Evaluating irrigation effects on wetland health using benthic macroinvertebrate indicators in drought-prone area of Kayonza district, Rwanda

Abstract

Freshwater ecosystems are multi-functional ecosystems, and whole communities are dependent upon their productivity and hydrological benefits. In Rwanda, the response of farmers to the pressure on land and the long dry season associated with a decline in productivity has been to expand their agricultural activities into these fragile wetlands. Human activities that demand wetland management, water diversion and damming, land alteration and use of agrochemicals, impose negative impacts not only on wetland value but also on the whole ecosystems particularly organisms depending on wetlands via stream change. With 11% of the total land of the country, Rwanda’s wetlands are the fastest lost and degraded compared to any other ecosystems. In Rwanda, irrigation systems are being implemented to address drought caused by climate change, aiming to reduce dependency on rain-fed agriculture, particularly in the Eastern Province. However, the effects of irrigation on wetlands are still not well documented; we aimed to evaluate the effect of irrigation on wetland health using benthic macroinvertebrates. With the spot sampling method, we identified the BMI community and indicator species of wetland health and we identified potential interaction between benthic macroinvertebrates and wetland use practices; we developed a baseline for monitoring wetland health based on indicator species in the study areas. As BMI are sedentary organisms sensitive to threats to wetland ecosystems, their assemblage gives information on biotic and abiotic factors interacting with them. This provides ecological information on the wetland ecosystem health upstream, downstream and within the dam differently during low and high precipitation. Precipitation periods combined with irrigation activities influence the distribution of pollutants from human activities, and this in turn impacts BMI assemblages in streams, especially related to adjacent land use practices. This was observed in areas dominated by agriculture where BMI tolerating agrochemical pollution was abundant while BMI affected by that pollution was found in areas of less agriculture and livestock activities. EPT sensitive group of BMI was mostly from upstreams of areas with less agricultural activities while tolerant were highly recorded downstream in areas dominated by agriculture. The poorer the stream health, fewer sensitive species were found during the sampling and the presence of numerous individuals of highly tolerant organisms is expected. There are opportunities to improve the management of irrigation practices, such as avoiding cement-lined water channels, restoring riparian zones, raising skills in the use of agrochemicals and reducing polluting run-off into wetlands; that support healthy functioning ecosystems. BMI are shown to be effective indicators of water quality in this wetland system.