Determination of water quality and eutrophication potential in the lower Goksu River

The river and estuarine ecosystems with the coastal marine regions are highly affected by the eutrophication problem caused by anthropogenic pressures. In this study, sampling and measurements of physical, biochemical and eutrophication-related parameters were carried out at the selected 5 stations in the Göksu River between September 2021 and August 2022. The study results indicated that physical and biochemical parameters in Göksu River showed significant regional and temporal (monthly) variations. According to water quality assessment based on the ecological standards of the Surface Water Quality Regulation, though the Göksu River waters are first class in terms of electrical conductivity, pH, dissolved oxygen, nitrate and total nitrogen values values/saturation levels and nitrate parameters, the visited stations were the second and third class in terms of orthophosphate and total phosphorus. Furthermore, trophic status was evaluated based on the annual averages of total nitrogen, total phosphorus and chlorophyll-a values obtained in this study and the calculated Trophic Index (TRIX) values. Although the visited stations in Göksu River were classified as mesotrophic (moderate) and eutrophic (bad) in terms of TN, they were classified as eutrophic (bad) in terms of total phosphorus and chlorophyll-a. According to TRIX values, all visited stations were classified as eutrophic (bad). Water quality classification and trophic status assessment in the study region indicated that the water quality of Göksu River waters, rich in phosphate and total phosphorus, deteriorated. By using the study findings, the nutrient inputs from the Göksu River into the Northeast Mediterranean coastal area were also determined. Nutrient fluxes from the Göksu River were calculated as 1079 tons/y for nitrate (NO3-N), 2158 tons/y for total nitrogen (TN), 224 tons/y for reactive phosphate (PO4-P) and 373 tons/y for total phosphorus (TP). The calculated TP/TN molar ratio of the nutrient fluxes was determined as 13, which is lower than the classical Redfield ratio (N/P=16), due probably to the result of mixing of phosphorus-contaminated wastewaters originated from agricultural activities into the Göksu River waters. This phosphorus enrichment from the Göksu River may affect the ecosystem dynamics in the coastal regions of phosphorus limited Northeastern Mediterranean Sea.