Hydrographic Stratification and Pollutant Retention at Constanța Port Roadstead, NW Black Sea: Five-Layer Dissolved Oxygen Structure and a CTD-Derived Retention Index from a Single-Station Profile

High-resolution CTD profiles, with SVP cross-validation of the sound speed field, were recorded at a single station in the outer roadstead of the Port of Constanța (northwest Black Sea; 44°07′41″ N, 28°53′15″ E; depth ≈ 25 m; June 2024), revealing a strongly stratified, five-layer water column driven by three combined forcing mechanisms: seasonal thermal stratification with an abnormally shallow Cold Intermediate Water layer (7.3–15.6 m), Danube-sourced freshwater input, and anthropogenic disturbances consistent with port and anchorage activity. A contextual hypothesis is proposed that conflict-related marine traffic intensification may contribute to observed signals, but physical measurements cannot establish causation. At the main pycnocline (7.31–15.62 m), a density difference of Δρ = 4.02 kg m−3 yields a maximum Brunt–Väisälä frequency of N2 = 2.37 × 10−3 s−2, reducing vertical eddy diffusivity by two orders of magnitude (Kz ≈ 10−6 m2 s−1). Physical conditions—a shallow mixed layer (~0.7–1.2 m) and strong pycnocline—support the theoretical expectation of surface-layer contaminant accumulation; however, no chemical measurements were carried out to confirm contaminant presence. All contamination inferences rely exclusively on physical proxies (turbidity, dissolved oxygen, and density gradients), and contaminant retention remains untested for lack of direct chemical evidence. A dimensional Stratification-Controlled Retention Index (SCRI = N2/Kz; units: m−2 s−1) is introduced, and its consistency with the observed hydrographic structure is demonstrated.