Dinoflagellate Ceratium symmetricum Pavillard ( Gonyaulacales : Ceratiaceae ) : Its occurrence in the Hooghly-Matla Estuary and offshore of Indian Sundarban and its significance

1,3,5,6 School of Oceanographic Studies, Jadavpur University, Kolkata, West Bengal 700032, India 2 Indian National Centre for Ocean Information Services (INCOIS), Ministry of Earth Science (MoES), Govt. of India, “Ocean Valley”, PB No. 21, IDA Jeedimetla PO, Hyderabad, Andhra Pradesh 500055, India 4 Department of Botany, Midnapore College, West Bengal 721101, India Email: 1 anirban_akhand@rediffmail.com (corresponding author), 2 srv_maity@rediffmail.com, 3 anirban_iirs@yahoo.com, 4 ms_indranidas@yahoo.co.in, 5 pranabes@gmail.com, 6 sugata_hazra@yahoo.com

Former Senior Deputy Director General, Geological Survey of India for his cooperation in this work.The authors are also grateful to National Aeronautics and Space Administration for providing the SST data through the Ocean Color website.
The Sundarban, a Biosphere Reserve, constitutes a complex ecosystem comprising one of the three largest single tracts of mangrove forests of the world.As neighboring countries, India and Bangladesh share the territories which cover the areas of the Sundarban.The Indian Sundarban (88 0 02'-89 0 06'E & 21 0 13'-22 0 40'N) including the forest and nonforest parts consist of three major estuaries.The biodiversity richness of the Sundarban has been reported by many researchers.However, there are very few studies on phytoplankton diversity (Santra et al. 1991;Mitra et al. 2003;Sen & Naskar 2003).Phytoplankton are the Abstract: The Sundarban is the largest mangrove ecosystem, which is presently vulnerable to climate change related impacts.The western part of it falls in the state of West Bengal between the estuaries of the Hooghly and Ichamati-Raymongal Rivers.The diversity of the genus Ceratium Schrank and the related physicochemical parameters such as Sea Surface Temperature (SST) was studied in the Hooghly-Matla estuary and offshore.Five species of bio-indicator dinoflagellate, Ceratium were identified in the bloom-forming season.The species are: C. furca, C. fusus, C. symmetricum, C. trichoceros and C. tripos.C. symmetricum was not previously reported from the Indian part of the Sundarban and is now found in low abundance.The other four species are less sensitive to warming or rise in SST.A comparative study of the day time SST from the satellite images of the year 2003 to 2009 of the months of January and February reveals a rising winter SST.Compared to the previous years, the increase in temperature can be one of the causative factors to explain the lower abundance of C. symmetricum compared to the others.With further rise of the SST, there is a possibility that this species may no longer be found in abundance in the western part of adjoining Hooghly-Matla estuarine system.
foundation of the foodweb in the marine ecosystem as they perform the critical ecological function of primary production (Nielsen & Jensen 1957;Banerjee & Santra 2001, Verlencer & Desai 2004).The knowledge of phytoplankton species diversity is crucial for any ecological or eco-physiological work on marine phytoplankton.Phytoplankton are highly sensitive to environmental changes.Their community composition, biomass and shifts therein represent an excellent tool to interpret the dynamics of a pelagic ecosystem, transformation, cycling of key elements and the impact on coastal water quality.Phytoplankton also help to detect variations induced by river discharge, eutrophication, pollution and even certain unusual climatic phenomena (Lepisto et al. 2004;Paerl 2006).Ceratium Schrank, an armoured dinoflagellate genus has been considered a biological model for a wide range of studies as utilized by Tunin-Ley et al. (2007, 2009).One of the several advantages offered by this genus is that identification of species level is more feasible than other phytoplankton group (Tunin-Ley et al. 2009).Apart from this, Ceratium is known for its sensitivity to temperature in terms of biogeography (Dodge & Marshall 1994), seasonality and morphology (Sournia 1967).Ceratium has been considered to be a biological indicator of water masses (Dodge 1993;Raine et al. 2002;Tunin-Ley et al. 2009), current regimes (Dowidar 1973) and climate change (Dodge & Marshall 1994;Johns et al. 2003).
In the present paper, diversity and record of the species Ceratium has been investigated in the Indian part of the Sundarban estuary to offshore Bay of Bengal and discussed in the context of ecological change, particularly the change in Sea Surface Temperature (SST).

Materials and Methods
Water samples were collected from different stations from the case 2 water of Hooghly estuary and offshore off Bakkhali-Frasergunje (Fig. 1).The depth of the water varies between 1 to 10 m along a 10km radial vector off the coast.The cruise was conducted in the months of January and February, 2009, as December, January and February are the bloom forming seasons for most of the phytoplankton in Sundarban (Biswas et al. 2004).Day time SST data, obtained from Moderate Resolution Imaging Spectro Radiometer (MODIS) with a wave length of 11µ and spatial resolution of
Four out of 10 species of Ceratium, reported from the Indian Sundarban earlier (Santra et al. 1991;Mitra et al. 2003;Biswas et al. 2009) was also found in the present study.The fifth species found in this study, C. symmetricum (Image 1) was not reported previously from the Indian Sundarban.The analytical data of the monthly composite SST for seven years (2003 to 2009) derived from satellite images within the study area for the months of January and February (24.114 and 25.495 0 C respectively) clearly depicts an increased winter SST in the year 2009, than the previous years (Figs. 2 & 3).The temperature difference is also maximum between 2008 and 2009 (during January and February), within the last seven years.

Discussion and Conclusions
The distributions of C. symmetricum are comparatively limited and are reported in warm temperate to tropical waters (Parke & Dixon 1976;Gil-Rodriguez et al. 2003).The species was reported from the water column of continental shelf of northern and northwestern Australia (Hallegraeff & Jeffrey 1984) and northwestern Mediterranean Sea (Tunin-Ley et al. 2009).
Several types of biological indicators of ecological change have been suggested, for example, abundance of individual taxa, functional attributes of the ecosystem, species assemblages and phenological traits (Beaugrand 2005).Among the species of Ceratium found in this study, available literature suggests: C. furca and C. tripos are perennial, whereas C. fusus is almost perennial since it may be absent only for a couple of months.C. trichoceros is one of the species which is not specifically associated with any depth or environment (Tunin-Ley et al. 2009).Among these  summer, 2003(Tunin-Ley et al. 2007) is comparable to the temperature (24.5-25.2 0C) of our study area in the winter as measured during the study period.Thus, similar inferences can be drawn from the present study of limited abundance of C. symmetricum in the estuary and offshore of Indian Sundarban.
Ceratium symmetricum was found to be much reduced in number (Relative Abundance: 3.2±0.2) compared to the other four species during winter (Table 1), indicating its lesser compatibility with the physico-chemical features, especially with SST in the study area.The other four species are less responsive to warming and tolerant of a wide range of physical condition, whereas the occurrence of a reduced number of C. symmetricum may be indicative of temperature rise in the Sundarban estuary and offshore.
It is proposed that, long term monitoring of thermal preference or sensitivity of species of Ceratium along with various physicochemical features of water can be a useful approach for the analysis of the impact of climate change on the phytoplankton community of the northern Bay of Bengal.This study, thus, opens some new avenues for generation of a long term dataset which would throw some light on various aspects of Sundarban's mangrove ecosystem and climate change induced impact on them.

Figure 1 .
Figure 1.Sampling location of phytoplankton in the Hooghly-Matla estuary and offshore of Indian Sundarban.

Figure 2 .Figure 3 .
Figure 2. Change of SST in the month of January from 2003 to 2009 around the study area (1˚ × 1˚)