Bionomics of the chironomidae of Barekese and Owabi reservoirs as a basis for formulating strategies for the control of the nuisance species

Chironomidae of two raw water reservoirs at Barekese and Owabi, in the Kurnasi Metropolis, were studied over an 18-month period with the objective of identifying the nuisance species. Seasonal variations of the physico-chemical parameters of the two reservoirs were examined with respect to the abundance, diversity and production of the five common chironomid species, namely, Nilodorurn fractilobus, Nilodorum brevibucca, Nilodorum brevipalpe, Dicrotendipes schoutedeni and Kiefferulus chioronotus. All together, seventeen chironomid species were recorded from the two reservoirs, three species more than previously reported by other workers. The large fluctuations observed in reservoir water level at Barekese were attributed to the higher rate at which raw water was extracted. Mean surface water temperature at Owabi and Barekese was 28.0°C and 27.9°C respectively. Surface water temperature exhibited seasonal fluctuations with the highest temperatures occurring in March, during the dry, humid season and the lowest in August during the rainy season, when the sky was heavily overcast. Water from both reservoirs was slightly alkaline (pH = ~ 7.0) and well buffered (alkalinity > 24.0 mg. 1-1 CaCO3). Concentration of nutrients in both reservoirs was high. Mean concentration of ammonia-nitrogen (NH3-N) was equal to that of nitrate-nitrogen (N03-N) but was greater than nitrite-nitrogen (NO2-N). Thus, the dominant pattern of nutrient concentration was of the order NFI3-N = NO3-N > NO2-N. Generally, the nutrients, major cations and anions followed a seasonal trend. Higher concentrations occurred at the start of, or during the rainy season whereas lower concentrations occurred during the dry season. In general, the concentration of calcium was greater than magnesium (i. e. Ca 2+ > Mg 2+), and Conformed to the pattern associated with fresh waters. Concentration of chloride in reservoir Water was 19.5 mg. 1-1 and 22.7 mg. 1-1 for Barekese and Owabi respectively. Chloride Concentration was higher than the limits (<10mg 1-1) found in pristine waters. Sulphate (SO4 2- ) was also higher than 10 mg 1-1 . The reservoirs were, therefore, classified as receiving organic pollutants. Chironornid production was estimated to be 170.66 ± 23.25 g m-2 yr-1 and 41.00 ± 55.40 gm-2 yr -1 in Barekese and Owabi reservoirs respectively. The production of Chironomidae in Barekese and Owabi reservoirs is also equivalent to 827.7 and 198.9 cal. of energy respectively that is locked up in the various species but is not utilised by the biocoenosis. The abundance of Chironomidae in the two reservoirs was related to thirteen physico-chemical variables, viz, dissolved oxygen (DO), Biochemical Oxygen Demand (BOD), chlorides (C1-), Acidity, ammonia-nitrogen (NH3-N), nitrite as nitrogen (NO2 -N), nitrate as nitrogen (NO3 -N), phosphate (PO4), chlorophyll a (Chl. A.), calcium (Ca2+ ) Turbidity, Conductivity and Total Hardness. Nine significant prediction models described these relationships (p < 0.05). However, there was no significant model that related abundance of Dicrotendipes schoutedeni to the physico-chemical characteristics of the waters of Barekese. Based on the observations made, during the study, a strategic plan for the control of the nuisance species in both reservoirs is provided. The plan involved the use of chemicals, biocides, and the introduction of fish species to exploit the vast amount of energy locked up in the nuisance chironomid species. The plan also recommended better management of the watershed by controlling inputs of nutrients from point sources.

Source: KNUST