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DYNAMICS OF NATURAL PHYTOPLANKTON ASSEMBLAGES IN THE INDIAN SUNDARBANS: AN INTEGRATED APPROACH

Dola Bhattacharjee1, Brajogopal Samanta1, Amit Kumar1, Anurag Danda2 and Punyasloke Bhadury1

¹Integrative Taxonomy and Microbial Ecology Group, Department of Biological Sciences, Indian Institute of Science Education and Research-Kolkata (IISER-K), Mohanpur Campus, Mohanpur-741252, Nadia, West Bengal, India.

²Sundarbans Programme and Climate Adaptation (Coastal Ecosystems), WWF-India, Kolkata- 700029, West Bengal, India.RESULTS AND DISCUSSIONABSTRACT

METHODS AND MATERIALSHIGHLIGHTS

REFERENCES

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Rising atmospheric carbon dioxide (CO2)concentration is causing global warming and oceanacidification, which increasingly are recognized asimportant drivers of change in biological systemsincluding in mangrove ecosystems. The IndianSundarbans located on the southern fringe of WestBengal at the apex of the Bay of Bengal has beenreported undergoing changes in the water quality.Plankton, in particular phytoplankton are excellentindicators to study the impact of climate change inthe Sundarban aquatic ecosystems. Since March of2010 we are undertaking systematic weekly samplingfor studying the natural phytoplankton assemblagesacross four sites in the Sagar Island of IndianSundarbarns. Key physico-chemical and nutrientparameters are measured as part of the study. APCR-clone library approach based on key functionalgenes (NR and RUBISCO) involved in nitrogen andcarbon metabolism in phytoplankton has been alsoattempted for studying the molecular dynamics ofphytoplankton assemblages. Natural phytoplanktonassemblages across all the study sites are found tobe overwhelmingly dominated by members ofBacillariophyceae followed by Dinophyceae.Picoplankton communities have been also detectedas part of the study. Generic compositions for majorphytoplankton functional groups show seasonalvariation with shifts in physico-chemical parametersand nutrient concentrations. Water temperature andpH values coupled with phytoplankton communitydynamics in the present study show significantvariation when compared with previous studiesindicating possible changes in the eco-region.Preliminary results from the clone library approachindicate the presence of several novel phytoplanktonlineages possibly playing an important role innutrient metabolism including in carbon cycling. Thisstudy is a part of an ongoing investigation tounderstand how phytoplankton communities mayrespond to changes in aquatic carbon chemistry inthe Sundarbans.

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Mangrove ecosystems are threatened globallydue to climate change¹. The IndianSundarbans has not been spared from theimpacts of climate change. The objectives ofthe present study are to track these changesusing phytoplankton as proxies. A PCR-clonelibrary approach based on key functionalgenes (NR and RUBISCO) involved in nitrogenand carbon metabolism in phytoplankton hasbeen also attempted as part of this study tocomplement with the conventional taxonomy.

METHODSSystematic sampling of phytoplanktonassemblages and measurements of relatedphysico-chemical parameters wereundertaken since March of 2010,encompassing 29 weeks (till 1st week ofFebruary, 2011), from four geographic stationsin and around the Sagar Island of IndianSundarbans (Fig 1). Following standardprotocols functional genes (NR and RUBISCO)from natural phytoplankton assemblages wereamplified and sequenced based on a PCR-clone library approach.

Natural phytoplankton assemblages across all the study sites were found to be overwhelmingly dominated bymembers of Bacillariophyceae (28 genera; 16 Centric and 12 Pennate forms) followed by Dinophyceae (5 genera).Pico-phytoplankton and Cyanobacterial communities have been also detected as part of the study. Numerically important diatom genera were Coscinodiscus, Thalassiosira, Cyclotella, Chaetoceros andSkeletonema; while for dinophytes, Ceratium, Dinophysis and Peridinium were dominant throughout the study period(Fig 2). Major phytoplankton functional groups at the generic level showed seasonal variation with shifts in hydrographicalparameters and nutrient concentrations (nitrate, phosphate and silicate) in the water column (Fig 3, a-d). Surface-water temperature and pH values coupled with phytoplankton community dynamics in the present studyvaried when compared with previous studies indicating possible changes in the eco-region. Clone L7 and L15 sequences showed 100% & 99% identity with rbcL sequence of Minutocellus polymorphus(centric diatom) at the amino acid level respectively, while clones L12 and L14 showed 97% & 99% identity with rbcLsequence of Amphora coffeaeformis (pennate diatom) at the amino acid level respectively.Preliminary results from the clone library approach indicate the presence of novel phytoplankton lineages possiblyplaying an important role in nutrient metabolism including in carbon and nitrogen cycling across the samplinglocations.

Shift in Chlorophyll-a concentration over the studyperiod matched with that of the dynamics ofphytoplankton cell densities. However, in late spring andsummer pico-phytoplankton cells were abundant that mayhave contributed to gross Chlorophyll-a concentrations.Also, in winter phytoplankton assemblages consisted ofdifferential size groups which might have contributedvariably in Chlorophyll-a concentrations (Fig 4, a-b). Spectrophotometric pigment (fucoxanthin and peridinin)profiles did provide division-level phylogenetic evaluationof large, short-term changes in phytoplankton communitycomposition.

Fig 3. Variation in physico-chemical parameters and nutrient concentration along the study period.

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Fig 4. Phytoplankton dynamics in terms of Chl a concentration and cell abundance.a

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Shift in phytoplankton community structure strongly linked with physico-chemicalparameters. Re-appearance of Triceratium sp. (polyhaline, indicating salinity tolerance) acrosssampling locations. Detection of Minutocellus polymorphus like rbcL sequences from the study site is anew finding in the Indian Sundarbans, as part of molecular diversity studies. Variations in salinity (0-22 psu), temperature profiles (22°C-34°C), average pH (7.99)and net-phytoplankton cell density (1.2X10³ Lˉ¹) in the study area project changes inthe water quality when compared with previous reports². Further, we have initiated detailed investigations towards understanding the role ofpico-phytoplankton in biogeochemical cycling using laboratory and field basedexperiments in the Indian Sundarbans.

1. Macintosh, D. J. and E. C. Ashton, 2002. A Review of Mangrove Biodiversity Conservation and Management. Report to WorldBank. Centre for Tropical Ecosystem Research, University of Aarhus, Denmark, 71 pp.

2. Saha, S.B. , S.B. Bhattacharyya and A. Choudhury, 2001. Photosynthetic activity in relation to hydrological characteristics of abrackishwater tidal ecosystem of Sundarbans in West Bengal, India. Tropical Ecology, 42, 111-115.

ACKNOWLEDGEMENTS: We thank Prof. A. Choudhury for scientific guidance, WWF-India for funding this project and Director, IISER-Kolkata for providing the facilities to undertake the study.

Chaetoceros sp. Thalassionema sp. Cyclotella sp.

Pleurosigma sp. Coscinodiscus sp.Thalassiosira sp.

Fig 2. Dominant diatoms from the study area.