What's the relationship between oxygen consumption and ambient oxygen concentration for a lobster?
Increasing the ambient oxygen concentration increases diffusion of oxygen across the gills, thereby increasing the oxygen uptake by the lobster.
This can be proven by the fick's law of diffusion-

r=[-DA(P2-P1)/L]

L=thickness of diffusion membrane

D=diffusion coefficient

A=cross sectional area of diffusion membrane

P2-P1=pressure difference