Mangroves: A Unique Gift of Nature

Abstract

Mangroves are salt-tolerant forest ecosystems found mainly in the tropical and subtropical intertidal regions of the world. They encompass swamps, forestland within, and the surrounding water bodies. It is a matter of great surprise that mangrove floral species can thrive luxuriantly in saline habitat (which is basically physiologically dry in nature) through orientation of their morphological, anatomical and physiological systems. Thus, this vegetation is the most efficiently adapted biotic community in response to climate-change-induced sea-level rise.

Mother Earth has created several gifts in the ecosystem vaults, which need to be priced properly. A free gift invites destruction, degradation, spoilage and conflict.

The Author

Appendices

Annexure 2A.1: Spatial Variation of Stored Carbon in Avicennia alba of Indian Sundarbans

• Kasturi Sengupta,
• Mahua Roy Chowdhury,
• G. Roychowdhury,
• Atanu Kumar Raha,
• Sufia Zaman &
• Abhijit Mitra 

1. Department of Marine Science, University of Calcutta, 35, B.C Road, Kolkata, West Bengal, 700 019, India

   Kasturi Sengupta, Mahua Roy Chowdhury & Sufia Zaman

2. Techno India University, Salt Lake Campus, Kolkata, West Bengal, 700 091, India

   G. Roychowdhury (Chancellor)

3. Department of Environment & Forest Science Techno India University, Salt Lake Campus, Kolkata, West Bengal, 700 091, India

   Atanu Kumar Raha

4. Department of Oceanography, Techno India University, Salt Lake Campus, Kolkata, West Bengal, 700 091, India

   Abhijit Mitra

Abstract

We evaluated stored carbon in the above-ground biomass (AGB), below-ground biomass (BGB) and total biomass (TB) of 12-year-old trees of Avicennia alba in the western, central and eastern sectors of Indian Sundarbans during premonsoon (June) of 2012. We also analyzed the soil organic carbon (SOC) simultaneously in these sectors with the aim to find the interrelationships between AGB, BGB, TB and SOC, which exhibited significant spatial variations (p < 0.05). In all the three sectors, significant positive correlations (p < 0.01) were observed between the mangrove carbon (both above-ground carbon or AGC and below-ground root carbon or BGC) and SOC indicating considerable contributions of stem, branch, leaf and roots of A. alba to SOC.

Keywords

Above-ground biomass • Below-ground biomass • Total biomass • Soil organic carbon • Indian Sundarbans

2.1.1 2A.1.1 Introduction

Forests form a major component of the carbon reserves in the world’s ecosystems (Whittaker and Likens 1975) and greatly influence the lives of other organisms as well as human societies. Tropical forests are an important compartment in the global carbon cycle and represent 30–40 % of the terrestrial net primary production (Clark et al. 2001). Although the area covered by mangrove forest represents only a small fraction of the tropical forests, their position at the terrestrial–ocean interface and potential exchange of nutrients with coastal water suggest that these forests make a unique contribution to carbon biogeochemistry in coastal oceans (Twilley et al. 1992). Mangroves are a taxonomically diverse group of salt-tolerant, mainly arboreal, flowering plants that grow primarily in tropical and subtropical regions (Ellison and Stoddart 1991). Estimates of mangrove area vary from several million hectares (ha) to 15 million ha worldwide (FAO 1981). The most recent estimates suggest that mangroves presently occupy about 14,653,000 ha of tropical and subtropical coastline (Wilkie and Fortuna 2003). The coastal zone (<200 m depth), covering ~7 % of the ocean surface (Gattuso et al. 1998), has an important role in the oceanic carbon cycle, and various estimates indicate that the majority of mineralization and burial of organic carbon, as well as carbonate production and accumulation, take place in the coastal ocean (Gattuso et al. 1998; Mackenzie et al. 2004). The ‘outwelling’ hypothesis, first proposed for mangroves by Odum (1968) and Odum and Heald (1972), suggests that a large fraction of the organic matter produced by mangrove trees is exported to the coastal ocean, where it forms the basis of a detritus food chain and thereby supports coastal fisheries. Despite the large number of case studies dealing with various aspects of organic matter cycling in mangrove systems (Kristensen et al. 2008), there is very limited consensus on the carbon sequestering potential of mangroves.

The present study aims to establish a baseline databank on the carbon storage by above-ground and below-ground structures of a dominant and most common mangrove species A. alba in the Indian Sundarbans along with soil organic carbon (SOC) in order to critically address two important issues:

• 1. Whether spatial difference in salinity causes variation in carbon storage capacity of the mangrove tree

• 2. Whether any interrelationships exist between mangrove biomass, mangrove carbon and soil carbon

2.1.2 2A.1.2 Material and Methods

2.1.2.1 2A.1.2.1 Study Site Description

The mighty River Ganga emerges from a glacier at Gangotri, about 7,010 m above mean sea level in the Himalayas, and flows down to the Bay of Bengal covering a distance of 2,525 km. At the apex of Bay of Bengal, a delta has been formed which is recognized as one of the most diversified and productive ecosystems of the tropics and is referred to as the Indian Sundarbans. The deltaic complex has an area of 9,630 km² and houses about 102 islands (Mitra 2000). Eighteen sampling sites were selected each in the western, central and eastern sectors of Indian Sundarbans (Fig. 2A.1.1). The western sector of the deltaic lobe receives the snowmelt water of mighty Himalayan glaciers after being regulated through several barrages on the way. The central sector, on the other hand, is fully deprived from such supply due to heavy siltation and clogging of the Bidyadhari channel since the fifteenth century (Chaudhuri and Choudhury 1994). The eastern sector of Indian Sundarbans is adjacent to the Bangladesh Sundarbans (which comprises 62 % of the total Sundarbans) and receives the freshwater from the River Raimangal and also from the Padma–Meghna–Brahmaputra river system of Bangladesh Sundarbans through several creeks and inlets. Samplings in these sectors were carried out in low tide period during June 2012.

Fig. 2A.1.1

Location of sampling stations of Indian Sundarbans

In each site, selected forest patches were ~12 years old. Fifteen sample plots (10 m × 10 m) were established (in the river bank) through random sampling in the various qualitatively classified biomass levels for each site. The population density of A. alba was evaluated to estimate the magnitude of stored carbon by the species in a particular site.

AGB of individual trees of the species in each plot was estimated, and the average values of 15 plots from each site were finally converted into biomass (t/ha) in the study area.

AGB is the sum total of stem, branch and leaves of the tree. Hence, the biomass of the vegetative parts was estimated separately.

2.1.2.2 2A.1.2.2 Above-Ground Stem Biomass (AGSB) Estimation

The stem volume for each tree of the species in every plot was estimated using the Newton’s formula (Husch et al. 1982) as per the expression:

$$ V=\frac{h}{6\left({A}_{\text{b}}+4{A}_{\text{m}}+{A}_{\text{t}}\right)}$$

where V is the volume (in m³), h the height measured with laser beam (BOSCH DLE 70 Professional model) and A _b, A _m and A _t are the areas of the selected tree at base, middle and top, respectively. Specific gravity (G) of the wood was estimated taking the stem cores, which was further converted into stem biomass (B _S) as per the expression B _S = GV. The stem biomass of individual tree was finally multiplied with the number of trees of the species in 15 selected plots for every site in the western, central and eastern sectors of Indian Sundarbans.

2.1.2.3 2A.1.2.3 Above-Ground Branch Biomass (AGBB) Estimation

The total number of branches irrespective of size was counted on each of the sample trees. These branches were categorized on the basis of basal diameter into three groups, namely, <6, 6–10 and >10 cm. Fresh weight of two branches from each size group was recorded separately using the equation of Chidumaya (1990).

