Diversity and Density of Aquatic Insects in the Lower Reach of River Moirang, Manipur, North East India

Abstract

A study on the diversity and density of aquatic insects in the lower reach of river Moirang, Manipur, India was conducted using aquatic insects as bio indicator to ascertain the pollution status of the river as it discharges into the Loktak lake (Ramsar site), the lifeline of people of Manipur. The study revealed the presence of three orders, nine families, twelve genera and eighteen species of aquatic insects in the lower reach of the river. The orders were Hemiptera, Odonata and Coleoptera. There was total absence of sensitive groups of aquatic insects like Ephemeroptera, Plecoptera and Trichoptera. According to Engelmann’s scale, Diplonychus rusticus (Fabricius) was found eudominant followed by five subdominant and six recedent species. No species was found in the dominant range. Species diversity and evenness indices fluctuated throughout the year and Shannon H′<1 in all the months indicated a stressed and disturbed environment. This is confirmed by the record of low dissolved oxygen (DO), high biological oxygen demand (BOD) and high phosphate in the water of the river. Among the heavy metals, the ranges of both lead and mercury were found to exceed the WHO maximum permissible limit. Significant positive correlation of rainfall with pH, DO, BOD, nitrate, phosphate, Berger Parker dominance index (d) and significant negative correlation with diversity index (Shannon H′) indicated that rainfall is one of the important parameters in the ecology of the system. Canonical correspondence analysis (CCA) revealed that the important physico-chemical attributes of the ecosystem were air temperature, water temperature, depth, rainfall, BOD, DO, salinity, sodium, total alkalinity, iron and lead.