  Periodic water waves generate Stokes drift as manifest from the orbits of
Lagrangian particles not fully closing. Stokes drift can contribute to the
transport of floating marine litter, including plastic. Previously, marine
litter objects have been considered to be perfect Lagrangian tracers,
travelling with the Stokes drift of the waves. However, floating marine litter
objects have large ranges of sizes and densities, which potentially result in
different rates of transport by waves due to the non-Lagrangian behaviour of
the objects. Through a combination of theory and experiments for idealised
spherical objects in deep-water waves, we show that different objects are
transported at different rates depending on their size and density, and that
larger buoyant objects can have increased drift compared with Lagrangian
tracers. We show that the mechanism for the increased drift observed in our
experiments comprises the variable submergence and the corresponding dynamic
buoyancy force components in a direction perpendicular to the local water
surface. This leads to an amplification of the drift of these objects compared
to the Stokes drift when averaged over the wave cycle. Using an expansion in
wave steepness, we derive a closed-form approximation for this increased drift,
which can be included in ocean-scale models of marine litter transport.
