Vertical chlorophyll a structure across density-interface morphologies in a turbid estuary: beyond the surface

Vertical chlorophyll a structure in turbid estuaries can be difficult to interpret because optical attenuation and density structure may decouple near-surface signals from water-column chlorophyll stocks. We analysed 58 high-resolution (0.1 m) fluorescence and hydrographic profiles collected during late July-early August across 10 midsummer field years (2014-2024) in the Neva River estuary. We examined associations of optical attenuation, density-interface morphology derived from potential density anomaly (σ_θ), and euphotic-depth geometry with near-surface chlorophyll a (Chl a_surf; 0-1 m), depth-integrated chlorophyll a (Chl a_int), and chlorophyll-maximum depth (Z_peak). Sharp interfaces were narrow, localized density transitions, whereas diffuse interfaces were broader gradients. Chl a_surf showed weak sensitivity to the diffuse attenuation coefficient for photosynthetically active radiation, K_d(PAR), and was a statistically significant but imperfect proxy for Chl a_int; this relationship did not become clearly stronger in unstratified profiles. Chl a_int was most consistently associated with density-interface morphology: sharp-interface profiles had lower integrated stocks than diffuse profiles, and continuous interface sharpness explained more variation than bulk stratification strength. The negative sharpness association persisted after accounting for year- and station-level clustering, although standardized 0-8 m integration provided weaker statistical support. Z_peak showed a central tendency near 2-3 m but retained substantial profile-level variability, while ratio Z_peak/Z_eu primarily reflected geometric scaling with euphotic depth. These results show that near-surface chlorophyll requires vertical-context interpretation. They motivate paired surface-profile validation but do not constitute a direct test of satellite products and should not be generalized beyond midsummer without additional seasonal observations.