Total branch biomass (dry weight) of individual tree was determined as per the expression:

$$ {B}_{\text{db}}={n}_{1}{\text{bw}}_{1}+{n}_{2}{\text{bw}}_{2}+{n}_{3}{\text{bw}}_{3}=\Sigma {n}_{i}{\text{bw}}_{i}$$

where B _db is the dry branch biomass per tree, n _i the number of branches in the ith branch group, bw _i the average weight of branches in the ith group and i = 1, 2, 3, …, n are the branch groups. The branch biomass of individual tree was finally multiplied with the number of trees of the species in all the 15 plots for each site.

2.1.2.4 2A.1.2.4 Above-Ground Leaf Biomass (AGLB) Estimation

Leaves from nine branches (three of each size group) of individual trees were plucked, weighed and oven-dried separately to a constant weight at 80 ± 5 °C. Three trees per plot were considered for estimation. The leaf biomass was then estimated by multiplying the average biomass of the leaves per branch with the number of branches in a single tree and the average number of trees per plot as per the expression:

$$ {L}_{\text{db}}={n}_{1}{\text{Lw}}_{1}{N}_{1}+{n}_{2}{\text{Lw}}_{2}{N}_{2}+\cdots {n}_{i}{\text{Lw}}_{i}{N}_{i}$$

where L _db is the dry leaf biomass of selected mangrove species per plot, n ₁ … n _i are the number of branches of each tree of the species, Lw₁ … Lw _i are the average dry weight of leaves removed from the branches and N ₁ … N _i are the number of trees of the species in the plots.

2.1.2.5 2A.1.2.5 Below-Ground Biomass (BGB) Estimation

An excavation method (Bledsoe et al. 1999) was used to estimate root biomass of the same trees that were selected for above-ground biomass (AGB) estimate. According to our observation, very few roots in our sampling plots were distributed deeper than 1 m in sediments. We also found canopy diameter of these trees was usually smaller than 2 m. Most roots of the selected species were distributed within the projected canopy zone. Therefore, for below-ground biomass (BGB, referring to root biomass in this study), we excavated all roots (of two trees/species) in 1 m depth within the radius of 1 m from the tree centre and then washed the roots. We excavated all the sediments within the sampling cylinder (2 m in diameter × 1 m in height) and washed them with a fine screen to collect all roots. The roots were sorted into four size classes: extreme fine roots (diameter < 0.2 cm), fine roots (diameter 0.2–0.5 cm), small roots (diameter 0.5–1.0 cm) and coarse roots (diameter > 1 cm). We did not separate live or dead roots. The roots after thorough washing were oven-dried to a constant weight at 80 ± 5 °C and biomass was estimated for each species.

2.1.2.6 2A.1.2.6 Carbon Estimation

Direct estimation of per cent carbon in the AGB was done by Vario MACRO elementar CHN analyzer, after grinding and random mixing the oven-dried stems, branches and leaves separately. For this, a portion of fresh sample of stem, branch and leaf from trees were oven-dried at 70 °C, randomly mixed separately and ground to pass through a 0.5 mm screen (1.0 mm screen for leaves). Carbon content of the oven-dried root system (BGB) was also estimated directly in the same instrument.

2.1.2.7 2A.1.2.7 Soil Organic Carbon (SOC) Estimation

Soil samples from the upper 5 cm were collected from all the 15 plots at each site and dried at 60 °C for 48 h. For analysis, visible plant particles were handpicked and removed from the soil. After sieving the soil through a 2-mm sieve, the samples of the bulk soil (50 g from each plot) were ground finely in a ball mill. The fine-dried sample was randomly mixed to get a representative picture of the study site. Modified version of Walkley and Black method (1934) was then followed to determine the organic carbon of the soil in %.

2.1.2.8 2A.1.2.8 Statistical Analysis

Data of the present study generated from 18 sampling sites were subject to sector-wise analysis of correlation coefficient (r) in order to evaluate the interrelationships between mangrove biomass, mangrove carbon and soil organic carbon. Analysis of variance (ANOVA) was performed to assess whether biomass, tree carbon and soil organic carbon content varied significantly between sectors; possibilities (p < 0.01) were considered statistically significant. All statistical calculations were performed with SPSS 9.0 for Windows.

2.1.3 2A.1.3 Results and Discussion

The recent thrust on global warming phenomenon has generated tremendous interest in the carbon-storing ability of mangroves. The carbon sequestration in this unique producer community is a function of biomass production capacity, which in turn depends upon interaction between edaphic, climate and topographic factors of an area. Hence, results obtained at one place may not be applicable to another. Therefore, region-based potential of different land types or substratum characteristics needs to be worked out. In the present study, spatial variations of stored carbon have been worked out separately for AGB, BGB and SOC, and statistical tools were employed to evaluate the significance of variation.

2.1.3.1 2A.1.3.1 Relative Abundance

The relative abundance of A. alba was highest in the eastern sector (45.73 %), followed by western sector (30.65 %) and central sector (23.62 %).The lowest relative abundance may be attributed to the hypersaline condition in the central sector, where the freshwater supply has been almost disconnected due to siltation of the Bidyadhari River since the late fifteenth century as well as upcoming of densely populated habitations and agricultural activities after reclamation of wetland in the upper catchment of the estuary.

2.1.3.2 2A.1.3.2 Above-Ground Biomass (AGB)

The AGB of the mangrove species was relatively higher in the sampling sites of the eastern sector compared to the western and central sectors.

In the present study, the AGB ranged from 50.75 t/ha (W₃) to 60.85 t/ha (W₆) in the western sector, 31.55 t/ha (C₅) to 58.75 t/ha (C₄) in the central sector and 48.25 t/ha (E₆) to 66.58 t/ha (E₁) in the eastern sector of the Indian Sundarbans. The AGB varied as per the order eastern sector (59.28 t/ha) > western sector (55.02 t/ha) > central sector (46.99 t/ha). ANOVA results also confirm significant spatial variations in AGB (p < 0.05) (Table 2A.1.1 and Fig. 2A.1.2).

Table 2A.1.1 Spatial variations of AGB, BGB, TB and AGC, BGC, TC of Avicennia alba and SOC of Indian Sundarbans

Fig. 2A.1.2

Spatial variations of biomass (t/ha) in western, central and eastern sectors of Indian Sundarbans

2.1.3.3 2A.1.3.3 Below-Ground Biomass (BGB)

The order of BGB in the study area is eastern sector (17.67 t/ha) > western sector (16.08 t/ha) > central sector (13.12 t/ha). In this study, the BGB ranged from 13.95 t/ha (W₃) to 17.85 t/ha (W₆) in the western sector, 8.27 t/ha (C₅) to 17.23 t/ha (C₄) in the central sector and 13.11 t/ha (E₆) to 20.64 t/ha (E₁) in the eastern sector in the Indian Sundarbans. These results are in accordance with the ANOVA that exhibit significant spatial variation in BGB (p < 0.05) (Table 2A.1.1 and Fig. 2A.1.2).

2.1.3.4 2A.1.3.4 Total Biomass

In this study the TB ranged from 64.70 t/ha (W₃) to 78.69 t/ha (W₆) in the western sector, 39.82 t/ha (C₅) to 75.98 t/ha (C₄) in the central sector and 61.36 t/ha (E₆) to 87.22 t/ha (E₁) in the eastern sector of Indian Sundarbans. The TB varied as per the order eastern sector (76.95 t/ha) > western sector (71.10 t/ha) > central sector (60.11 t/ha). ANOVA results confirm significant spatial variation in TB of A. alba between western, central and eastern sectors (p < 0.05) (Table 2A.1.1 and Fig. 2A.1.2).

2.1.3.5 2A.1.3.5 Stored Carbon in A. alba Biomass

In this study, the stored carbon in AGB of the selected species ranged from 23.08 t/ha (W₃) to 27.45 t/ha (W₆) in the western sector, 14.43 t/ha (C₅) to 26.60 t/ha (C₄) in the central sector and 22.24 t/ha (E₆) to 30.22 t/ha (E₁) in the eastern sector. Stored carbon in below-ground biomass ranged from 6.53 t/ha (W₃) to 8.34 t/ha (W₆) in the western sector, 3.99 t/ha (C₅) to 8.34 t/ha (C₄) in the central sector and 6.42 t/ha (E₆) to 10.13 t/ha (E₁) in the eastern sector. The total stored carbon in A. alba thus followed the order eastern sector > western sector > central sector. ANOVA results indicate significant spatial differences in carbon content in AGB, BGB and TB (Table 2A.1.1 and Fig. 2A.1.3).

