ABSTRACT
Agricultural development is critical for economic growth and food security. However, sediment and nutrient runoff generated by farming activities may constitute an important source of pollution and water quality deterioration. The objective of this study was to assess the effects of the conversion of wetlands to agriculture on the water quality in valley bottoms in the Migina catchment, southern Rwanda. Three valley bottom land use/land cover (LULC) types (fishponds, rice and wetland plots) were studied in a replicated mesocosm setup. LULC characteristics (hydrology, biomass growth) and farming practices (land preparation, water use, feed/fertilizer application) and the associated changes in total nitrogen (TN), total phosphorus (TP), total suspended solids (TSS), dissolved oxygen (DO), conductivity (EC), pH, and temperature were measured during two periods from 2011 to 2013. Results showed that fish farming used 7–8 times less water than rice farming (101 mm m−2 d−1 compared with 191 mm m−2 d−1). Biomass increase in fish farming was much lower (30 g m−2 in 8-months) than in rice (2500 – 4500 g m−2 in 4 months) and wetland plots (1300 – 1600 g m−2 in 8–11 months). Over the two seasonal periods studied, higher concentrations of TSS, TP and TN in inflows and outflows were mainly associated with human activities (cleaning of water supply canals, rice plot ploughing, weeding and fertilizer application, and fishpond drainage and dredging). As a result, fishponds and rice plots generally had consistently higher TSS concentrations in surface outflows (5-9506 and 4-2088 mg/L for fishponds and rice plots, respectively) than in inflows (7-120 and 5-2040 mg/L for fishponds and rice plots, respectively). For TN and TP, results were more mixed, but with peaks associated with the periods of land ploughing, weeding and fertilizer application, fishpond drainage and dredging. In wetland plots, TSS and TN significantly decreased from the inlet to the outlet, owing to the absence of disturbances of the plots and probably other mechanisms (higher settling/adsorption, nutrient uptake and denitrification). Adoption of conservation farming techniques and efficient use of water and fertilizers would promote environmental protection and the sustainability of agricultural production. It is worth exploring the integration of fishponds for temporary storage of water, sediments and nutrients for reuse in crop farming, and of natural wetlands as buffer zones for sediments and nutrients from farming effluents during the critical periods of agricultural activities.
