Water Purifying Capacity of Natural Riverine Wetlands in Relation to Their Ecological Quality

Wetlands are among the crucial nature based solutions for river water purification. The ecological quality of these multi-purposed ecosystems could be disrupted due to overloading of nutrients and other pollutants. We investigated the water purifying efficiency and ecological quality of three natural riverine wetlands located in urban area in southern Ethiopia. Of the three wetlands, Boye wetland is located in a densely populated area with intensive human activity, and the other two, Fisho and Kitto located in a relatively less populated area of Jimma town. We sampled water, macroinvertebrates, and diatoms, to compare sites before joining the wetland, within the wetland and after passing through the wetland. Considering both seasons, up to 74% TP, 73% DIN, and 77% BOD reduction was recorded. The lower concentration of nutrients, and BOD in sites after joining the wetland showed the presence of pollution attenuation. Macroinvertebrate and diatom based bio-indices depicted higher biodiversity and lower relative abundance of tolerant taxa in sites after joining the wetland, which implied the potential of studied wetlands to reduce pollutants and sustain biodiversity. The incoming streams of Boye showed bad water quality and heavily degraded ecological status (Ethbios score 8–10). Most of the sites associated with Boye wetland depicted a major ecological disturbance (Ethbios score < 44). The incoming streams of Fisho revealed moderate (Ethbios score = 58) and poor (Ethbios score = 20) water quality. Most of Fisho sites had a moderate water quality with significant ecological disturbance. Sites associated with Kitto wetland, had a slight ecological disturbance (Ethbios score of 93 and 72) and some with significant ecological disturbance (Ethbios index score between 45 and 59). The wetland fed by heavily polluted streams showed the most degraded ecological quality compared to the other two that are fed by less polluted streams. An RDA model visualized pollution gradient among sites. Generally, this study confirmed the potential of natural wetlands to mitigate nutrients and organic pollutants and sustain biodiversity. However, when the incoming water is heavily degraded, the retention of pollutants seriously affect the wetland’s ecological quality.