The Effect of Değirmendere on Hydrography and Nutrient Salt Distribution in the Southeastern Black Sea (Trabzon) Coastal Waters

In this study, the physical properties and nutrient salt composition of coastal waters affected by Değirmendere along the Trabzon coast of the Southeastern Black Sea were compared with stations representing open sea conditions relatively independent of river influence. Fieldwork was planned for March and August 2025 to capture two contrasting hydrographic conditions; sampling was carried out at depths of 0–200 m along the river-influenced TM (Trabzon-Center; TM1–TM4) line and the reference TY (Trabzon-Yomra; TY1–TY4) line. Temperature, salinity, and density profiles of the water column were obtained using CTD; dissolved inorganic nitrogen species (nitrate, nitrite, ammonium), orthophosphate, reactive silicate, and total nitrogen and total phosphorus concentrations were measured using an auto-analyzer and statistically evaluated using the Month×Line approach. In March, a cold and vertically well-mixed water column of 9 °C was observed in both channels, while in August, a warmer and less dense layer developed at the surface and a distinct thermocline/pycnocline formed between 20 and 40 m. While no significant difference was observed between TM and TY in terms of salinity and density, nutrient salts showed a significant change from the coast to the open sea and from the surface to the depths. Near the coast, TM showed a higher tendency for ammonium and total nitrogen, while total phosphorus and silicate were relatively higher in TY in most cases. The effects of month, line, and interaction were pronounced on total nitrogen, with enrichment being more prominent in TY in March and in TM in August. DIN:PO₄³⁻ ratios above the Redfield ratio generally indicate phosphorus limitation; although Değirmendere increases enrichment near the coast, this effect weakens within a few nautical miles, indicating that the trophic status of the region remains relatively balanced in terms of eutrophication. This study provides a local reference dataset that comparatively documents river effects during two distinct periods rather than representing an annual cycle.