The Svržnica canal was initially built as a second-order melioration canal to drain surface water from agricultural land. However, in the early 1980s, the canal was repurposed as the primary recipient of untreated municipal wastewater from the settlement of Donji Andrijevci. Ecophysiological research was conducted from June to December at four locations in the southern part of the canal to increase public interest in adequate management of this water resource: pre-discharged site (S1, high abundance of macrophytic vegetation); immediately after the discharge of municipal wastewater (S2, complete absence of macrophytes); oak forest (S3, ruderal coastal vegetation and weak representation of macrophytes); and site at the mouth of the canal into the river Biđ (S4, significant macrophyte cover). Research has shown that untreated municipal wastewater significantly contributes to increased concentrations of nutrients, total organic carbon, and bacterial pollution, especially fecal pollution. Macrophytic vegetation and natural water purification processes downstream of the wastewater discharge gradually reduce the microbiological and nutrient loads. Nevertheless, the biological quality elements and the physico-chemical elements of water quality point to the poor ecological condition of the Svržnica canal. Lemna test showed that a lack of nutrients (before the discharge) can limit plant growth. In contrast, high concentrations of nutrients (downstream of the discharge) can contribute to the better resistance of duckweeds to toxicants and pathogens. Therefore, the Lemna test showed to be a valuable tool that can help assess the ecological health of aquatic ecosystems.