The present study deals with the composition and abundance of Rotifera in a tropical lake of Belgaum city. Rotifera (14 species) comprised the dominant zooplanktons of the highly polluted lake. This group formed 52.38 and 54.6 percent of total zooplanktons   during 1998 and 1999 respectively. Maximum population density was recorded during summer and minimum in monsoon during    the study period. Brachionus (four species) constituted the dominant genus, followed by Keratella (two species), Filinia , Asplanchna, Polyarthra, Trichocerca as sub-dominant genera. Other two genera Ascomorpha and Anuraeopsis were   found in very low numbers in the lake. The possible reasons for the fluctuations   in the physico-chemical factors of the lake have been discussed.

Rotifers are pseudocoelomate microorganisms with intricate structure and well organised bodies. These organisms are also commonly called “wheel-animalcules” because of their characteristic wheel-organ. These fascinating organisms represent one of the oldest group of invertebrates (Sladecek, 1983). Rotifera comprises a very small group of the animal kingdom but their qualitative and quantitative importance is beyond question in freshwater environment as they colonise littoral, limnetic and benthic regions. About 95 per cent of the known species of rotifers occur in freshwater, which is considered their original habitat. The rotifers inhabit practically every inland aquatic biotope polluted or non-polluted.

Investigation on the ecology of freshwater rotifers of India was initiated by Sewell (1935) and since then there have been studies by a large number of workers. A review of Indian literature indicates that the autecological data is confined to fewer papers (Qudri and Yousuf, 1982; Rao and Sharma, 1985, Deb et al ., 1987; Sharma and Rao, 1991 and Sharma and Sharma, 2001), comprising   part of the general limnological work,   which often lack detailed analysis of these communities and hence, provide limited information. However, some important contributions on this aspect are those by George (1966), Tiwari and Sharma (1977), Laal (1984), Haque et al . (1988) and Sharma (1992). The Rotifera is an important group of zooplankton but little is so far known about their abundance in the lakes of this region of Karnataka state. Hence, the present study deals with composition and abundance of Rotifera in one of the lakes of Belgaum district of North Karnataka state and with factor(s) affecting their production.

The observations, a part of general limnological survey, were undertaken in Fort lake (4.15 sq. km.) located (longitude: 74°37'E, latitude: 15°51'N) in the Belgaum district, Karnataka (Table 1 and Figure 1). The lake is covered with luxuriant growth of rooted and submerged hydrophytes. Water analysis was done as per the   standard methods (APHA, 1991).

Table 1. Topography of Fort lake

Qualitative and quantitative samples were collected with a   nylon plankton net (No.25) from the lake at regular monthly intervals from February 1998 to January 2000. The former were obtained by towing plankton net and after collection were preserved in 4% formalin and were examined for identification of various rotifera taxa. Quantitative samples was collected by filtering 100 ml of lake water and each sample was concentrated to 25 ml. Population densities (individual/litre) were enumerated with Sedgwick-Rafter counting cell. The trophic status of the lake was assessed applying Q B/T quotient, where B = number of Brachionus spp occurring in lake T = Trichocerca spp occurring in the same lake.

Abiotic factors of the lake exhibited range of water temperature (21-34.6°C), the lake was alkaline in   nature (pH 8.2-10.6)   and the dissolved oxygen concentration ranged between 5.04 – 8.6 mg/l and BOD between 42.6 – 64.8 mg/l. Total alkalinity (88.4 – 225.8 mg/l) attributed to carbonate and bicarbonate ions depicted in the hardness   of lake water and correlated with the total hardness levels (59.8 – 217.4 mg/l). It also indicated   increased concentrations of organic pollution and high nutrient concentrations. The rotifer biocoenosis (14 species) appeared to be fairly rich and diversified and depicted typical tropical character (Sharma, 1996). This group comprised the most important qualitative component of zooplankton and hence, coincided with dominance pattern in several Indian lakes (Sharma, 2000). Rotifera   was the dominant group among zooplankton population and contributed almost 53 percent, depicting considerable variations in the community structure (Figure 2).

Rotifer richness and diversity were found to be maximum in summer (Table 2 and 2a) indicating the influence of temperature, which was supported by positive correlation between summer temperature and rotifer population. This observation is in concurrence with the work of Kaushik and Sharma (1994) and Singh (2000). Alkaline water of the lake is characterised by the presence of Brachionus, Keratella , Anuraeopsis and Testudinella . Brachionus constituted the dominant genus over other forms and includes four species of which B. forticula was fairly abundant and inhabited the lake throughout the study period. Other species include B. caudatus , B. angularis and B. calyciflorus . The genus Brachionus is of Gondwanian origin and invaded Eurasia and North America   by dispersal from Africa and India (Dumont, 1983). It is missing in the cold regions like Arctic but dominates in the tropics and subtropics. An abundance of Brachionus spp in tropical regions has been registered (Fernando, 1980, Sharma and Michael, 1980 and Sharma, 1983). Various species of this genus dominate plankton community in the warmer part of peninsular India. Further, they are conspicuous for their absence or rare occurrence at northern altitudes in India.

Table 2. Monthly variation of rotifer species (no/l)

Table 2a. Monthly variation of total rotifer population (no/l)

Keratella was also abundantly observed in the alkaline lake. In the present investigation the occurrence of this genus along with Brachionus , Ascomorpha and Testudinella (Figure 3), obviously indicate eutrophic nature of the water body. This is further supported by direct correlation with BOD (r=0.7374), cyanophytes (r=0.8589) and diatoms (r=0.7548). Rotifers are considered as good indicators of water quality (Berzins and Pejler, 1987) and indicate situations only within the limits of limnosaprobity. This aspect has subsequently drawn attention of several other investigators (Karikal, 1995 and Sharma, 2000).

Table 3. Seasonal variations of rotifer population (no/l)

Sladecek's Q B/T quotient was useful to indicate trophic condition of individual water bodies. According to this quotient values less than 1.0 meant oligotrophy, between 1.0 and 2.0 indicate mesotrophy and those above 2.0 show eutrophy. The values obtained for the lake (Table 4) reflects eutrophic to hypereutrophic nature. More work is still required to designate regional indicator species from different parts of India and elsewhere. Hence, special attention needs to be focused on rotifers to examine their specific ecological requirements.

Table 4. Values of Q B/T quotient

To sum up, the rotifer fauna of the polluted lake appeared rich and diverse. The rotifer communities reflected notable qualitative and quantitative differences in time and between sampled stations in response to habitat variations influenced by marginal and submarginal macrophytes.

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