The Sakarya and Yeşilırmak Rivers' presence had a major effect on the surface waters (at a depth of 10 m), resulting in variations in salinity and nutrient levels across the stations. However, a more consistent picture appeared 10 m and 50 meters below, with nutrient variations remaining within a narrower range. Nutrient composition shifts in the Black Sea were shown to be reliant on density variations at stations deeper than 50 meters.
The surface temperature values of the stations differed primarily seasonally, as expected (Fig. 2). With only minor regional fluctuations, the winter and summer observations' average temperatures were 11°C and 26°C, respectively. However, because of the impact of river inlets, salinity values fluctuated spatially. Salinity readings dropped to 16.5 psu at stations along the Sakarya and Yeşilrmak Rivers, whereas 18.0 psu was the average at the other stations. While the salinity drops were seen in Yeşilrmak River readings only during the winter, they were seen in both seasons at the Sakarya River. During the summer research expedition, the Sakarya River stations showed the most pronounced river influence on salinity (Fig. 2).
The surface mixed layer of the coastal waters of the Sakarya River had NO2 + NO3-N concentrations that ranged from 0.16 to 11.40 M in the winter and from 0.05 to 9.56 M in the summer (Fig. 3a). There was a close correlation between the Sakarya River's winter and summer concentrations, but the Yeşilırmak River had significantly higher concentrations in the winter than in the summer due to differences in nitrogen loads across the two river basins (Fig. 3a). The highest concentrations of NO2 + NO3-N coincided with the regions where the NH4-N concentrations were the most apparent, with values ranging from 0.63 to 2.84 µM during the winter and from 0.041 to 2.58 µM during the summer. Similar to this study, Alkan et al., 2013 reported elevated nitrate concentrations in the rivers during the winter.
Orthophosphate (PO4-P) concentrarions ranged from 0.07 to 0.64 µM (0.10–0.69 M for TP) in the winter and from 0.02 to 0.44 µM (0.33–1.20 µM for TP) in the summer in the surface waters of the Sakarya River site. In contrast, PO4-P concentrations in the Yeşilırmak River ranged from 0.07 to 0.19 µM (0.07 to 0.46 µM for TP) in the winter, compared to concentrations ranged from 0.02–2.97 µM (0.09 to 3.07 µM for TP) in the summer (Fig. 3b). Increased concentrations of PO4-P and TP in the Yeşilırmak River are likely the result of agricultural operations taking place in the river basin. Increased terrigenous input is also responsible for the increased levels of silicate (SiO2) in coastal waters. The average concentration of silicate in both rivers was similar in the winter, while it was nearly three times greater in the Sakarya River region than in the Yeşilırmak River during the summer.
The highest concentrations of NH4-N were found at SAAT locations that were closest to the shore, with concentrations ranging from 0.04 to 11.3 µM in the winter and from 0.04 to 2.9 µM in the summer. Ortho-phosphate (PO4-P) concentrations ranged vastly in the surface waters of the Samsun coastal area, from 0.02 to 3.48 µM in the winter to 0.02 to 8.3 µM in the summer. Areas impacted by municipal wastewater discharge were found to have high levels of PO4-P. In the coastal region of the Samsun site, silicate concentrations varied from 1.75 to 6.2 µM in the winter and from 0.06 to 3.77 µM in the summer. SAAT > SN > SLI was the trend in silicate concentrations, indicating that there are variations depending on geography features and pressures (Fig. 3b).
Marine organisms and the chemical structure of their habitat depend on oxygen. The concentrations of dissolved oxygen (DO) at the surface of the Sakarya and Yeşilrmak Rivers were measured between 7.74 and 9.38 mg/l at both locations in the winter, whereas summer values were as measured lower (between 5.39 and 6.66 mg/l). DO concentration of Samsun Port and WWTP area is ranged from 5.3 to 9.5 mg/l in the summer and 7.4 to 9.2 mg/l in the winter. At the water column around the SAAT stations, slightly lower DO concentrations were recorded. Concentrations of chlorophyll-a in the surface mixed layer near the Sakarya River varied from 0.16 to 1.57 µg/L in the winter and from 0.05 to 0.93 µg/L in the summer (Fig. 4a). In both seasons, but especially in the summer, the chlorophyll-a concentrations in the Yeşilırmak River were higher than in the Sakarya River due to the increased phosphorus influx into the Yeşilırmak region during that time of the year. Chlorophyll-a concentrations in the surface mixed layer at Samsun varied from 0.82 to 2.19 µg/L in the winter and from 0.08 to 23.4 µg/L in the summer (Fig. 4a). The eastern nearshore stations (SN01, SN04) and the treatment plant discharge area (SAAT01, SAAT03, SAAT04) were found to have highest chlorophyll-a concentrations.
The Secchi disc depths (SDD) were recorded nearly the same for the Sakarya and Yeşilırmak rivers. SDD of the rivers were measured between 2–8.5 m in the summer and 0.5–6 m in the winter. Summer brought about depth shifts of 1–8 m at SAAT01–04, SN01–04 and SN05, while the Secchi disc depth (SDD) showed a uniform pattern in the winter (Fig. 5). Total Suspended Solids (TSS) concentrations were found to be changing between 3.2 and 8.3 mg/l, while TSS levels in the Yeşilırmak region ranged from 0.7 to 4.0 mg/l. TSS values in the summer months were detected between 0.1 and 2.7 mg/l, with the Yeşilırmak region's shallow sites showing somewhat higher concentrations (Fig. 5). The range of TOC concentrations was 2.22 mg/L to 5.48 mg/L, with 3.06 mg/L representing the mean value. In the Yeşilırmak area, TOC concentrations were found to be substantially greater in the summer than in the winter (Fig. 4c). Under the urban wastewaters pressures in the region (SAAT stations), TSS concentrations during the winter months had a range between 0.7 and 5.3 mg/l, while summer concentrations had a range between 0.2 and 3.85 mg/l (Fig. 5). TOC contents varied from 2.32 to 2.67 mg/l during the winter and 2.75 to 6.2 mg/L during the summer (Fig. 4c).
Analyses predict that the SAAT site, which is closest to the WWTP outflow, will show higher nutrient concentrations than the other two locations. Spearman's correlation analysis revealed strong seasonal and regional correlations when applied to the surface layer data of several pelagic variables. Chlorophyll-a showed no associations with other variables during the winter at both sites, but showed strong positive relationships with inorganic nutrients during the summer at the Sakarya River site. Chlorophyll-a showed positive relationships with temperature, phosphorus, total soluble solids, and total organic carbon throughout the summer at the Yeşilırmak site, but it represented negative correlations with salinity, dissolved solids, and silicate. No link to nitrogen was discovered.
Across the both sites and seasons, nutrient concentrations were negatively correlated with salinity, further demonstrating the impact of river inputs. The expected positive connection between SDD and salinity was observed at both sites and during both seasons. Low flow rate and low oxygen content in relation to urban wastewater and agricultural operations are assumed to be under high nitrite and ammonium concentrations. The Yeşilırmak river distinguished out from the rest due to its increased chlorophyll-a content.