Fig. 2A.1.3

Spatial variations of stored carbon (t/ha) in biomass of species in western, central and eastern sectors of Indian Sundarbans

2.1.3.6 2A.1.3.6 Soil Organic Carbon

The order of SOC in the study area is eastern sector (1.41 %) > western sector (1.18 %) > central sector (1.05 %). The SOC ranged from 1.02 % (W₃ and W₄) to 1.61 % (W₆) in the western sector, 0.79 % (C₅) to 1.30 % (C₄) in the central sector and 1.15 % (E₆) to 1.66 % (E₁) in the eastern sector of Indian Sundarbans. Significant spatial variations (p < 0.05) were observed in SOC of deltaic Sundarbans (Table 2A.1.1 and Fig. 2A.1.4).

Fig. 2A.1.4

Spatial variations of SOC (%) in western, central and eastern sector of Indian Sundarbans

It is observed that AGB, BGB and TB exhibit significant spatial variations with highest value in the eastern sector followed by the western and central sectors. Similar trend is also observed in case of stored carbon. Two plausible reasons behind such spatial variations may be attributed to aquatic salinity and anthropogenic influence.

A long-term study conducted on salinity variation by several researchers in the Indian Sundarbans exhibits three distinct regimes in terms of salinity. The western sector is relatively low saline owing to freshwater discharge from the Farakka barrage. Ten-year surveys (1999–2008) on water discharge from Farakka dam revealed an average discharge of (3.7 ± 1.15) × 10³ m³/s. Higher discharge values were observed during the monsoon with an average of (3.81 ± 1.23) × 10³ m³/s, and the maximum of the order 4,524 m³/s during freshet (September). Considerably lower discharge values were recorded during premonsoon with an average of (1.18 ± 0.08) × 10³ m³/s, and the minimum of the order 846 m³/s during May. During postmonsoon discharge, values were moderate with an average of (1.98 ± 0.97) × 10³ m³/s.

The central sector, on contrary, exhibits hypersaline condition and an increasing trend in salinity through time. This is due to complete siltation of the Bidyadhari channel since the fifteenth century that has almost cut off the freshwater supply from the upstream region (Chaudhuri and Choudhury 1994). The eastern sector of Indian Sundarbans adjacent to Bangladesh Sundarbans receives freshwater from several channels and creeks from the Padma–Meghna–Brahmaputra river system and their tributaries.

The mangroves are salt-tolerant species, but under hypersaline condition, they exhibit stunted growth (Mitra et al. 2004). The order of biomass and stored carbon in A. alba in the present study is basically the reflection of salinity, which is highest, and exhibits an increasing trend in the central sector of Indian Sundarbans compared to the other two sectors. In the western and eastern sectors, the salinity decreased by 21.91 and 16.35 %, respectively, over a period of 24 years, whereas in the central sector there has been a steady increase in the salinity by 9.32 % during the same period (Fig. 2A.1.5).

Fig. 2A.1.5

Spatio-temporal variations of surface water salinity (in psu) in Indian Sundarbans

The degree of anthropogenic stress is also less in the eastern Indian Sundarbans compared to western and central sectors. This is because of the location of eastern sector in the Reserve Forest area, where human entry is highly restricted. This causes more natural growth rate of mangroves in the eastern sector compared to the western and central sectors, where the mangroves are mostly cut down for timber, fuel, etc.

The SOC in the mangrove ecosystem is contributed by the vegetative and reproductive parts of the halophytes, although the contributions of riverine inputs pose a regulatory effect on the SOC budget of the mangrove soil (Banerjee et al. 2012). The present study reveals significant contribution of AGB and BGB of A. alba in the matrix of SOC (Table 2A.1.3). The stored carbon in AGB (24.96 % in the western sector, 21.22 % in the central sector and 26.82 % in the eastern sector) and BGB (7.51 % in the western sector, 6.35 % in the central sector and 8.67 % in the eastern sector) are the main players in regulating the SOC in the intertidal mudflats of Indian Sundarbans. The significantly high correlation coefficient values of SOC with AGB, AGC, BGB, BGC, TB and TC of A. alba support the dependency of SOC on mangrove biomass and carbon (Tables 2A.1.2 and 2A.1.3).

Table 2A.1.2 Interrelationships between biomass and SOC in western, central and eastern sectors of Indian Sundarbans

Table 2A.1.3 Interrelationships between stored carbon content and SOC in western, central and eastern sectors of Indian Sundarbans

The overall discussion thus explains a congenial environment for the growth of A. alba in eastern Indian Sundarbans and also indicates the necessity of freshwater supply to accelerate the biomass and subsequently the stored carbon in A. alba thriving in the Indian Sundarbans region in the lower Gangetic delta complex.

2.1.4 2A.1.4 Conclusion

This study has demonstrated that dwarfing of the mangrove species A. alba in the central Indian Sundarbans is a complex phenomenon influenced by a variety of hydro-edaphic conditions. Lack of freshwater supply from the upper catchments coupled with tidal influence from the Bay of Bengal has increased the salinity of central Indian Sundarbans over a period of time. The conditions in the western and eastern Indian Sundarbans are comparatively congenial for the growth of the mangrove species due to freshwater supply from the upstream region. These findings tend to support several other researches that point towards the necessity of dilution of saline water for proper growth and survival of mangroves. Additional research is needed to test and further define these conclusions.

Annexure 2A.2: Influence of Anthropogenic and Natural Factors on the Mangrove Soil of Indian Sundarbans Wetland

• Kakoli Banerjee,
• Mahua Roy Chowdhury,
• Kasturi Sengupta,
• Saurov Sett &
• Abhijit Mitra 

1. Department of Marine Science, University of Calcutta, 35, B.C. Road, Kolkata, West Bengal, 700 019, India

   Kakoli Banerjee, Mahua Roy Chowdhury, Kasturi Sengupta & Saurov Sett

2. Department of Oceanography, Techno India University, Salt Lake Campus, Kolkata, West Bengal, 700 091, India

   Abhijit Mitra

Abstract

Soil organic carbon, pH and salinity were monitored in mangrove ecosystem of Indian Sundarbans in five successive years (2006–2010). Samplings were carried out at 14 stations in four different depths (0.01–0.10, 0.10–0.20, 0.20–0.30 and 0.30–0.40 m) during premonsoon period. High organic carbon load is observed in the stations of western Indian Sundarbans (mean = 1.02 wt%) which are near to the highly urbanized city of Kolkata. The central and eastern sectors under the protected forest area show comparatively less soil organic carbon (mean = 0.64 wt%). A unique spatial variability in soil salinity and pH was observed with lower values in the western and eastern sectors compared to central sector. Soil pH exhibited a lower value (7.47 ± 0.071) in reserve forest zone (central and eastern sectors) compared to western sector (7.57 ± 0.067). The soil salinity increased with depth, while organic carbon and pH decreased with depth in all the stations. The paper depicts the increase of soil organic carbon and pH due to anthropogenic activities in western Indian Sundarbans, which if continued may decrease the potential of Sundarban soil as carbon sink and make the soil highly saline. Hence, curbing of anthropogenic activities may keep the soil characteristics ecologically safe.

