Dinoflagellate Ceratium symmetricum Pavillard (Gonyaulacales: Ceratiaceae): Its occurrence in the Hooghly-Matla Estuary and offshore of Indian Sundarban and its significance
Anirban Akhand 1, Sourav Maity 2, Anirban Mukhopadhyay 3, Indrani Das 4, Pranabes Sanyal 5 & Sugata Hazra 6
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, ŅOcean ValleyÓ, PB No. 21, IDA Jeedimetla PO, Hyderabad, Andhra Pradesh 500055, India
4 Department of Botany, Midnapore College, West Bengal 721101, India
Email: 1 email@example.com (corresponding author), 2 firstname.lastname@example.org, 3 email@example.com, 4 firstname.lastname@example.org, 5 email@example.com, 6 firstname.lastname@example.org
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 (88002Õ–89006ÕE & 21013Õ–22040ÕN) 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 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 4km has been used in a 10×10 grid around the study area. The monthly composite data were derived from the year 2003 to 2009 for the months of January and February. Salinity, pH and temperature were measured using a portable refractometer (Model no.RHS-10(ATC)), a digital pH meter (Model no. pHTestr 1, Eutech instruments, Oakton Instruments) and a digital thermometer respectively. Dissolved oxygen was measured using WinklerÕs titrimetric method. The transparency of the water column was measured with the help of a Secchi disc. Phytoplankton samples were collected by plankton net of 20µ mesh size fitted to a wooden stick. Known volume of water was passed through it with a five litre bucket. Plankton samples were preserved in LugolÕs iodine solution and sent back to laboratory within 48 hours where it was counted by a Sedgewick Rafter counting cell. The identification of phytoplankton species was done with the help of standard manuals and literature (Tomas 1996; Verlencar & Desai 2004). Relative abundance (RA) was calculated using the formula:
RA of species X (%) of Ceratium = (No. of species X of Ceratium in each known volume of sample × 100) / No. of total Ceratium species in the same volume.
All together, five species of Ceratium were identified in the months of algal bloom. Ceratium furca (Ehrenberg) Clapar~de & Lachmann, Ceratium fusus (Ehrenberg) Dujardin, Ceratium trichoceros (Ehrenberg) Kofoid, Ceratium tripos (O.F. Mialler) Nitzsch and Ceratium symmetricum Pavillard have been found when the day temperature of the surface water ranged between 24.5–25.2 0C, pH between 8.0–8.3, salinity between 26–27 ppt, dissolved oxygen between 4.7–5.2 mg/L and total alkalinity (TA) between 125–150 mg/L. The least transparency of the water column was observed to be 39cm during the course of sampling (Table 1). Relative abundance was calculated for each species (Table 2), and showed highest abundance of Ceratium furca, followed by C. fusus, C. tripos, C. trichoceros and C. symmetricum.
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 0C 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 species, mixotrophic behavior has been observed in C. furca (Smalley et al. 1999) and C. fusus (Mikaelyan & Zavyalova 1999). Mixotrophic organisms are theoretically supposed to be, less dependent on nutrient availability and irradiance (Tunin-Ley et al. 2009).
Tunin-Ley et al. (2009) have revealed in their work that the diminished occurrence of certain Ceratium species during the warm season (in case of northwestern Mediterranean Sea) in surface water is an important sign of change. Among these, C. teres, in the main and C. symmetricum and C. horridum to a lesser extent, could be proposed as indicators of warming since they seemed to be limited by a maximum temperature threshold. The temperature of the northwestern Mediterranean Sea in the warm season (230C in summer, 2002, and above 250C in 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 SundarbanÕs mangrove ecosystem and climate change induced impact on them.
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