Insight into the Changes of Urban Wetland Biogeochemical Functions in Rwanda: Variation of Nutrient Levels in Rubirizi Wetland, Kigali City

Wetlands are important in terms of supporting biodiversity, cycling of nutrients and the filtration of pollutants, but urban development is threatening the role played by these wetlands. This research projects the interaction of the concentration of nutrients and associated biogeochemical indicators over space and seasons in Rubirizi Wetland, Kigali City. The water was sampled on three occasions (inflow, midstream, outflow) between September 2024 and February 2025 and measured in terms of pH, COD, BOD, TSS, total nitrogen (TN) and total phosphorus (TP). Geospatial mapping determined the routes of flow dominance and possible sources of pollution. Findings indicate that the runoff and domestic wastewater are the major sources of pollutants: COD and BOD maximized at the inlet during the rainy season (mostly in November), whereas filtration adequacy in the wetland reduced COD/BOD at the outflow in the dry season. The highest TSS values were observed in high-runoff months and the lower ones were observed in drier months. High TN and TP at outflow points are signs of accumulation of nutrients and eutrophication. The majority of the parameters were within the WHO standards, and there were excesses of TN (September, November), COD (November) and BOD (some rainy-season and a few dry-season points). The trends suggest that there is a powerful seasonal effect and intense impacts of adjacent residential activities. We suggest specific wastewater control, local education, and regular surveillance to safeguard the ecological processes of Rubirizi and decrease the risk of eutrophication.