Keywords

Indian Sundarbans • Soil organic carbon • Soil pH • Soil salinity

2.2.1 2A.2.1 Introduction

In mangrove ecosystem, organic carbon usually originates via the riverine introduction of pollutants, including industrial and domestic wastes; agricultural, aquacultural and mining run-off; accidental spillages; and decomposition of debris from marine organisms. However, different factors may control the partitioning and also the bioavailability of the organic compounds within the benthic ecosystem. These factors include sediment characteristics, such as grain size distribution, mineral composition and organic content (Lambert 1967; Forstner 1977; Khalaf et al. 1981). Surface sediments may be resuspended and redistributed by the action of waves and currents (Cahoon and Reed 1995). As these phenomena trigger the process of erosion and accretion, therefore, the top most layers of the sediments contain recently deposited organic matter. Total organic carbon has a major influence on both chemical and biological processes that take place in sediments (Kamaruzzaman et al. 2010). The amount of organic carbon has a direct role in determining the redox potential and pH in sediment, thus regulating the behaviour of other chemical species such as metals (Eshleman and Hemond 1985; Kerekes et al. 1986). Natural processes and human activities have resulted in elevated content of total organic carbon in mangrove soils and adjacent estuaries and creeks. These include diverse input through fall, stream flow, inappropriate animal waste applications and disposals, forest clearance, agricultural practices and changes in land uses (Moore and Jackson 1989). Also mangrove litter fall and decomposition of organisms regulate the organic carbon budget in the intertidal mudflats (Mitra et al. 2004).

The mangrove ecosystem of Indian Sundarbans, at the apex of Bay of Bengal, covers an area of about 4,266.6 km². On the basis of satellite imagery, the Forest Survey of India (1999) estimated the area of Indian Sundarbans as 2,125 km², excluding the network of creeks and backwaters, which are the vital matrix of mangrove ecosystem. Mangrove communities often exhibit distinct patterns of species distribution (Chapman 1976; Lugo and Snedaker 1974; Macnae 1968; Tomlinson 1986) that contribute to the organic carbon level in the intertidal soil through decomposition of litter and organisms. Since the mangrove habitat is basically saline, several studies have attempted to correlate salinity with the standing crop of vegetation and productivity (Chen and Twilley 1998, 1999; Lugo 1980; Mall et al. 1987; Ukpong 1991; Mitra et al. 2011a). Sundarbans shelters one of the most important mangrove communities of the world. A few published works deal with the community structure of this forest (Joshi and Ghose 2002; Matilal et al. 1986). However, very few reports are available on the organic carbon profile of mangrove soil (Mitra et al. 2004) that can reflect the status of this unique ecosystem in terms of natural (Mitra and Banerjee 2005) or anthropogenic influences (Mitra 1998, 2000; Mitra et al. 2011b). The aim of this paper is to determine what role the anthropogenic and natural factors have on mangrove soil and how the soil characteristics change over time.

2.2.2 2A.2.2 Materials and Methods

2.2.2.1 2A.2.2.1 The Study Area

The Indian Sundarbans (between 21°13′ N and 22°40′ N latitude and 88°03′ E and 89°07′ E longitude) is bordered by Bangladesh in the east, the Hugli River (a continuation of the River Ganga) in the west, the Dampier and Hodges line in the north and the Bay of Bengal in the south. The temperature is moderate due to its proximity to the Bay of Bengal in the south. Average annual maximum temperature is around 35 °C. The summer (premonsoon) extends from the middle of March to mid-June and the winter (postmonsoon) from mid-November to February. The monsoon usually sets in around the middle of June and lasts up to the middle of October. Rough weather with frequent cyclonic depressions occurs during mid-March to mid-September. Average annual rainfall is 1920 mm. Average humidity is about 82 % and is more or less uniform throughout the year. Thirty-four true mangrove species and some 62 mangrove associate species have been documented in Indian Sundarbans (Mitra 2000). The ecosystem is extremely prone to erosion, accretion, tidal surges and several natural disasters (Mitra and Banerjee 2005), which directly affect the topsoil of the intertidal mudflats encircling the islands. The average tidal amplitude is around 3.0 m. Some sea-facing islands experience high tidal amplitude (~5.0 m).

We conducted survey at 14 stations in the Indian Sundarbans region during premonsoon (May) from 2006 to 2010. Station selection was primarily based on anthropogenic activities, salinity and mangrove floral richness. The western Indian Sundarbans is a stressed zone (stations 1–7). On the contrary, stations 8–14 are within the reserve forest areas with luxuriant mangrove vegetation and diversity and have been considered as control zone in this study. The major activities influencing the nature of soil in the selected stations are highlighted in Table 2A.2.1.

Table 2A.2.1 Major activities influencing the organic carbon pool in Indian Sundarbans

2.2.2.2 2A.2.2.2 Sampling and Analysis

Table 2A.2.1 and Fig. 2A.2.1 represent the study site in which sampling plots of 10 m × 5 m were considered for each station. Care was taken to collect the samples within the same distance from the estuarine edge, tidal creeks and the same micro-topography. Under such conditions, spatial variability of external parameters such as tidal amplitude and frequency of inundation (Ovalle et al. 1990), inputs of material from the adjacent bay/estuary and soil granulometry and salinity (Lacerda et al. 1993) are minimal.

Fig. 2A.2.1

Map of the study region showing the sampling stations. R1–R7 are the seven rivers of Sundarbans starting from west to east, namely, Hugli, Mooriganga, Saptamukhi, Thakuran, Matla, Gosaba and Harinbhanga

Ten cores were collected from the selected plots in each station by inserting PVC core of known volume into the soil to a maximum depth of 0.40 m during low tide condition. Each core was sliced into four equal parts, placed in aluminium foil and packed in ice for transport. In the laboratory, the collected samples were carefully sieved and homogenized to remove roots and other plant and animal debris prior to oven-drying to constant weight at 105 °C. Total organic carbon was analyzed by rapid dichromate oxidation method of Walkley and Black (1934).

Measurement of soil pH was done with fresh samples in the field with a Systronics pH Meter with glass–calomel electrode (sensitivity ± 0.01) and standardized with buffer 7.0 to avoid oxidation of iron pyrites (a common constituent of mangrove soils) to sulphuric acid (English et al. 1997). Soil salinity was determined in supernatant of 1:5 soil–water mixtures using a refractometer.

2.2.3 2A.2.3 Results

2.2.3.1 2A.2.3.1 Soil Organic Carbon

The soil organic carbon differs significantly between stations and years considering the mean values (0.99 ± 0.07) of all four depths (Fig. 2A.2.2). We observe relatively higher values of organic carbon in the stations of anthropogenically stressed western sector (Stn. 1–7) compared to those in the central (Stn. 8–11) and eastern sectors (Stn. 12–14) of Indian Sundarbans that encompass mainly the reserve forest with almost no human intrusion. The mean value of soil organic carbon in the western sector (stressed zone) is 1.02 wt%. In the central and eastern sectors (control zone), the value is 0.64 wt%. In all the selected stations, the soil organic carbon content decreases with depth (Fig. 2A.2.3). The gradual increase of organic carbon (composite figure of four depths) through years in all the stations clearly reflects the efficacy of Sundarban mangrove as potential sink of carbon. In western Indian Sundarbans, the rate of increase is 0.031 % per year, whereas in the stations of central and eastern sectors that are mostly within the reserve forest, the value is 0.026 % per year.

Fig. 2A.2.2

Spatial and temporal variations of soil organic carbon (wt%) during premonsoon 2006–2010 (composite value of the depths)

Fig. 2A.2.3

Yearly variations of soil organic carbon (wt%) with depth in the selected stations

2.2.3.2 2A.2.3.2 Soil pH

Soil pH decreases significantly at those sites which fringe the salt-marsh grass (Porteresia coarctata) bed and sustain rich mangrove vegetation particularly in the reserved forest area (Stn. 8–14). The average soil pH in this zone (considering all depths and years) is 7.47 ± 0.071. In the western sector of Indian Sundarbans (Stn. 1–7), comparatively higher soil pH is observed (average pH value of all four depths considering 5 successive years = 7.57 ± 0.067). We also observe a decreasing trend in soil pH with depth in all the stations (Fig. 2A.2.4). The uppermost layers are alkaline and slightly acidic soil is observed within the depth of 0.20–0.40 m mainly in the stations 8–14. Significant yearly variations of soil pH are observed in the present study area (Fig. 2A.2.5). In stations 1–7, the mean values (of all depths) are 7.67 ± 0.23, 7.60 ± 0.21, 7.55 ± 0.21, 7.53 ± 0.21 and 7.49 ± 0.19 in 2006, 2007, 2008, 2009 and 2010, respectively. In stations 8–14, the values are 7.57 ± 0.23, 7.51 ± 0.21, 7.43 ± 0.16, 7.44 ± 0.21 and 7.40 ± 0.19 in 2006, 2007, 2008, 2009 and 2010, respectively.

