Rise and fall of dissolved phosphate in South African rivers

Neil J Griffin

doi:10.17159/sajs.2017/20170020

Abstract

   Eutrophication of water resources following nutrient loading is a global threat to water quality, and has been found to be one of the major threats to water quality in South Africa. Eutrophication is large-scale autotroph growth following nutrient enrichment and has several consequences, including loss of biodiversity, oxygen depletion, taste/odour generation and algal toxin production. Phosphate enrichment is often (but not always) the cause of freshwater eutrophication, and limitation of phosphate is commonly used as a means of controlling eutrophication. This study reports on a survey of trends in nutrient levels in South African freshwater resources. The research reported on here shows a significant decrease in dissolved phosphate levels in recent years, following a long period during which phosphate levels had been increasing with time. While changes in inorganic nitrogen were found, these changes did not match those in phosphate levels. Several potential causes of these changes were assessed, and it is concluded that no one cause can explain the changes observed. While the decrease in freshwater phosphate levels bodes well for water quality management, internal phosphorus cycling and other mechanisms are likely to mask the short-term impact of phosphate decreases.     Significance:     Eutrophication, caused by nutrient loading, is a threat to water quality. Phosphate levels in South African fresh water have risen steadily with time, but recently have shown a sharp decrease, which cannot easily be attributed to one phosphate source. The decrease has promise for eutrophication management but changes may not occur in the short term.   

Highlights

A number of studies have commented on global trends of increasing nutrient loads in fresh water that drive eutrophication of freshwater and coastal ecosystems.[1,2] Eutrophication, recognised as a major threat to surface waters globally, is excessive autotroph growth consequent to nutrient enrichment.[1]
Management of phosphorus levels may be modified by legacy phosphate and other mechanisms[7,8], phosphate limitation is a common approach to managing eutrophication in fresh water
Data on major nutrients in rivers and canals collected at the Department of Water and Sanitation’s (DWS) 333 priority monitoring points were sourced from the Department’s WMS database

Summary

Introduction

A number of studies have commented on global trends of increasing nutrient loads in fresh water that drive eutrophication of freshwater and coastal ecosystems.[1,2] Eutrophication, recognised as a major threat to surface waters globally, is excessive autotroph growth consequent to nutrient enrichment.[1]. Nutrient enrichment effectively removes control over growth and production by limiting nutrients. The predominant paradigm suggests that freshwater systems are phosphorus-limited[3,4], both nitrogen and phosphorus enrichment may increase growth in freshwater systems, and concomitant enrichment of nitrogen and phosphorus may have synergistic effects leading to the greatest increase in autotroph production[5,6]. Control of nitrogen in water resources is confounded by the potential of changes to available nitrogen forms and levels as a result of nitrogen cycling, among other factors. Management of phosphorus levels may be modified by legacy phosphate and other mechanisms[7,8], phosphate limitation is a common approach to managing eutrophication in fresh water. Nutrient limitation is a simple and obvious approach to eutrophication management, and has in some cases been successful.[5]

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Conclusion