Oceans and lakes are typically inhomogeneous in temperature and salinity, and host a significant part of their biomass in the form of microscopic organisms. Their individual and collective dynamics are affected by the resulting gradients in viscosity and density. The purpose of this internship is to model and analyse the effects of such spatial variations on the swimmer's trajectory and reciprocally, the effect of the swimming motion on the swimming dynamics. The project can be continued onto a PhD where the focus will be extended to the interactions of swimmers and how these are affected by the environment's inhomogeneities.