Time series constituents output

I have a running model and have set the time series output for one segment at various elevations. Bathymetry has 1m depths for all segments For the time series output i have set ESTR to the surface layer (o) and layer 10 (ESTR=-10) which corresponds to 1500m. The model runs but when the water level drops below 1499m (surface elevation) i am still getting output for constituents at layer 10 (1500m). Surely I should only be getting output at the surface? Is layer 10 fixed as 10 layers below the surface layer or is it fixed at 1500m? Am i understanding ETSR correctly as the layer in the bathymetry file and its corresponding depth?