Effect of Waste Heat from Egbin Thermal Plant on the Plankton and Macrobenthos of Lagos Lagoon

Abstract

The effect of waste heat discharge on the water chemistry, plankton and benthic fauna in the Lagos lagoon at Egbin was investigated for six months (February – July 2000) over three dry month (February - April) and three wet months (May - July).Water temperature were high (270C - 400C) and decreased spatially from station A to C. Transparency was higher in the dry months than wet months. The highest transparency (196cm) was recorded in May at station A while the lowest (66cm) was recorded in June at station B. Similarly, the highest Total Dissolved Solids (394mgL-1) was recorded in July. The site was acidic (<6.93) all through the sampling period Conductivity (<2.5cm-1) and salinity (<12.1‰) values followed seasonal trends and were higher at station B. The lowest dissolved oxygen value (3.2mgL-1) was recorded in May while higher nutrient values (NO3-N 3.98mg/L-1, PO4-P 0.64mg/L-1, SO4 1135mg/L-1) were recorded in March at station B. Heavy metals and oil and grease values were higher in the dry months. The biota spectrum comprised 83 phytoplankton species, 23 zooplankton species and five benthic fauna. High cyanobacteria specie, richness was recorded in elevated water temperatures (>270C), high transparency and low dissolved oxygen content (3.50 mg/L-1) at stations A and B. Plankton species were more abundant in the wet than dry months. Aulacosera, Spirogyra, Acartia, Paracalanus and Cyclops were prevalent plankton forms while Pachymalania, Tymapanotho and Aloides were frequent benthic taxa especially in the wet months. The high number of empty shells was more discernable in stations A and B.