Do diurnal dissolved oxygen cycles have diagnostic value for assessing pollution status in lakes?
The effects of pollutant burdens on natural cyclic patterns in aquatic ecosystems have been studied, to test the hypothesis that a parameter or parameters describing the pattern of variation in a single determinant, such as DO or pH, may be used to describe the status of a body of water more meaningfully than results from single spot measurements. In this study the background water quality in a Scottish and a Ghanaian lake was assessed, then rafts were built on the lakes (Myrtle Dam and Weija Lake respectively) creating three small lakes to test the effects of pollutant loads (e.g. organics and nutrients) on them. Laboratory investigations to study the behaviour of the pollutants on DO or pH cycles were carried out, and then a dynamic DO model was developed to describe the cyclic behaviour of DO. Finally, the model was used to investigate the effects of added pollutant burdens on the DO cycles. Water quality assessment of Myrtle Dam and Weija Lake by using the Weighted Water Quality Index proved that Weija Lake has doubtful quality with WQI 51, which needs improvement. Myrtle Dam WQI of 73 shows that the lake waters are unpolluted (or recovered from pollution). It is also well oxygenated at the surface and receives no toxic discharge. The raft experiments for Myrtle Dam and the Weija Lake have been used to demonstrate that, in the raft tubes, organic loadings do have effects on the oxygen cycles, and showed that there are two effects. One is that, with a small organic loading, the mean DO value dropped, then increased as the organic loading was consumed. The second is that the amplitude of the DO cycle increased as the organic loading increased from 2.5 to 5.0 mg L-1 TOC for Myrtle Dam, and decreased for Weija Lake as the organic loading increased from 2.5 to 5.0 mg L-1 TOC. The laboratory experiments in both Ghana and Aberdeen proved to be useful in explaining the effects of nutrients and trace elements on the DO cycles. It was evident that the higher the algal concentration, the greater was the DO cycle and 30% v/v (c.a. 254 mg L-1 chlorophyll 'a' content ≍ 17 g L-1 biomass aqueous algal suspension) proved to be the most suitable concentration for investigations, because, with higher organic loadings, the 100% aqueous algal suspension proved to be too concentrated and caused total deoxygenation.