system. But in Bhola-Baleswar river system Bacillariophyta dominated the composition in monsoon and winter (81.81%-98.48%); but during summer Cyanophyta dominated the composition (54.42%-78.76%) (Figure 2).

Figure 2: Spatial distribution of relative abundance of phytoplankton at different measuring stations of (a) Rupsha-Pashur, (b) Khalpatua-Arpangachia and (c) Bhola- Baleswar river system during monsoon, winter, and summer respectively.

Discussion

Phytoplankton community in the Sundarbans river systems showed with noted abundance and diversity with spatial and temporal variation to some extent. The observed differences are likely related to local forcing functions such as transparency; salinity and in some cases with nutrient concentrations variations in addition with some unknown reasons in this study.

Phytoplankton community in relation to water quality condition

Concentration of nitrate (NO₃^-); phosphate (PO₄^3-); ammonium (NH₄⁺); and silicate (SiO₄^4-) play important role as nutrient both for phytoplankton growth and production. These were estimated on a seasonal basis during the study. Results showed no uniform seasonal trend of all these nutrient elements. Nitrate (NO₃^-) followed a seasonal trend of lower concentration in monsoon and increased through winter and summer in Rupsha-Pashur; and Bhola-Baleswar. But in Khalpatua-Arpangagchia; it showed no clear seasonal trend. Phosphate didn´t show any seasonal pattern in Rupsha-Pashur; and Bhola-Baleswar; but in Khalpatua-Arpangachia it showed a trend of lower amount in monsoon then rose in winter and again dropped in summer. Only in Bhola-Baleswar a seasonal pattern of Ammonium (NH₄⁺) was observed with lower values in monsoon; and gradual higher values through winter and summer. In Rupsha-Pashur silicate (SiO₄^4-) showed a pattern of lower concentrations in monsoon and higher in winter and summer. But in Khalpatua-Arpangachia it was different; higher concentrations were found in winter and lower in monsoon and summer. This significant variation suggests the coexistence of different processes in water quality conditions. In marine coastal systems; there can be many sources of nutrients like upwelling; river input; sediment resuspension or remineralization;aquaculture effluents; and urban; agricultural; and industrial wastewater; thereby making it difficult to determine the relative contribution of nutrient sources to coastal water quality [23,24].

Concentration of Chlorophyll a (algal biomass) is normally used as an index of the productivity [25] and trophic condition of estuaries; coastal and oceanic waters [26-28]. It reflects the net result (standing stock) of both growth and loss processes. There is generally a good agreement between planktonic primary production and algal biomass. Algal biomass is associated with the visible symptoms of eutrophication. It is considered the principal variable to use as a trophic state indicator. Present study analyzed algal biomass production in terms of chlorophyll a concentration on seasonal basis. Throughout the study area highest biomass production (5.94 μgL^-1) was observed in the Bhola-Baleswar river system in summer. Rupsha-Pashur and Bhola- Baleswar river systems showed the similar trend of seasonal variation with lower concentrations in monsoon and gradually increased value through winter and summer. But in Khalpatua-Arpangachia river system concentrations were lower in monsoon; rose in winter; and again fell in summer. Growth and production of phytoplankton in an area is a function of environmental factors prevailed in that area [29]. By pearson correlation analysis (Figure 3) we found biomass production i.e. chlorophyll a was positively correlated with transparency; salinity; and nitrate (NO₃^-) in Rupsha-Pashur river system. In Khalpatua- Arpangachia it was positively correlated with transparency; salinity and silicate (SiO₄^4-); and negatively correlated with temperature and phosphate (PO₄^3-). But in Bhola-Baleswar positive significant correlation was observed with transparency; salinity; nitrate (NO₃^-); ammonium (NH₄⁺); and silicate (SiO₄^4-). That is; in all three river systems chlorophyll a didn´t show strong and uniform correlation with temperature and all nutrient elements.

Figure 3: Pearson correlation between phytoplankton biomass and different water quality parameters in (a) Rupsha-Pashur, (b) Khalpatua-Arpangachia, and (c) Bhola- Baleswar river systems (* indicate significant correlation at 0.05 level (2 tailed) & ** indicate significant correlation (2 tailed) at the 0.01 level).

Water temperature and transparency are most important among various physical factors affecting the distribution and seasonal variation of phytoplankton growth [30]. In the present study we have found significant positive correlation with transparency but no significant positive correlation with temperature. It may be due to the cross interaction between temperature and transparency. In summer temperature and transparency both are high and biomass production was also high; but in monsoon temperature was high but low transparency limited the light penetration and as a result limited the growth [31].

Phytoplankton abundance and biomass production both showed similar trend of seasonal variation in Rupsha-Pashur and Bhola- Baleswar river systems with highest production in summer but differed in Khalpatua-Arpangachia with lower production in summer. This noncompliance may be due to the late sampling time for the summer season in Khalpatua-Arpangachia (26^th May-2011); when early monsoonal impact (144mm rainfall in May-2011) has already started.

