N:P ratios to estimate nutrient limitations and trophic state in a shallow freshwater lake

Understanding nutrient limitation is essential for managing shallow freshwater ecosystems experiencing eutrophication. Kovada Lake, a National Park in Türkiye, suffers from severe nutrient pollution and excessive macrophyte growth, yet its specific limitation status is unclear. This study aimed to diagnose the nutrient limitation and current trophic state of the lake and its inlet canal. Epilimnetic water samples were collected from five stations during the growing season (May–October 2024). Samples were analyzed for Total Nitrogen (TN), Total Phosphorus (TP), Dissolved Inorganic Nitrogen (DIN), Dissolved Inorganic Phosphorus (DIP), and Chlorophyll-a (Chl-a). Molar stoichiometric ratios (TN:TP and DIN:DIP) were then calculated to identify limitation patterns. Results revealed extremely high nutrient concentrations throughout the system. The lake-canal average molar TN:TP ratio was 7.77:1, indicating strong and persistent N limitation. In contrast, the average molar DIN:DIP ratio was 13.97:1, suggesting a N-P co-limitation for immediately bioavailable nutrients. Significant spatial and monthly variations confirmed the system's dynamic nature. The average Chl-a concentration (14.69 µg L-1) confirmed a eutrophic trophic state. Crucially, Chl-a did not correlate with TN or TP but showed significant relations with DIP and DIN:DIP ratio. This study demonstrates that the Kovada Lake system is primarily N-limited. Furthermore, it highlights that in this macrophyte-dominated shallow lake, dissolved nutrient parameters (DIP, DIN:DIP) are more reliable predictors of trophic response than total nutrient ratios. These findings provide a critical diagnostic basis for developing nutrient management strategies.