Effects of wetland conversion to farming on water quality and sediment and nutrient retention in a tropical catchment

The study used a combination of landscape-scale synoptic surveys (catchment, reaches) and mesocosm surveys (experimental plots) to assess the impacts of conversion of natural valley-bottom wetlands to farming land on the water quality and retention of sediment and nutrients. The results showed that temperature, pH, electrical conductivity and dissolved oxygen concentration decreased, and total suspended solids (TSS) increased with storm water increase. Nitrogen (TN) and phosphorus (TP) accumulated in the catchment during the dry season and washed into the water courses during the early stages of the higher flows, with subsequent lower concentrations at the end of the rains due to dilution. Large proportions of the annual loads of TSS, TP and TN (93%, 60% and 67%, respectively) were transported during rainfall events that occurred in 115 days. Fishponds acted as temporal traps of TSS, TN and TP at the early stages of farming, and were a source of and TN and TP at the end of the farming period, in contrast to rice farming that generated sediments and nutrients early in the farming period and trapped them at the end of the farming season. Wetlands mostly acted as sinks but sometimes as a source of sediment and nutrients.</p

Fluxes and retention of sediment and nutrients in valley bottom fish and rice farms and wetlands : impacts on surface water

This study compares nutrient and sediment retention among rice and fish farms and wetlands in valley bottoms in southern Rwanda. Small-scale wetland, rice and fishpond experimental systems were established to measure sediment, nitrogen (N) and phosphorus (P) fluxes during a 9-month period. There were significant differences in the processes contributing to sediment, N and P retention in the three systems related to system characteristics, management practices, and seasons. Overall nutrient retention was higher in the rice and fish systems, but these systems had higher inputs and outputs of sediment and nutrients. In rice plots, resuspension and discharge of sediment and nutrients to the outflow were caused by ploughing and weeding during the first 3 months of the culture period. In fishponds, nutrients and sediment discharge were associated with water renewal and sediment removal during the last 5 months of the farming period. The undisturbed wetland plots had the lowest outflows of sediment and nutrients. Nutrient uptake and accumulation in biomass was much higher in rice and wetland vegetation than in fish biomass. In fishponds and wetlands, nutrients accumulated in soil, whereas rice plots showed a decrease or depletion in nutrient storage. To increase nutrient utilization at the plot level, sediment and nutrient discharge from land preparation and rice transplanting should be reduced by better farm practices. Within a catchment, nutrient flows can be integrated by using fishpond sediments for crop farming, by incorporating natural wetlands in crop rotations or using them as buffer zones