Temporal changes in water quality index of polluted lagoon ecosystems: a case study on the Küçükçekmece Lagoon

For lagoon ecosystems, it is crucial to conduct long-term monitoring of the water quality parameters and predict their potential effects. This study aimed to analyse the changes in the water quality index (WQI) profile of the Küçükçekmece Lagoon in Istanbul, Turkey, which has been facing ecological problems, and determine the increased stress in aquatic biota due to different pollutants. To begin, the sampling data were obtained from the Küçükçekmece Lagoon throughout a 13-month period in 2018-2019. The estimated WQI values were compared to studies conducted in the same study area within the last 20 years. Then, the relationship between the acute toxicity of surface waters, which is the recommended parameter for analyses in this field, and the WQI was determined. Although the water class has generally been defined as 'good quality' (WQI = 86), the study found it to be 'very poor quality' (WQI = 112 and 97, respectively) with bloom events and toxicity effects in spring and autumn. When compared with the WQIs calculated from the last 20 years, a significant improvement in water quality at station L1 (decreased from WQI = 288 to WQI = 161) and a deterioration at station L2 (increased from WQI = 71 to WQI = 100) were observed. The acute toxicity and water quality classes were highly positively correlated (r = 0.773; p < 0.01). The method, used for the first time in this area, was able to interpret the acute toxicity of lagoon surface waters and WQI data. Furthermore, the use of this method was recommended for rapid analysis of the increased stress in aquatic biota.