ECOLOGY OF AQUATIC INSECTS IN OPI LAKE, ENUGU STATE, NIGERIA.
ABSTRACT
The ecology of aquatic insects of Opi Lake was carried out to determine their composition, abundance and diversity from February to July, 2014. Adult insects of different species were collected from the water surface using a dip-net with Nytex® netting of 500µm mesh. In addition, adult Insects and nymphs were collected from the vegetation around the lake using a sweep net with mesh size of 250µm, while bottom dwellers were sampled using a scoop net. The lake was divided into three sampling stations as a result of the nature and amount of the vegetation, and the type of substratum found in each location. Station 1 had vegetation, shade and detritus, Station 2 had no shade, very little detritus and vegetation, while Station 3 had shade, detritus with no vegetation. The physico-chemical parameters and heavy metals concentrgations of the lake were determined while the climatic data of the area was collected from the Center for Space Science University of Nigeria Nsukka.A total number of 1,042 insects representing 30 species, belonging to 26 families and 8 orders were recorded. Odonata had the highest mean abundance (44.52%) in all the stations, followed by Hemiptera (23.32%) which was the most diverse group. Hemiptera had the highest number of families (8 out of the 26 families collected). Other insect orders collected with their abundance include: Coleoptera (12.28%), Orthoptera (10.29%), Hymenoptera (5.09%), Diptera (3.36%), Trichoptera (1.06%) and Lepidoptera (0.01%). Station 1 recorded the maximum number (46.35%) of aquatic insects throughout the sampling season. However, stations 2 and 3 recorded 28.98% and 24.66% of aquatic insects respectively. The abundance of insects was maximum in the month of July (20.44%) and minimum in April (8.16%). The abundance and distribution of insect species varied and were not constant from one month to another during the period of study, due to biotic and abiotic factors. There was high species diversity of aquatic insects in the different strata of the lake, indicating the rich and diverse group of insects in the study area. Dissolved Oxygen had an inverse relationship with Orthoptera (r = -0.63, p < 0.01) and Hymenoptera (r = -0.54, p < 0.05. Diptera also had negative relationship with depth (r = -0.48, p < 0.05). There was positive correlation between Hemiptera and Copper (r = 0.78, p < 0.01), while Iron also correlated positively with Coleoptera (r = 0.47, p < 0.05) and Lepidoptera (r=0.59, p < 0.05).Among the insects and zooplankton, Odonata had positive correlations with Rotifera (r=0.502, p < 0.05), Cyclops (r = 0.541, p < 0.05), Bosmina (r=0.53, p < 0.05) and Daphnia (r = 0.595, p < 0.01). Orthoptera also showed positive relationship with Fish egg (r = 0.684, p < 0.01). Also, with phytoplankton, Odonata had positive relationship with Chlorophycaea (r = 0.505, p < 0.05) and Xanthophycaea (r = 0.499, p < 0.05). Orthoptera correlated positively with Cryptophycaea (r = 0.491, p <g 0.05) and Xanthophycaea (r = 0.487, p < 0.05).This therefore, adds to the fact that undisturbed habitat quality is more suitable for insects to breed and multiply under the natural ecosystem with abundant food supply.