  Improved observational technologies have enabled the resolution of
substructures and the measurement of chemical abundances in discs.
Understanding the chemical composition of the inner disc allows us to infer the
building blocks available for planet formation. Recently, the depletion of
water in the inner disc has been suggested to be linked to the presence of
substructures, such as gaps and rings, further out. We investigate this
hypothesis further by running 1D semi-analytical models of a disc with a gap to
understand the combined effects of disc viscosity, gap depth, gap location, and
gap formation timescales on the inner disc composition. Our results show that
for a specific value of disc viscosity, the simulation outcome can be
classified into three regimes: shallow gap, "traffic jam", and deep gap. While
deep gaps may already be distinguishable with moderate-resolution techniques,
it is still challenging to resolve shallow gaps with the current capabilities.
On the other hand, discs with traffic jams have a higher chance of being
resolved when observed with a high resolution, but they may appear as an
intensity enhancement or even featureless when observed with moderate to low
angular resolution. In this regard, information on the inner disc composition
is useful because it can help to infer the existence of traffic jams or
distinguish them from deep gaps: discs with deep gaps are expected to have a
low water content and thus high C/O ratio in the inner disc due to the
effective blocking of pebbles, while discs with shallow gaps would demonstrate
the opposite trend. Furthermore, discs with a traffic jam would have a constant
inward flux of water-rich pebbles resulting in a moderate water content and
sub-stellar C/O ratios. Finally, we find that the effectiveness of gaps as
pebble barriers diminishes quickly when they form late, as most of the pebbles
have already drifted inwards.