Fig. 2A.2.4

Spatial and temporal variations of soil pH during premonsoon 2006–2010 (composite value of the depths)

Fig. 2A.2.5

Temporal variations of soil pH with depth in the selected stations

2.2.3.3 2A.2.3.3 Soil Salinity

The soil salinity exhibits a unique spatial and temporal trend (Fig. 2A.2.6). In the western Indian Sundarbans (Stn. 1–7), the values are relatively lower (mean value of all depths and years = 9.75 psu), while in stations 8–11, adjacent to Matla River in the central sector of Indian Sundarbans, the values are relatively higher (mean value of all depths and years = 13.85 psu). Interestingly, in the eastern Indian Sundarbans encompassing stations 12–14 (adjacent to Bangladesh Sundarbans), the soil salinity again decreases significantly (mean value considering all depths and years = 6.98 psu). An apparent increase in soil salinity with depth is observed in all the stations (Fig. 2A.2.7). It is observed that the soil salinity exhibits a decreasing trend with years in stations 1–7 (10.92, 10.14, 9.80, 9.16 and 8.71 psu in 2006, 2007, 2008, 2009 and 2010, respectively) and stations 12–14 (7.70, 7.35, 7.04, 6.64 and 6.18 psu in 2006, 2007, 2008, 2009 and 2010, respectively), but the values increase in stations 8–11 (13.32, 13.52, 13.64, 14.26 and 14.49 psu in 2006, 2007, 2008, 2009 and 2010, respectively).

Fig. 2A.2.6

Spatial and temporal variations of soil salinity (psu) during premonsoon 2006–2010 (composite value of the depths)

Fig. 2A.2.7

Yearly variations of soil salinity (psu) with depth in the selected stations

2.2.4 2A.2.4 Discussion

2.2.4.1 2A.2.4.1 Soil Organic Carbon

The significant variation (p < 0.0001) of soil organic carbon between anthropogenically stressed western zone and non-disturbed central and eastern zones may be attributed to a large extent by human activities, mangrove floral richness and physical factors like accretion and erosion. Anthropogenic activities like fish landing, tourism and unplanned urban development and shrimp farms contribute appreciable amount of organic load in stations like Kachuberia (Stn. 1) and Frazergaunge (Stn. 5). The presence of shrimp farms at Chemaguri (Stn. 6) along with a 12-year-old mangrove vegetation (17 species) may be attributed to highest organic carbon level in the soil core. The western Indian Sundarbans (encompassing stations 1–7) is under severe stress due to intense industrialization, rapid urbanization and unplanned tourism and aquaculture activities (Mitra et al. 1994; Mitra 1998) which contribute appreciable organic carbon in the soil. The relatively low organic carbon at Sagar South (Stn. 3) is due to its location at seafront where wave action and tidal amplitude are maximum (range 3.0–5.0 m and mean = 3.5 m). Continuous erosion of this island may be the reason behind minimum retention of organic matter in the intertidal zone. The low organic carbon at stations 8–14 confirms the anthropogenic origin of organic load, which is almost absent in these stations (control zone). Being located within the reserve forest area, stations 8–14 receive complete legal protection, and hence, the major source of organic carbon in this zone is primarily the mangrove detritus. The variation of organic carbon in the Indian Sundarbans is thus regulated through an intricate interaction of biological, physical and anthropogenic activities (Table 2A.2.1).

The decrease in soil organic carbon with increased depth (p < 0.0001) is in accordance with the findings of Lacerda et al. (1995), where the organic carbon levels under Rhizophora mangle soil were 2.80, 2.70 and 2.70 % in the 0.01–0.05, 0.05–0.10 and 0.10–0.15 m depth, respectively. Similar trend was also observed under Avicennia soil (Lacerda et al. 1995). Report of decreasing mangrove soil organic carbon below 1 m is presented by Donato et al. (2011). The factor governing variation of below-ground carbon storage in mangrove soils is difficult to pinpoint (Bouillon et al. 2009; Alongi 2008) as it is not a simple function of measured flux rates but also integrates thousands of years of variable deposition, transformation and erosion dynamics associated with fluctuating sea levels and episodic disturbances (Chmura et al. 2003).

The present study is significant from the point that the area has not yet witnessed the light of documentation of soil carbon content although AGMB and carbon storage have been studied by several workers (Mitra et al. 2011a). Donato et al. (2011) quantified whole-ecosystem C storage in mangroves across a broad tract of the Indo-Pacific region, which includes the Bangladesh Sundarbans. The study however did not cover the lower Gangetic soil sustaining 38 % of the total Sundarbans in the Indian part. The present approach is thus an attempt to fill this gap area and establish a continuous 5-year baseline data of soil carbon in the mangrove-dominated Indian part of Sundarbans.

2.2.4.2 2A.2.4.2 Soil pH

The acidity of the soil influences the chemical transformation of most nutrients and their availability to plants. Most mangrove soils are well buffered, having a pH in the range of 6–7, but some have a pH as low as 5 (Clarke 1985). In this study, soil pH (7.47 ± 0.071) is lower in the reserved forest area (Stn. 8–14) that sustains rich mangrove and several associate floral species. The organic acids released from these vegetations may drive the pH of soil to lower value. In anthropogenically stressed western Indian Sundarbans, the soil pH is comparatively higher (7.57 ± 0.067). The spatial variation of soil pH is highly significant (p < 0.0001). The spatial variation of soil pH is highly significant (p < 0.0001).

A significant decrease in soil pH with depth at all locations (p < 0.0001) may indicate the production of organic acids and carbon dioxide by actively metabolizing mangrove roots. The surface soils are usually neutral to slightly acid in mangrove ecosystem due to the influence of alkaline estuary water (Clarke 1985), and in the present system, the value ranges from 7.90 to 8.30 depending on season (Mitra et al. 2011b). Soil pH in all the stations exhibit significant yearly variations (p < 0.0001), which may be attributed to climatic factors that regulate the ambient aquatic pH through precipitation, run-off and biological phenomenon like mangrove litter fall and their subsequent decomposition in the soil of intertidal mudflats.

2.2.4.3 2A.2.4.3 Soil Salinity

Soil salinity reflects the geophysical features of the ecosystem. It is also an indicator of dilution caused by run-off, stream discharge, barrage discharge and other anthropogenic activities. The relatively low soil salinity in the stations at western sector (stations 1–7) is the effect of Farakka barrage discharge that release freshwater through the main Hugli channel. The Hugli estuary in the western Indian Sundarbans marked by the outer drainage of Ganga River system receives high volume of freshwater discharge all round the year. The annual freshwater discharge through the estuary accounts for 6,7200, 16,200 and 62,100 million ft³ from the main channel of the River Ganga, Damodar and Roopnarayan covering an aggregate of about 11,900 km² of catchment area. The siltation of the Bidyadhari River since the late fifteenth century blocked the flow of freshwater in the central and eastern Indian Sundarbans. Interestingly stations 12, 13 and 14 in the eastern Indian Sundarbans exhibit low soil salinity profile and also a decreasing trend with time. This is due to proximity of these stations to Bangladesh Sundarbans that receive the maximum freshwater flow from the Himalayan glacier through the River Padma. The presence of numerous creeks and channels in the eastern most part of Indian Sundarbans may act as conveyer belt of freshwater from the Bangladesh part to the eastern Indian Sundarbans. The increase of soil salinity with depth (p < 0.0001) is the effect of percolation during tidal inundation of the intertidal mudflats (twice daily). The bottom layer is not washed away unlike the topsoil layer by daily tidal action which results in accumulation of salts in the bottom layer.

