Rotifer communities of Deepor Beel , Assam , India : richness , abundance and ecology

Rotifera are important qualitative and quantitative components of zooplankton in freshwater ecosystems, comprising integral links of aquatic food-webs (primarily as fish food) and contributing significantly to secondary productivity. These organisms have been studied in a wide variety of Indian freshwater biotopes for more than a century, yet there is limited information on their ecology and role in aquatic productivity in floodplain lakes (Sharma & Sharma 2008). Earlier studies of Rotifera in the floodplains of Northeastern India mainly relate to biodiversity (Sharma 2000a, 2005; Sharma & Sharma 2001, 2008) and ecology in specific locations in Assam (Sharma 2000b, 2006), and the present study extends this work to characterize the Rotifera of Deepor Beel, an important floodplain lake of the Brahmaputra River basin. Qualitative and quantitative analyses of the rotifer communities are presented describing temporal variations in richness, community similarities, abundance, species diversity, dominance and evenness. In addition, the influences of abiotic parameters on occurrence and abundance of Rotifera are analyzed in order to understand ecological relationships.

II) and were analyzed for various abiotic factors.Water temperature, specific conductivity and pH were recorded by field probes, transparency was noted with Secchi disc, dissolved oxygen was estimated by modified Winkler's methods and other parameters were analyzed following APHA (1992).Monthly qualitative (by towing) and quantitative (by filtering 25 l water each) plankton samples were collected by nylobolt plankton net (No. 25) and were preserved in 5% formalin.The former were screened for the rotifer species and quantitative collections were analyzed for their abundance.Rotifera species were identified following Koste (1978), Segers (1995), Sharma (1998), and Sharma & Sharma (1999, 2000).
Community similarities (Sorensen's index), species diversity (Shannon's index), dominance (Berger-Parker's index) and evenness (E1 index) were calculated following Ludwig & Reynolds (1988) and Magurran (1988).ANOVA (two-way) was used to analyse significance of temporal variations of biotic communities.Simple correlation coefficients (r 1 and r 2 ) were calculated between abiotic and biotic parameters while multiple regressions (R 2 1 and R 2 2 ) were computed with ten abiotic factors i.e., water temperature, rainfall, pH, transparency, specific conductivity, dissolved oxygen, alkalinity, hardness, phosphate and nitrate for both sampling stations respectively.

