Biomonitoring Macrophytes Diversity and Abundance for Rating Aquatic Health of an Oxbow Lakeecosystem in Ganga River Basin

Objective: Evaluation of the biological community of a water body provides sensitive and cost effective means of assessing its water quality. Aquatic macrophytes diversity and its role in understanding the oxbow lake ecosystem dynamics have tremendous significance. The study is aimed for quantitative biomonitoring the ecological quality through macrophytes diversity.
Methods: A survey of macrophytes communities of freshwater Chhariganga oxbow lake of West Bengal in eastern India along with analysis of diversity indices, dominance trends with importance value index (IVI) and composition trends was carried out.
Results: Estimating annual average IVI values, we found that emergent (154.91±12.59) were dominant, followed by the freefloating (78.22±25.12), submerged (37.78±11.82) and rooted floating-leaved genus (29.09±13.84). Among emergent, Cynodon sp was the most dominant during pre monsoon and post monsoon and Ipomoea sp during monsoon. Among the free-floating genus, Eichhornia sp was highly dominant throughout the year while Lemna sp topped the group during pre monsoon. Among the submerged genus, Hydrilla sp was observed to be the most dominant genus throughout the year. Among rooted floating-leaved Nymphaea sp was highly dominant during monsoon and post monsoon and Brasenia sp during pre monsoon. Highest values of density (120no/m2), average area coverage (69.34%) and average wet biomass (1816.72 g/m2) were observed during post monsoon while lowest values of density (27no/m2), average area coverage (38.95%) and average wet biomass (281.78g/m2) were estimated during the monsoon. We found genus richness values higher during post (44) and pre (36) monsoon and lower in monsoon (22) with mean genus Evenness value of 0.79±0.05. In our study lower average diversity values of Shannon Wiener index (2.74 ±0.36) and Simpson index (0.90±0.04) throughout the year render poor to moderate pollution status to the oxbow lake while presence of certain bio-indicators species like Lemna, Eichhornia, Myriophyllum, Potamogeton, Trapa, Marsilea and Cyperus also confirm pollution. Similar pollution status was observed when assessed with rotifer, whole zooplankton and macroinvertebrate diversity indices on the same oxbow lake during the same period. The growth of the macrophytes reveals the lower productive nature of the lake. The Throughout dominance of emergent among other growth forms indicates the slight encroachment of littoral vegetation, reducing the core area of the lake and showing the slow trend of succession towards marsh meadow condition. Lower richness values also do affirm its oligotrophic status. Conclusion: We find selected lake has much anthropogenic activity and not suitable for fish growth especially during monsoon. So we opine that highest priority must be given to removal of macrophytes especially water hyacinth; and regulation and prevention in jute retting process, intensity and density are needed for its sustainable management and conservation of aquatic environment.

Author(s): Dipankar Ghosh and Jayanta Kumar Biswas

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