Zooplankton diversity of Loktak Lake , Manipur , India

Zooplankton communities of Loktak Lake showed rich and speciose biocoenosis (162 and 142 species), high monthly richness (91 ± 13 and 80 ± 10 species) and by higher similarities (51.1–82.0 and 51.8–78.3 %) and peak richness during winter and autumn over two years of study. Zooplankton (267 ± 41 n/l) formed a significant quantitative component (56.0 ± 6.3 %) of net plankton and showed annual peak abundance during winter. Rotifera > Cladocera are dominant quantitative groups while Copepoda > Rhizopoda are sub-dominant groups. We observed significant annual and monthly variations of zooplankton richness and abundance. This study showed limited influence of individual abiotic factors on zooplankton, with richness showing a significant inverse correlation with water hardness and chloride, and abundance inversely correlated with nitrate. Multiple regressions indicated higher cumulative effects of 15 abiotic factors on richness and abundance. Our results exhibited no definite periodicity of richness and abundance of zooplankton and their constituent groups during two annual cycles. Zooplankton is characterized by highest species diversity (4.172 ± 0.237), higher evenness and lower dominance.


INTRODUCTION
Zooplankton are integral components of aquatic food webs and contribute significantly to aquatic productivity in freshwater ecosystems.They have been studied from various inland aquatic environs of India, but a review of the limnological literature indicates limited information on their composition, ecology and role in aquatic productivity in the floodplain lakes in particular (Sharma & Sharma 2008).The related contributions from the floodplain lakes of northeastern India (Sharma & Hussain 2001;Sharma 2005;Sharma & Sharma 2008) are as yet restricted to the beels of the Brahmaputra river basin of Assam.
The present study on synecology of zooplankton of Loktak Lake assumes special limnological significance in view of a lack of investigations in the floodplain lakes (pats) of Manipur.The observations are made on temporal variations in richness, community similarities, abundance, species diversity, dominance and evenness of zooplankton of this important floodplain lake, a Ramsar site of India.In addition, the influence of abiotic parameters on richness and abundance of zooplankton are analyzed.

Abiotic parameters
Mean water temperature affirms sub-tropical range of Loktak Lake.Slightly acidic and soft waters of this Ramsar site are characterized by low specific conductivity (

Zooplankton richness and community similarities
Among 189 species of Zooplankton documented from Loktak Lake (Sharma unpublished), 169 species observed at the sampled site reflect speciose and diverse nature of their biocoenosis.Zooplankton comprise dominant qualitative component of net plankton (237 species) and significantly influence temporal variations of the latter (r = 0.960, p < 0.0001).Overall zooplankton richness is the highest known till date from any floodplain lake or any individual aquatic ecosystem of India and, hence, reflects greater environmental heterogeneity of this Ramsar site.The richness is, however, notably higher than an unpublished report of 55 species (including undetermined and doubtful species) from this lake (Singh 1991).Besides, it is distinctly higher than the records from other Indian floodplain lakes i.e., 51 species (Khan 1987) and 26 species (Yousuf et al. 1986) from Kashmir; 19 species (Baruah et al. 1993) and 31 species (Sanjer & Sharma 1995) from Bihar; 49 species (Sharma & Hussain 2001) from Assam, and 71 species (Khan 2003) from West Bengal.Qualitative dominance of zooplankton in net plankton communities of Loktak Lake concurs with the findings of Sharma & Sharma (2008) but differs from higher phytoplankton richness observed by Baruah et al. (1993), Sinha et al. (1994) and Sharma & Hussain (2001).In general, zooplankton biocoenosis exhibits typical 'tropical character' and greater richness of cosmopolitan species while cosmotropical and pantropical species are well represented.

Table 2. Temporal variations of Zooplankton of Loktak Lake
throughout first year except during July.In general, this study records (Figs. 1 & 2) relatively lower number of species from February-July during first year while this trend is noticed from January-August in the succeeding year.Individual abiotic factors exert limited influence on zooplankton richness; it registers only significant negative correlations with hardness (r = -0.650,p = 0.0011) and chloride (r = -0.723,p = 0.0002).On the other hand, multiple regression (Table 3) indicates higher cumulative effect of 15 abiotic factors on richness (R 2 = 0.863).

Species diversity, evenness and dominance
Zooplankton show the highest species diversity (3.750-4.639,4.172±0.237)so far known from any floodplain lake or aquatic ecosystem of northeastern India and elsewhere from this country (Sharma unpublished).This salient feature affirms higher environmental heterogeneity of Loktak Lake.It registers significant monthly (F 11, 23 = 3.762, p < 0.01), shows (Table 2) relatively higher values (4.186±0.278)during the second year and exhibits broadly bimodal but different annual patterns (Fig. 8) with peaks during winter (December 2002) and post-monsoon (October 2004) and minima during summer (May).In general, higher species diversity (< 4.0) is observed during November-February, July and September-October during both years and again during May and June (2 nd year) and August (1 st year).The notable feature of higher species diversity with relatively lower densities of majority of species noticed in this study may be ascribed to fine niche portioning amongst zooplankton species in combination with high micro-and macroscale habitat heterogeneity as hypothesized by Segers This study depicts (Table 2) higher evenness (0.813-0.992, 0.939±0.032) of zooplankton; this feature affirms equitable abundance of various species and concurs with the results of Sharma & Hussain (2001) and Sharma & Sharma (2008).It follows (Fig. 9) bimodal and multimodal annual patterns with peaks during August and October during the two years respectively while minima are noticed during summer (May).Zooplankton show (Table 2) lower dominance (0.044-0.170, 0.079±0.033)indicating lack of quantitative importance of individual species.This feature again concurs with the results of Sharma & Hussain (2001) and Sharma & Sharma (2008).It registers only significant monthly variations (F 11, 23 = 3.896, p < 0.01), and follows (Fig. 10) different annual patterns with maxima during April and May during two years respectively.
To sum up, zooplankton form important qualitative and quantitative components of net plankton, exhibit highly speciose character with richest diversity and quantitative dominance of Rotifera > Cladocera, and indicate no definite periodicity of richness or abundance of zooplankton or their constituent groups.zooplankton are characterized by highest species diversity, higher evenness and lower dominance, and exhibit lower densities of majority of species.This study registers limited influence of individual abiotic factors but records higher cumulative influence of 15 abiotic factors on richness and abundance.The present observations limited to one sampling station, though provide useful information on composition, production and ecology of zooplankton yet may not reflect full view of heterogeneity of this interesting Ramsar site.
Further studies in different parts (pats) of Loktak basin are, hence, desired and have been initiated.

Figure 5 .
Figure 5. Monthly variations in abundance of zooplankton

Figure 6 .
Figure 6.Monthly variations in abundance of different groups of zooplankton (2002-03) Figure 8. Monthly variations in species diversity of zooplankton

Table 1. Abiotic factors of Loktak Lake concentrations
. Our results also indicate moderate dissolved oxygen, low free CO 2 , low concentrations of micro-nutrients and other abiotic factors.