MSc

The study entitled ‘Aquatic insects as bio-indicators of water pollution in
Vellayani lake, Kerala’ was carried out at Department of Entomology, College of
Agriculture, Vellayani during the period 2021-2023. The objectives of the study were to
assess the diversity of aquatic insects, to analyse the physicochemical parameters of
water and to detect heavy metals and pesticide residues in water and aquatic insects of
Vellayani lake.
A total of 195 aquatic insects under five orders viz., Ephemeroptera, Odonata,
Hemiptera, Coleoptera and Diptera belonging to 12 families such as Baetidae,
Libellulidae, Gomphidae, Coenagrionidae, Gerridae, Nepidae, Belostomatidae,
Corixidae, Hydrophilidae, Dytiscidae, Curculionidae and Chironomidae were
documented from three sites viz., S1, S2 and S3 of Vellayani lake during October to
December (2022), January to March (2023), April to June (2023) and July to September
(2023). Identity of specimens were confirmed by taxonomists. Study revealed that the
dominant order was Odonata (65) followed by Hemiptera (59), Diptera (49),
Ephemeroptera (17) and Coleoptera (5). The dominant family was Chironomidae (49)
followed by Gerridae (45), Libellulidae (43), Baetidae (17) and Coenagrionidae (16).
Using the data generated on the population of aquatic insects, different indices
were calculated. Margalef’s species richness was calculated and it was 0.96 in S1 which
was considered as severely polluted whereas S2 (1.65) and S3 (1.78) were regarded as
moderately polluted.
Biotic Index (BI) was calculated to assess the degree of organic pollution and
compared with the biotic index range proposed by Hilsenhoff. The BI range of 6.56 to
7.90 was recorded in all sites during entire sampling seasons which indicated poor to
very poor water quality with substantial to severe organic pollution. Shannon-Wiener
Index was calculated to assess the diversity of aquatic insects and the index range was
used to evaluate the level of pollution. According to Shannon-Wiener Index, S1 (0.30 to
1.70) came under highly polluted condition whereas S2 (0.90 to 1.60) and S3 (1.00 to
1.60) under moderately polluted condition.
Physicochemical parameters of water viz., pH, Total Dissolved Solids (TDS),
Electrical Conductivity (EC), water temperature (WT), Dissolved Oxygen (DO),
salinity, calcium, magnesium, nitrates, phosphates, and silicates were estimated
twice/year using suitable protocols. All physicochemical parameters were found to be
within the permissible limits except DO. The DO was low which ranged from 1.04 to
2.20 mg L-1 in all three sites.
Correlation analysis was performed to find out the relationship between
physicochemical parameters of water and abundance of aquatic insects. pH exhibited
negative correlation with Belostomatidae (-0.269), Corixidae (-0.246), Coenagrionidae
(-0.185) and Chironomidae (0.358). Water temperature showed positive correlation with
Gerridae (0.349) while negative correlation with Libellulidae (-0.712) and
Coenagrionidae (-0.044). Dissolved oxygen exhibited negative correlation with
Chironomidae (-0.297). TDS and EC showed positive correlation with Gerridae (0.149,
0.399) while negative correlation with Hydrophilidae (-0.417, -0.270). Phosphate
exhibited positive correlation with Baetidae (0.132) whereas negative correlation with
Dytiscidae (-0.197).
Heavy metals were analyzed in water, sediments and aquatic insects once/year
using ICP-AES. Fe, Mn, Zn, Cu, Cd, Cr, Pb, Ni and Al were within the standard limits
in three matrices. Geo-accumulation index was calculated to determine the magnitude
of heavy metal pollution in sediments and since the values were <1, no heavy metal
pollution was noticed. Bioaccumulation Factor was calculated by dividing the metal
concentration in insects by sediment metal concentration and was well within the limit
which revealed no heavy metal pollution in three sites in Vellayani lake.
Pesticide residue analysis was carried out twice/year in water, sediments and
aquatic insects from S1, S2 and S3. The mean residue level was below the limit of
quantification in all sites.
To conclude, among the twelve families documented, Libellulidae, Gerridae and
Chironomidae were the dominant ones, and they were considered as the effective bioindicators of water pollution. The dominance of these families and the values of various
indices revealed that the Vellayani lake water is moderately polluted, which was
substantiated by low DO. No heavy metal pollution and pesticide residue contamination
were observed in any of the sites in Vellayani lake