Effect of dust settling on the disc chemical abundances. Left column indicates abundances in a fully mixed model, and right column represents a strongly-settled model with otherwise identical parameters. Top row: CO abundance with dust (blue dashed lines) and gas (white dashed line) temperature contours. Middle row: H₂ abundance with 300 K gas temperature contour (black dashed line) and visual extinction contour (white dashed line). Bottom row: H₂O abundance. White dashed lines indicate T_gas contours enclosing “hot water” region, red dashed lines indicate contours of UV field strength per hydrogen nucleus, log (χ/n_H) as defined in Draine & Bertoldi (1996), enclosing the cool water belt.