It is to be noted that increase of salinity in the stations adjacent to the Matla River (Stn. 8–11 in the central sector) may pose serious threat to certain mangrove species like Heritiera fomes (locally known as Sundari, from which the name Sundarbans has originated). Symptoms of excess chloride include burning and firing of leaf tips or margins, bronzing, premature yellowing, abscission of leaves and, less frequently, chlorosis. Smaller leaves and slower growth also are typical. Symptoms of excess sodium also include necrotic areas on the tips, margins or interveinal areas. High salinity also results in the stunted growth of mangroves (Mitra et al. 2009, 2010, 2011a). This may have far-reaching impact on the aquatic subsystem of central Indian Sundarbans as mangrove litter and detritus, which are the primary sources of soil organic carbon, may reduce in quantum. This may eventually lead to poor productivity of the adjacent water bodies. Therefore, efforts should be made to develop better understanding of the problem so that appropriate management strategy could be adopted for improved and sustainable ecological management of the central sector of Indian Sundarbans with particular reference to siltation problem that has cut off the freshwater supply in the region.

2.2.5 2A.2.5 Conclusions

Few core findings are listed below:

• 1. The estimated mean soil organic carbon in the western Indian Sundarbans is 1.02 wt%, where the soil salinity is low due to dilution of the system with Farakka barrage discharge and run-off from highly urbanized townships and agricultural fields around the area. The average pH (7.57 ± 0.067) is relatively higher in this sector.

• 2. The stations in the central and eastern part of Indian Sundarbans are free from anthropogenic influences due to their locations within the reserve forest area. The luxuriant mangrove vegetation in these areas associated with salt-marsh grass (Porteresia coarctata) has caused low pH in the soils of intertidal mudflats (7.47 ± 0.071). The organic carbon is comparatively low (0.64 wt%) in the absence of any human activities like aquaculture, agriculture and sewage disposal.

• 3. It can be concluded from the soil organic carbon data that the carbon budget in the soil is mostly influenced by physical (waves, tides, erosion, accretion, etc.), biological (vegetation type and density) and anthropogenic (urbanization, barrage discharge and nature of livelihood) factors.

• 4. The gradual increase of soil organic carbon with time in the western Indian Sundarbans is a clear signature of anthropogenic role in regulating soil organic carbon in the present geographical locale.

Annexure 2A.3: Carbon Content in Phytoplankton Community of a Tropical Estuarine System

• Abhijit Mitra,
• Subhro Bikash Bhattacharyya,
• Sufia Zaman,
• Kakoli Banerjee,
• Subhasmita Sinha &
• Atanu Kumar Raha 

1. Department of Oceanography, Techno India University, Salt Lake Campus, Kolkata, West Bengal, 700 019, India

   Abhijit Mitra

2. Department of Marine Science, University of Calcutta, 35 B.C. Road, Kolkata, West Bengal, 700 091, India

   Subhro Bikash Bhattacharyya, Sufia Zaman, Kakoli Banerjee & Subhasmita Sinha

3. Department of Environment & Forest Science, Techno India University, Salt Lake Campus, Kolkata, West Bengal, 700 091, India

   Atanu Kumar Raha

Abstract

Seasonal variations of cell carbon content in diatoms, dinoflagellates, cyanobacteria and green algae were studied during 2011 in the western and central sectors of Indian Sundarbans, a mangrove-dominated tropical estuary in the lower Gangetic delta region. Twelve geometric shapes were assigned to 47 species documented from 12 stations to calculate the cell volume and carbon content. The carbon content of the species varied significantly with seasons and sites (p < 0.01). This variation may be attributed to unique seasonal variations of salinity and contrasting environmental conditions prevailing in the selected sectors of Indian Sundarbans. The western sector is relatively less saline and favourable for the growth of the phytoplankton species. The central sector is hypersaline on account of massive siltation that prevents the mixing of freshwater of the River Ganga with the River Matla. Such a hypersaline condition posed adverse effect on the cell volume and carbon content of the phytoplankton species. Among 47 phytoplankton species, cell volume and carbon content of 6 species exhibited significant negative relationships, and 2 species exhibited significant positive relationships with aquatic salinity. Our results imply that rising salinity in the central sector of Indian Sundarbans may be a threat to phytoplankton cell carbon reservoir by way of shrinking their volumes.

Keywords

Cell carbon • Indian Sundarbans • Phytoplankton • Seasonal variation

2.3.1 2A.3.1 Introduction

In the marine and estuarine ecosystems, free-floating microscopic photoautotrophic floral communities referred to as phytoplankton account for approximately half the production of organic matter on Earth. This community strongly influences climate processes (Boyce et al. 2010) and biogeochemical cycles (Sabine et al. 2004; Roemmich and McGowan 1995) particularly the carbon cycle (Boyce et al. 2010). Despite this far-reaching importance, empirical estimates of carbon content in this community remain limited. Biovolume and surface area calculations for phytoplankton cells are important for studying many related ecological parameters (Malone 1980; Sournia 1981; Chisholm 1992), such as biomass, growth, photosynthesis, respiration, assimilation, sinking and grazing. Mullin et al. (1966) determined cell carbon, cell volume and surface area for a variety of phytoplankton organisms and concluded that cell volume gave a better estimate of cell carbon than did surface area. Phytoplankton cell size varies greatly among different genera or even between different individuals. Sizes range from a few micrometres (or even less than 1 mm) to a few millimetres. Hence, there is a wide range of nine orders in magnitude for cell biovolume of phytoplankton. Several automated and semi-automatic methods for biovolume estimation have been described in the literature, such as the Coulter Counter (Hastings et al. 1962; Maloney et al. 1962; Boyd and Johnson 1995), the micrographic image analysis system (Gordon 1974; Krambeck et al. 1981; Estep et al. 1986), flow cytometry (Olson et al. 1985; Wood et al. 1985; Steen 1980; Cunningham and Buonnacorsi 1992) and holographic scanning technology (Brown et al. 1989). Indian Sundarbans, being a major tropical estuarine system and a World Heritage site at the apex of Bay of Bengal, has no baseline databank on cell volume and carbon content of phytoplankton although 106 species have been documented till date (Mitra et al. 2004).

In this study, the carbon content in diatoms, dinoflagellates, cyanobacteria and green algae was compared in two different physiographic regions of Indian Sundarbans (western and central sectors) having contrasting salinity levels.

2.3.2 2A.3.2 Methods

2.3.2.1 2A.3.2.1 Study Sites

Sundarbans delta is one of the dynamic mangrove-dominated estuarine deltas of the world (Banerjee et al. 2012), which is situated at the apex of Bay of Bengal. A major portion of this delta (62 %) lies in Bangladesh and the remaining 38 % is within the Indian subcontinent. In the Indian Sundarbans, approximately 2,069 km² of area is occupied by the tidal river system or estuaries, which finally end up in the Bay of Bengal. The seven main riverine estuaries from west to east are Hugli, Muriganga, Saptamukhi, Thakuran, Matla, Gosaba and Harinbhanga. The flow of the mighty Ganges River through the Hugli estuary in the western sector of the Indian Sundarbans, ending up in the Bay of Bengal, results in an ecological situation totally different from that of the central sector, where five major rivers have lost their upstream connections with the Ganges (Fig. 2A.3.1) due to heavy siltation and solid waste disposal from the adjacent cities and towns (Chakrabarti 1998; Mitra et al. 2009, 2011). Presently, the rivers in the western sector (Hugli and Muriganga) are connected to the Himalayan glaciers through the Ganges originating at the Gangotri Glacier, whereas the five central and eastern sector rivers like Saptamukhi, Thakuran, Matla, Gosaba and Harinbhanga are all tide fed. The tidal flow from the Bay of Bengal (mean salinity = 32 psu) in the south of this deltaic complex sustains these five tide-fed rivers.

