**1. Introduction**

Coastal lagoons are complex and dynamic ecosystems that show sharp distributional gradients and short-term and seasonal variability in their physical, chemical, and biological properties [1]. As a result of long hydrologic retention times, lagoons are ecosystems which are very vulnerable to chemical contamination [2]; pollutants are consequently retained longer [3].

Küçükçekmece Lagoon, located in Istanbul, Turkey, suffers from many problems, which might lead to environment degradation, differences in salinity levels, and substantial changes in its ecosystem. It has been affected by population pressure around the lagoon, wastewater discharges, algal blooms, and salinity increases due to the cutoff of freshwater inputs [4].

In this study, the spatial and seasonal variability of physicochemical parameters and the chlorophyll-*a* were assessed in a coastal lagoon connected to the sea. Our main goal was to understand the nutrient dynamics, including the importance of water exchanges between the lagoon and the adjoining area, and determination of the effect of anthropogenic activities and the closure of freshwater inlets on the trophic conditions of the lagoon for more than ten years.

### **2. Materials and Methods**

Küçükçekmece Lagoon is located in the south-west of Istanbul (Figure 1). It has a direct connection to the Marmara Sea with a canal. It has a 15.22 km<sup>2</sup> surface area and a maximum depth of 18.5 m. Because of rapid urbanization and industrialization, a significant deterioration in water quality was observed due to intense wastewater discharge.

Faculty of Aquatic Sciences, Istanbul University, Istanbul 34134, Turkey; gozde.ozbayram@istanbul.edu.tr (E.G.O.); ayca.oguzcam@istanbul.edu.tr (A.O.Ç.); akcaalan@istanbul.edu.tr (R.A.); merbay@istanbul.edu.tr (M.A.)

**<sup>\*</sup>** Correspondence: latife.koker@istanbul.edu.tr

A gradual increase in eutrophication has resulted in cyanobacterial blooms from the 1990s to the 2010s [5,6].

**Figure 1.** Location of sampling sites in Küçükçekmece Lagoon.

To compare water quality variations, samples were collected from June to December 2012 and 2022 at regular monthly intervals. Water samples were collected from the surface in two stations (first and third station), and to observe the depth profile, the samples were collected throughout the water column (surface, 9 m, and 18 m) from the second station. Dissolved oxygen (DO), salinity, and temperature were measured in situ using the YSI multiparameter instrument (YSI 650 MDS). Chlorophyll-*a* (chl-*a*) was determined using the method of ISO 10260 (1992) [7]. Nutrient analyses for nitrite (N02), nitrate (N03), total phosphorus (TP), and silicate were performed according to APHA-AWWA WPCF (1989) [8].

### **3. Results and Discussion**

The physicochemical characteristics of the surface waters of Küçükçekmece Lagoon are given in Figure 2. The water temperature ranged between 8.1 and 28.2 ◦C. The salinity diagrams (Figures 2 and 3) showed that seawater inflow influences the lagoon's characteristics. It was determined that the salinity level doubled from 2012 (mean: 7.44 ppt) to 2022 (mean: 14.1 ppt). Due to the increase in salinity, the detected cyanobacteria members between 1990 and 2015 were left to marine species instead. This revealed that chl-*a* decreased in 2022. While the mean value of total phosphorus concentration was 712.1 μg/L in 2012, it was measured as 370 μg/L in 2022. The mean value of nitrate and nitrite concentrations increased from 0.72 mg/L to 2.9 mg/L in 2022. All these measurements showed that the trophic status of Küçükçekmece Lagoon was hypertrophic since the 1990s and did not show any sign of enhancement.

**Figure 2.** Physicochemical characteristics of surface waters of stations.

**Figure 3.** Depth profile of physicochemical characteristics of station 2.

4 of 5

In the water column, data showed that as a quality deteriorated at the bottom of the lake because of slow water exchange between layers [9]. As an important indicator of trophic status, oxygen concentration or depletion rates in hypolimnetic water have long been studied [10]. In all sampling periods, the bottom layer was characterized by anoxic conditions (Figure 3). Gürevin et al. reported that the significant release of nutrients from sediment caused long-term eutrophication in the lagoon [11].
