engleski

Estimation of the wastewater impact on the Krka River by Daphnid Acute toxicity testing

INTRODUCTION: In addition to chemical water analyses, assessment of water quality might involve toxicity testing, as a biological tool that reflects toxic impact on aquatic organisms. The commonly used testing organism is crustacean Daphnia magna Straus, 1820, which is sensitive to a wide range of contaminants and important consumer in the food chain. Water quality was assessed in the karst Krka River (Croatia), whose lower part was proclaimed national park due to its exceptional natural beauty. Only 2 km upstream from the northern border of the park industrial and municipal wastewaters from the Town of Knin are released in the Krka River without proper purification. Their impact on the river water was assessed at five sites, Krka River source (KRS) as reference location and at four locations downstream of the wastewater impact (industrial (IWW) and municipal (MWW) wastewaters, Orašnica (TOR) and Butišnica (TBU) rivers near outlets) in April 2021. OBJECTIVES: The potential toxicity of industrial and municipal wastewaters and nearby Krka River water was assessed by an acute toxicity test in order to estimate pollution impact on aquatic organisms and establish protection plans. METHOD / DESIGN: Acute toxicity was evaluated after hatching of the ephippia in a specific media supplemented with salt nutrient (Fig. 1a). The obtained neonates were used immediately for ecotoxicity testing of water samples from five sites, which was manifested as an immobilization (meaning death) of the crustacean. Probit analysis was used to calculate 50% toxic effect thresholds (EC50) for 24-48h exposure and the lowest ineffective dilution (LID) for which at least 90% of Daphnids are mobile. Potassium dichromate (K2Cr2O7) was applied as a reference toxicant. RESULTS: The percentage of mobile daphnids was up or equal to 90% for all freshwaters except IWW in the first 24h in both seasons. After 48h all daphnids remained alive in water from KRS and TBU, justifying good ecological status of these locations. For TOR and MWW, 5% of daphnids were immobile in the 50% diluted and 20% were immobile in the non-diluted freshwater, resulting in LID= 2, which confirmed moderate pollution in sites nearby wastewater outlets. The highest impact on daphnids was found for IWW, in which a decreasing vitality rate in increasing concentration of the sample was observed. Moreover, the lowest ineffective dilution calculated by Probit analysis was significantly higher compared to the other samples (LID= 18, 48h). Such higher mortality was in agreement with the poor physico-chemical water parameters and a blackish color, high viscosity and strong odor related to fuel oil (Fig. 1b) at site IWW. CONCLUSIONS: A statistically significant increase, meaning a high LID value, was observed for IWW, pointing to toxic influence of the industrial wastewater from the screw factory and the importance of proper purification before discharging it in the environment. Therefore, strict and continuous biomonitoring plans must be established if serious consequences want to be avoided on the whole ecosystem, biota and the national park itself.

Daphnia magna ; industrial and municipal wastewaters ; Krka National Park ; toxic impact

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