Fig. 2A.3.1

Map of Indian Sundarbans highlighting the sampling stations

We conducted seasonal survey at 12 stations in the Indian Sundarbans region (between 21°40′ N and 22°40′ N latitude and 88°03′ E and 89°07′ E longitude) during 2011 in May (premonsoon), September (monsoon) and December (postmonsoon). Station selection was primarily based on aquatic salinity (Table 2A.3.1 and Fig. 2A.3.1). The discharge of Farakka barrage (the biggest barrage in the Gangetic plain) through Hugli channel has made the western sector of the study area (stations 1–6) relatively low saline (Mitra et al. 2009). On the contrary, stations 7–12 are high-saline zone due to complete blockage of the freshwater because of siltation of the Bidyadhari River (Chaudhuri and Choudhury 1994; Mitra et al. 2011) that used to contribute freshwater to the tidal rivers of central Indian Sundarbans in the fifteenth century (Chaudhuri and Choudhury 1994).

Table 2A.3.1 Sampling stations with coordinates

2.3.2.2 2A.3.2.2 Salinity

The surface water salinity in the selected stations was recorded during high tide condition by means of an optical refractometer (Atago, Japan) and cross-checked in laboratory using Mohr-Knudsen method. The correction factor was found out by titrating silver nitrate solution against standard seawater (IAPO Standard Seawater Service Charlottenlund, Slot Denmark, chlorinity = 19.376 psu). Our method was applied to estimate the salinity of standard seawater procured from NIO, and a standard deviation of 0.02 % was obtained for salinity. The average accuracy for salinity (for triplicate sampling) was ± 0.24 psu.

2.3.2.3 2A.3.2.3 Cell Volume

Net samples were collected during high tide condition (around 12.00 noon) with a conical nylon net bag (30 cm diameter) made of a 30 No. bolting silk from 12 selected stations and preserved in 4 % neutral formaldehyde. Phytoplankton samples were observed with a ZEISS research microscope coupled with an image analyzing system. Phytoplankton cell identifications were based on standard taxonomic keys (Verlencar and Desai 2004; Botes 2003). Linear dimensions of the phytoplankton species were measured on the basis of taxonomic information and shape code as provided by Sun and Liu (2003). For each species, the best fitting geometric shape and corresponding equation were used to calculate the cell volume.

2.3.2.4 2A.3.2.4 Cell Carbon

Standard mathematical expressions (specific for different phytoplankton group) were used to transform cell volume into cell carbon.

The cell volume of diatoms was converted into cell carbon as per the expression cell carbon (pg) = 0.288 [live cell volume (μm³)]^0.811. For dinoflagellates, the expression cell carbon (pg) = 0.760 [live cell volume (μm³)]^0.819 (Montagnes and Berges 1994; Menden-Deuer et al. 2001; Davidson et al. 2002) was used. For phytoplankton species other than dinoflagellates and diatoms, the expression Log₁₀ C = 0.76 Log₁₀ V − 0.29 (Mullin et al. 1966) was used to estimate carbon content (pg) per cell. The phytoplankton population (in No.l⁻¹) was enumerated simultaneously using Sedgewick Rafter Cell Counter as per the method of McAlice (McAlice 1971). This approach is appropriate for larger phytoplankton species (>10–15 μ) having relatively higher population densities (≥10⁵cells/l). The species-wise carbon content per unit volume of water was calculated by the product of population and mean cell carbon content of each species.

2.3.2.5 2A.3.2.5 Statistical Analysis

To explore the relationships between phytoplankton cell volume and cell carbon scatter plots, allometric equations and correlations were computed separately for diatoms, dinoflagellates, cyanobacteria and green algae (n = 225 for each group). To assess whether cell volume and carbon content varied significantly between sectors (western vs. central) and seasons, two-way ANOVA was performed. All statistical calculations were performed with SPSS 9.0 for Windows.

2.3.3 2A.3.3 Results

2.3.3.1 2A.3.3.1 Salinity

The stations in western and central sectors of Indian Sundarbans exhibited significant differences in aquatic salinity during the study period. In the western sector, salinity of surface water ranged from 1.89 psu (at station 1 during September 2011) to 27.99 psu (at station 6 during May 2011), and the average salinity was 14.98 ± 9.16 psu. In the central sector, the lowest salinity was recorded at station 7 (4.01 psu during September 2011) and the highest salinity was at station 10 (29.98 psu during May 2011) with an average value of 20.60 ± 8.82 psu. In both the sectors, the seasonal trend in salinity was premonsoon > monsoon > postmonsoon (Fig. 2A.3.2).

Fig. 2A.3.2

Spatial variations of salinity during 2010–2011 in selected stations

ANOVA computed for aquatic salinity (Table 2A.3.2) exhibits significant spatial (western vs. central Indian Sundarbans) and temporal (seasonal) variations (p < 0.05).

Table 2A.3.2 ANOVA (spatial and seasonal variations) of aquatic salinity, phytoplankton cell volume and phytoplankton cell carbon content

2.3.3.2 2A.3.3.2 Cell Volume

A total of 47 phytoplankton taxa were identified to species level, which exhibited 12 different geometric shapes. The cell volume ranged from 86.89 μm³ (Hemidiscus hardmanninus) to 271,405.63 μm³ (Planktoniella sol) in the western sector and from 85.39 μm³ (Asterionella japonica) to 227,153.21 μm³ (Planktoniella sol) in the central sector (Table 2A.3.3).

Table 2A.3.3 Seasonal variation of cell volume (in μm³) of phytoplankton in Indian Sundarbans during 2010–2011

We observed relatively higher cell volume in the hyposaline western Indian Sundarbans compared to the hypersaline central sector. Almost all the phytoplankton species (except Rhizosolenia alata, Ceratium trichoceros, Ceratium furca, Nitzschia closterium, Trichodesmium erythraea and Chlorella marina) exhibited relatively higher cell volume in the western sector (Table 2A.3.3).

In most cases, the cell volume showed unique seasonal variations in both the sectors as per the order monsoon > postmonsoon > premonsoon (exceptional species: Rhizosolenia crassipina, Chaetoceros peruvianus and C. compressus in central Indian Sundarbans).

ANOVA results indicate significant seasonal and sectoral differences in cell volume for all the three groups of phytoplankton (Table 2A.3.2).

2.3.3.3 2A.3.3.3 Cell Carbon

The cell carbon content of the phytoplankton ranged from 10.76 pg (Hemidiscus hardmanninus) to 7,346.10 pg (Planktoniella sol) in the western sector and 10.61 pg (Asterionella japonica) to 6,358.68 pg (Planktoniella sol) in the central sector (Table 2A.3.4). The relatively higher cell carbon content in the western sector (with some exceptions like Rhizosolenia alata, Ceratium trichoceros, Ceratium furca, Nitzschia closterium, and Trichodesmium erythraea) compared to the central sector is due to higher cell volume of the phytoplankton in the estuaries of western Indian Sundarbans.

Table 2A.3.4 Mean seasonal variation of phytoplankton cell carbon content (in pg) in estuarine water of Indian Sundarbans during 2010–2011

With few exceptional species like Rhizosolenia crassipina, Chaetoceros peruvianus and C. compressus in central Indian Sundarbans, all species in the study area exhibited a sharp seasonal trend in cell carbon content with highest value during monsoon followed by postmonsoon and premonsoon.

ANOVA results indicate significant differences in cell carbon content (p < 0.01) between seasons and sectors in all three groups of phytoplankton (Table 2A.3.2).