RESULTS AND DISCUSSION
Water samples analyzed from Deepor Beel are characterized (Table 1) by low ionic concentrations and thus warrant inclusion of this Ramsar site under 'Class I' category following Talling & Talling (1965).Mean water temperature affirms tropical range concurrent with its geographical location.The circum-neutral and marginally hard waters of this wetland record moderate dissolved oxygen, low free CO 2 and low concentration of micronutrients.In general, the ranges of the recorded abiotic factors are generally concurrent at both sampling stations (I and II).
Plankton samples examined from Deepor Beel reveal 110 species of Rotifera belonging to 35 genera and 19 families, comprise about 54.5% of the species known from Northeastern India (Sharma & Sharma 2005) and represent about 30.5% of the Indian Rotifera and, hence, highly speciose and diverse nature of the rotifer biocoenosis.The examined diversity reflects greater environmental heterogeneity and habitat diversity of Deepor Beel and, in turn, affirms hypothesis of Segers et al. (1993) indicating (sub) tropical floodplain lakes to be the world's richest habitats for the rotifer diversity.Further, the present report follows next to the highest Indian record of 120 species from Loktak Lake (Sharma 2009), Manipuranother Ramsar site.The rotifers comprise the dominant qualitative component of zooplankton (171 species) of Deepor Beel (Table 2).The present report even exceeds earlier lists of 67-103 species (Sharma 2005) and 69-93 species (Sharma & Sharma 2008) recorded from 15 floodplain lakes (beels) each of Assam.Rotifera richness is, however, significantly higher than the reports of 48 species from 37 beels (Sarma 2000), 64 species from five beels (Sharma 2000a) and 64 species from twelve beels of the Pobitora Wildlife Sanctuary (Sharma 2006) of Assam state; 27 species from two floodplain lakes of Kashmir (Khan 1987) and 38 species from four ox-bow lakes and nine floodplain lakes of South-eastern West Bengal (Khan 2003).
The rotifer communities indicate 48.9-88.1 % similarity at Station I and marginally higher range (53.1-89.7 %) at Station II (Tables 3 & 4).This study shows few instances of < 60.0% or > 80.0% similarities while it varies between 60-70% in 38.4 and 34.8% instances and between 70-80% in 43.9 and 53.3 % instances at two sampling stations respectively, thereby, indicating relatively higher similarities in their species composition.Peak similarities are observed between November-February and December-January at two stations respectively.Cluster analysis indicates more differences of their monthly groupings at station II in general.The rotifer communities at station I show (Fig. 2) higher affinities between November-February and again between July-August while greater divergence is noticed during April.On the other hand, higher affinities at station II are noticed (Fig. 3) between December-January and February-October while June, July, March, May and April communities show greater divergence in their composition.
Interestingly, amongst rich Rotifer biodiversity (110 species) observed in Deepor Beel, only a few species namely Asplanchna priodonta, Keratella cochlearis, Platyias quadricornis, Lecane leontina, Polyarthra vulgaris, Keratella tropica and Brachionus falcatus indicate certain degree of quantitative importance while no individual species shows distinct dominance.On the other hand, the present results are characterized by lower densities of majority of species as well as lack of any definite pattern of quantitative periodicity of any particular family, genus or species.This salient feature stands in contrast to importance of certain species indicated by Sharma (2000b) and Sharma & Sharma (2008).The rotifer communities of Deepor Beel (Table 2) are characterized by higher species diversity (3.480 ± 0.150 and 3.445 ± 0.116) which, in turn, indicates only marginal differences at two stations and registers insignificant variations between months as well as stations.It follows (Fig. 6) multimodal but different annual patterns with peaks during July and October and minima during December and January at two sampling stations respectively.The present results indicate significantly higher species diversity than the report of Sharma (2000b) and Sharma & Hussain (2001) and, even higher values than those of the rotifer communities of various floodplain lakes of Assam (Sharma 2005(Sharma , 2006;;Sharma & Sharma 2008).The rotifer diversity is negatively correlated with their abundance (r 1 = -0.510,r 2 = -0.414).The salient feature of higher species diversity with relatively lower numbers (density) of majority of species noticed in this study may be ascribed to fine niche portioning amongst rotifers species in combination with high micro-and macro-scale habitat heterogeneity as hypothesized by Segers (2008).This generalization affirms earlier remarks of Sharma (2009).
The present study indicates (Table 2) lower Rotifera dominance (0.125 ± 0.037 and 0.133 ± 0.039) which, in turn, registers significant temporal variations between months (F 11, 23 = 6.285, p < 0.005) and insignificant variations between stations.The salient feature of lower dominance concurs with the findings of Sharma (2005Sharma ( , 2006) ) and Sharma & Sharma (2008) from other floodplain lakes of Assam as well as those of Sharma (2009) from Loktak Lake, Manipur.Dominance follows multimodal annual patterns with peaks during summer (April) but records minima during March (station I) and February (station II).It is negatively correlated with species diversity (r 1 = -0.491,r 2 = -0.443)at both stations and only with Rotifera evenness at station II ( r 2 = -0.458).Sladecek (1983) proposed Q B / T quotient based on the ratios between Brachionus: Trichocerca species to depict trophic status of different ecosystems or even individual samples.Sharma & Dudani (1992) and Sharma (2000a) successfully applied it to certain aquatic ecosystems under the Indian conditions.Application of the stated quotient to the rotifer communities of Deepor Beel indicates Q B / T ratios of 3.01 ± 1.0 and 2.87 ± 0.8, thereby, reflecting eutrophic status of this floodplain lake.
To sum up, Rotifera communities of Deepor Beel are highly diverse and speciose, form important qualitative and quantitative components of zooplankton and indicate no definite periodicity of occurrence or abundance of any family, group or species.They are characterized by lower densities of majority of species, relative quantitative importance of fewer species, higher species diversity, higher evenness and lower dominance.The present results indicate limited influence of individual abiotic factors, while ten abiotic factors exert higher cumulative influence on Rotifera richness and abundance.Q B / T

Figure
Figure 1.Monthly variations in richness of Rotifera Months

Figure
Figure 3. Hierarchical cluster analysis of Rotifera (Station II)

Figure 4 .Figure 5 .
Figure 4. Monthly variations in abundance of Rotifera and important families (Station I)N D J F M A M J J A S O

Figure 6 .
Figure 6.Monthly variations in species diversity of RotiferaN D J F M A M J J A S O

Figure
Figure 8. Monthly variations in dominance of Rotifera

Table 1 . Temporal variations of abiotic factors
which are characterized by higher abundance during December-