  We present the results of hydrodynamic simulations using the method of
multi-particle collision dynamics for a system of squirmer microswimmers moving
under the influence of gravity at low Reynolds numbers. In addition, the
squirmers are bottom-heavy so that they experience a torque which aligns them
along the vertical. The squirmers interact hydrodynamically by the flow fields
of a stokeslet and rotlet, which are initiated by the acting gravitational
force and torque, respectively, and by their own flow fields. By varying the
ratio of swimming to bulk sedimentation velocity and the torque, we determine
state diagrams for the emergent collective dynamics of neutral squirmers as
well as strong pushers and pullers. For low swimming velocity and torque we
observe conventional sedimentation, while the sedimentation profile becomes
inverted when their values are increased. For neutral squirmers we discover
convective rolls of circulating squirmers between both sedimentation states,
which sit at the bottom of the system and are fed by plumes made of
collectively sinking squirmers. At larger torques porous clusters occur that
spawn single squirmers. The two latter states can also occur transiently
starting from a uniform squirmer distribution and then disappear in the
long-time limit. For strong pushers and pullers only weak plume formation is
observed.