Silicate concentrations varied a great deal among the sites and seasons. It was found within the range of 3.201 mgL^-1-26.122 mgL^-1. These higher concentrations of SiO₄^4- may result from the shrimp-farm effluents and nutrient export from the harbor that enriched the coastal water with nutrient [32]. Often; SiO₄^4- acts as a limiting nutrient for diatom growth; and it could therefore control replacement of diatoms by dinoflagellates in conditions of Si deficiency; which means that SiO₄^4- can play an important role in phytoplankton community-structure changes [33]. High inputs of SiO₄^4- could also cause an imbalance of the normal phytoplanktonic communities by stimulating diatom growth; some of which may have harmful effects.

The salinity is also one of the main parameters that can be attributed to the phytoplankton diversity and acts as a limiting factor which influences the distribution of planktonic community [34-37]. Generally; changes in the salinity of the brackish water habitats such as estuaries; backwaters and mangrove are due to the influx of freshwater from land run off; caused by monsoon or by tidal variations. Presently recorded higher values in dry season could be attributed to the higher degree neritic water dominance from sea [38,39].

Differences among sites in dissolved oxygen concentrations; as well as seasonal changes within sites; result from the balance between physical (e.g. turbulence; diffusion and solubility of oxygen) and biogeochemical(e.g.; consumption; production; remineralization) processes [40]. In the study area comparatively higher values were observed in monsoon and lower in summer. In monsoon it may be due to precipitation and higher turbulence and in summer which could be a function of lower turbulence and warmer water temperatures that increase respiration and decrease solubility of oxygen.

Phytoplanktonic community composition as bioindicator

The phytoplankton community structures in coastal Sundarbans were determined as significantly dynamic with respect to spatial and temporal level. These differences in phytoplankton production may be related to a variety of environmental factors in aquatic environment [29].

The number of phytoplankton species identified in major three river systems of Sundarbans; reflecting the high species richness characteristics of tropical coastal areas [41]. In general diatoms dominate the whole area. But in Bhola-Baleswar Cyanophyta dominate the composition in summer. Dinoflagelates are common only in Khalpatua-Arpangachia. Chlorophytes are abundant in Bhola- Baleswar. These variations in species composition suggests; however hydrological connectivity exists among these river systems; it was local hydrological conditions that determined phytoplankton community structure at each site.

Silicate concentrations were much higher in the study area. It probably acted as a factor for stimulating diatom growth [33]. In Bhola- Baleswar community composition was dominated by Cyanophyta and in summer; when highest NO₃^- concentrations were observed among all sites. This NO₃^- enriched condition probably stimulated the growth of Cyanophyta [42]. This community composition indicated high nutrient status and the presence of toxic contaminants [43-45]. It also reflects the possibility of anthropogenic activities for the enrichment of nutrient [42].

Phytoplankton community structural changes are a good indicator of water quality or aquatic ecological status as they show complex and rapid responses to fluctuations of environmental conditions [46]. Skeletonema costatum is an abundant species in the study areas. It is a eurytharmal and euryhaline species. So its presence is an indicator of stress environmental condition. Thalassionema nitzschioides was also abundant throughout the area; and its presence indicates resuspension and remineralization processes happening there. Amphora sp. and Nitzschia sp. etc. are benthic diatoms. These were mostly abundant in Khalpatua-Arpangachia river system. Their presence indicates higher concentrations of SiO₄^4- and resuspension of sediments. Nitzschia longissima and Protoperidinium were also abundant throughout the area. These are opportunistic and indicator species of shrimp farm effluents [24,47].

Conclusion

The Sundarbans which environmental conditions are highly dynamic and variable in nature is rich in phytoplankton community. Variation both in abundance and diversity is due to rainfall; transparency; salinity and nutrient concentartions. Variable water quality conditions at spatial and temporal level indicates the influence of various climatic and local forcing functions. A relationship among local water quality characteristics with phytoplankton community is noticed. Favorable climatic condition and nutrient status of water led to a blooming condition in summer. The study will provide a number of information regarding water chemistry of the Sundarbans river systems and phytoplankton community structure in order to find factors responsible for biodiversity loss.

Acknowledgements

The study was supported by the Grants for Advanced Research in Science from the Ministry of Education (MoE) of Bangladesh. The authors are thankful to the laboratory staff of Fisheries & Marine Resource Technology Discipline and Environmental Science Discipline of Khulna University for their cooperation during sample analysis. Thanks are due to the graduate and undergraduate students for their generous cooperation during field visit and laboratory analysis. The writers acknowledge the support given by the authorities of Forest Department of Bangladesh for providing necessary support to carry out field observation and sampling in the Sundarbans Mangrove Forest. The writers would also like to thank the many others who have been involved in the field observations.

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