2.3.4 2A.3.4 Discussion

Aquatic salinity seems to be the key player in regulating the cell volumes and carbon content of phytoplankton in the present study area. The relatively lower salinity in the western sector of the Sundarban delta region (Indian part) may be attributed to Farakka barrage that releases freshwater on regular basis through Ganga–Bhagirathi–Hugli River system. The central sector, on the contrary, does not receive the riverine discharge due to massive siltation of the Bidyadhari River that has blocked the freshwater flow in the region (Mitra et al. 2009; 2011; Raha et al. 2012). Ten-year survey (1999–2008) on water discharge from Farakka barrage revealed an average discharge of (3.1 ± 1.2) × 10³ m³/s. Higher discharge values were observed during the monsoon with an average of (2.9 ± 1.2) × 10³ m³/s, and the maximum of the order 4,185 m³/s during freshet (September). Considerably lower discharge values were recorded during premonsoon with an average of (1.0 ± 0.09) × 10³ m³/s, and the minimum of the order 820 m³/s during May. During postmonsoon discharge, values were moderate with an average of (1.9 ± 0.95) × 10³ m³/s. The lower Gangetic deltaic lobe also experiences considerable rainfall (1,400 mm average rainfall) and surface run-off from the 60,000 km² catchment areas of Ganga–Bhagirathi–Hugli system and their tributaries. All these factors (dam discharge + precipitation + run-off) increase the dilution factor of the Hugli estuary in the western sector of Indian Sundarbans (Mitra et al. 2011). The central sector does not receive the freshwater input on account of siltation of the Bidyadhari River since the fifteenth century, and the stations in this sector (stations 7–12) receive only the tidal waters from the Bay of Bengal. Such significant variations in salinity within the same deltaic lobe may be the probable reason for variation in phytoplankton cell volume. We observed significant negative relationships between aquatic salinity and cell volume of Coscinodiscus eccentricus, C. jonesianus, C. lineatus, C. radiatus, C. oculusiridis and Planktoniella sol (Table 2A.3.5).

Table 2A.3.5 Inter-relationship between phytoplankton cell volume and salinity during 2010–2011

It is possible that the salinity has a direct effect on cell morphogenesis of certain phytoplankton species. Hildebrandt et al. (2006) observed that the height of the centric diatom is reduced at increased salinity. Similar observations were also reported by other researchers (Hildebrand et al. 2006; Roubeix and Lancelot 2008) who observed a lower size of the species Thalassiosira pseudonana when grown at higher NaCl concentration. Many researchers put forward several views on the negative impact of salinity on phytoplankton cell volume. According to some researchers (Pickett-Heaps et al. 1990; Harold 2002), the elongation of diatom cells during the interphase preceding division is driven by turgor pressure, which makes the siliceous components of the cell walls slide apart. At increased salinity, freshwater diatom might not be able to produce the intracellular osmolarity needed to generate the same turgor pressure as at low salinity. Thus, if cell elongation is less efficient before each cell division, cell height might decrease faster in high-saline water (Roubeix and Lancelot 2008). This may be a possible cause for lowering of cell volume of the species Coscinodiscus eccentricus, C. jonesianus, C. lineatus, C. radiatus, C. oculusiridis and Planktoniella sol with the increase of aquatic salinity. However, two species Fragilaria oceanica and Chlorella marina exhibited significant positive relationships with aquatic salinity (Table 2A.3.5) and confirm the tolerance of the species to high salinity. Similar results were obtained while conducting the growth experiment on Cyclotella meneghiniana (Pickett-Heaps et al. 1990). According to a classification type of phytoplankton (Harold 2002), few species of phytoplankton are holoeuryhaline in nature that are able to grow from almost freshwater to marine conditions with a wide range of tolerance. Such species may serve as ideal indicators of salinity through variation of their cell size and volume. The cell volume may, however, vary depending on the adaptive efficiency of the species through variation of turgor pressure and cell morphogenesis at varying salinity.

One of the interesting aspects in the present estuarine system is the dominancy of diatoms (85.10 %) due to which the group plays the key role in carbon storage. The relative abundances of dinoflagellates and other phytoplankton (like green algae and cyanobacteria) are 10.64 and 4.26 %, respectively. Converting the carbon values cited in into carbon dioxide equivalent, it is observed that in the western sector for diatoms, dinoflagellates, blue green algae and green algae, the mean carbon dioxide equivalents per cell are 2,467.39, 624.08, 1,017.05 and 259.00 pg, respectively. Similarly in the central sector, the mean values of carbon dioxide equivalents for diatoms, dinoflagellates, blue green algae and green algae are 2,211.43, 595.80, 1,188.49 and 243.25 pg, respectively. Considering the standing stock of phytoplankton community and species-wise cell carbon content, it is observed that carbon content per litre of estuarine water (Table 2A.3.6) is maximum in diatoms (4,069.07 pg in western sector and 4,997.22 pg in the central sector) followed by blue green algae and green algae (2,512.33 pg in the western sector and 3,288.23 pg in the central sector) and dinoflagellates (186.98 pg in western sector and 483.13 pg in the central sector). It is also interesting to note that the monsoon period is most congenial for storing carbon in the phytoplankton cell (as revealed from significant negative relationship between salinity and phytoplankton cell carbon in Table 2A.3.7) due to increase in volume that may be attributed to enrichment of the present tropical estuarine waters with nutrients from mangrove litter and run-off from adjacent landmasses and agricultural fields (Banerjee et al. 2002, 2003; Chowdhury et al. 2011). The highly significant positive correlations between cell volume and carbon content of phytoplankton (Fig. 2A.3.3a, b, c, d, e, and f) confirm the contribution of cell volume to stored carbon in phytoplankton cell. The variations of allometric equations between the western and central Indian Sundarbans (Fig. 2A.3.1) emphasize the role of environmental variables (preferable salinity) in regulating the volume and carbon content in phytoplankton cell.

Table 2A.3.6 Mean seasonal variation of phytoplankton carbon (in pg) × 10⁵/l of estuarine water in Indian Sundarbans during 2010–2011

Table 2A.3.7 Inter-relationship between phytoplankton cell carbon and salinity during 2010–2011

Fig. 2A.3.3

(a) Interrelationship between cell carbon content (pg) and cell volume (μm³) for cyanobacteria and green algae in central sector during 2010–2011. (b) Interrelationship between cell carbon content (pg) and cell volume (μm³) for cyanobacteria and green algae in western sector during 2010–2011. (c) Interrelationship between cell carbon content (pg) and cell volume (μm³) for dinoflagellates in central sector during 2010–2011. Fig. 2A.3.3 (continued) (d) Interrelationship between cell carbon content (pg) and cell volume (μm³) for dinoflagellates in western sector during 2010–2011. (e) Interrelationship between cell carbon content (pg) and cell volume (μm³) for diatoms in central sector during 2010–2011. (f) Interrelationship between cell carbon content (pg) and cell volume (μm³) for diatoms in western sector during 2010–2011

2.3.5 2A.3.5 Conclusion

The carbon sequestration in phytoplankton community is a direct function of cell volume that depends on the interaction between salinity, climate and several hydrological parameters. Hence, results obtained at one location may not be equally applicable to another location, and site-specific allometric models need to be developed to predict the stored carbon on the basis of cell volume.

The present study demonstrates that the cell volume and carbon storage capacity of phytoplankton species vary with spatial locations due to varying salinity, perhaps moderated by anthropogenic factors (like barrage discharge) and climatic factors (like monsoon). Effective freshwater flow (through artificial canalization or channelization from rainwater harvested pools) into the tropical mangrove system is important for increasing the carbon storage potential of phytoplankton.

2.3.6 Acknowledgements

The authors are grateful to Department of Science and Technology, Govt. of India, for financial support [Project Sanction No. DST/IS-STAC/CO2-SR-59/09 (Pt. II) dated 24.05.2011] and Directorate of Forest, Government of West Bengal, for providing infrastructural support during the tenure of the work. One of the authors (Sufia Zaman) is grateful to Department of Science and Technology Govt. of India for financial assistance (under Women Scientist Scheme-B Project, Sanction No. SSD/SS/028/2010/G; dated 28.09.2011).