  Aims: Accretion rates in low-mass protostars can be highly variable in time.
Each accretion burst is accompanied by a temporary increase in luminosity,
heating up the circumstellar envelope and altering the chemical composition of
the gas and dust. This paper aims to study such chemical effects and discusses
the feasibility of using molecular spectroscopy as a tracer of episodic
accretion rates and timescales.
  Methods: We simulate a strong accretion burst in a diverse sample of 25
spherical envelope models by increasing the luminosity to 100 times the
observed value. Using a comprehensive gas-grain network, we follow the chemical
evolution during the burst and for up to 10^5 yr after the system returns to
quiescence. The resulting abundance profiles are fed into a line radiative
transfer code to simulate rotational spectra of C18O, HCO+, H13CO+, and N2H+ at
a series of time steps. We compare these spectra to observations taken from the
literature and to previously unpublished data of HCO+ and N2H+ 6-5 from the
Herschel Space Observatory.
  Results: The bursts are strong enough to evaporate CO throughout the
envelope, which in turn enhances the abundance of HCO+ and reduces that of
N2H+. After the burst, it takes 10^3-10^4 yr for CO to refreeze and for HCO+
and N2H+ to return to normal. The chemical effects of the burst remain visible
in the rotational spectra for as long as 10^5 yr after the burst has ended,
highlighting the importance of considering luminosity variations when analyzing
molecular line observations in protostars. The spherical models are currently
not accurate enough to derive robust timescales from single-dish observations.
As follow-up work, we suggest that the models be calibrated against spatially
resolved observations in order to identify the best tracers to be used for
statistically significant source samples.
