Rivers and lakes in Africa are under growing pressure from farming. As agriculture expands, fertilizers and pesticides wash into waterways, and the combination can have greater ecological impact than previously assumed.

This thesis investigates what happens to freshwater ecosystems when both types of pollution are present at the same time. Using Kenya’s Lake Naivasha as a field laboratory, the research produces findings that challenge how we assess water quality and ecological risk across inland waters in tropical regions.

The central discovery is that ecosystem effects from fertilizers and pesticides are not simply additive. At lower concentrations they can partially cancel each other out, a phenomenon known as antagonism. But at higher concentrations they amplify each other’s harm (a synergistic effect), damaging aquatic life beyond what either pollutant would cause alone. This concentration-dependent interaction can explain why ecosystems continue to degrade even when pollution levels appear to fall within accepted safety limits. The research also uncovers evidence of illegal pesticide use, including DDT, that is still actively contaminating the catchment today.

The thesis further shows that the biomonitoring tools currently used to assess river and lake health, largely designed for European conditions, perform poorly in tropical Africa. Macroinvertebrate indices that reliably detect nutrient pollution struggle to capture pesticide toxicity, and a single index is never sufficient. The author argues for multi-metric frameworks and mixture-aware water quality standards developed specifically for African ecosystems and farming realities.