  The next generation of spectroscopic surveys will target emission-line
galaxies (ELGs) to produce constraints on cosmological parameters. We study the
large scale structure traced by ELGs using a combination of a semi-analytical
model of galaxy formation, a code that computes the nebular emission from HII
regions using the properties of the interstellar medium, and a large-volume,
high-resolution N-body simulation. We consider fixed number density samples
where galaxies are selected by either their H$\alpha$, [OIII]$\lambda 5007$ or
[OII]$\lambda \lambda 3727-3729$ emission line luminosities. We investigate the
assembly bias signatures of these samples, and compare them to those of stellar
mass and SFR selected samples. Interestingly, we find that the [OIII]- and
[OII]-selected samples display scale-dependent bias on large scales and that
their assembly bias signatures are also scale-dependent. Both these effects are
more pronounced for lower number density samples. The [OIII] and [OII] emitters
that contribute most to the scale dependence tend to have a low gas-phase
metallicity and are preferentially found in low-density regions. We also
measure the baryon acoustic oscillation (BAO) feature and the $\beta$ parameter
related to the growth rate of overdensities. We find a slight tendency for the
BAO peak to shift toward smaller scales for [OII] emitters and that $\beta$ is
scale-dependent at large scales. Our results suggest that ELG samples include
environmental effects that should be modelled in order to remove potential
systematic errors that could affect the estimation of cosmological parameters.
