Impacts of agricultural activities on the diversity of aquatic macroinvertebrates as an indicator of ecosystem health along the Lower Komati River, Swaziland

Abstract

At present, sugar cane production requires more land than any anthropogenic activity in the lower Komati catchment. Despite its threat to water quality, not much is known about the impact of the increased agricultural land for sugar cane production on ecosystem health within the subcatchment. The main objective of this research was to assess macro-invertebrates diversity as an indicator of aquatic ecosystem health in the lower Komati River as well as determine whether this diversity is a significant function of physico-chemical water quality parameters. Richness, Shannon -Weiner and Simpson’s diversity indices were used to analyse and determine aquatic macro-invertebrates diversity. Water samples were also collected and analysed for pH, dissolved oxygen, electrical conductivity, turbidity, nitrates, ammonia and ortho-phosphates according to standard methods. Results show that diversity along agricultural fields decreased in diversity but increased at certain lengths after sugar-cane fields. However, the difference was not significant (p>0.05) between sites. It was found that some water quality parameters such as turbidity, dissolved oxygen, and ortho-phosphates showed a significant difference (p<0.05) between sites upstream and adjacent to the sugar cane fields. However, no significant relationship (p>0.05) was found between diversity and water quality parameters except turbidity. Based on PCA analysis and relating the PCs to diversity of aquatic macro-invertebrates results showed that a combination of increased turbidity and dissolved oxygen coupled with a decrease in temperature significantly explained the diversity along the Lower Komati River. It can be concluded that agriculture for sugar-cane production seem to have an limited influence on aquatic macroinvertebrates diversity due to increased turbidity which was shown to be the primary water quality parameter influencing diversity in the Lower Komati River. The study therefore recommends that further studies be undertaken during the winter season when flow has reduced to determine concentration of nutrients and the response of the macro-invertebrates during the winter season when the flows have been reduced.