  Earth's aquatic environments are inherently stratified layered systems where
interfaces between layers serve as ecological niches for microbial swimmers,
forming colonies known as Active Carpet (AC). Previous theoretical studies have
explored the hydrodynamic fluctuations exerted by ACs in semi-infinite fluid
media, demonstrating their capability to enhance thermal diffusion and mass
transport in aquatic systems. Yet, little is understood about the fluid
dynamics and impact of ACs residing in confined layered environments, like
slicks floating on water bodies. In this study, we report novel solutions for
the hydrodynamic fluctuations induced by ACs geometrically confined between a
free surface and a fluid-fluid interface characterized by a jump in fluid
viscosity. Combining theory and numerical experiments, we investigate the
topology of the biogenic hydrodynamic fluctuations in a confined, thin fluid
environment. We reveal that within this thin layer, ACs gives shape to three
characteristic regions: Region I is the closest zone to the AC and the
fluid-fluid interface, where hydrodynamic fluctuations are dominantly vertical;
Region II is further up from the AC and is characterized by isotropic
hydrodynamic fluctuations; Region III is the furthest region, near the free
surface and is dominated by horizontal flow fluctuations. We demonstrate that
the extent of these regions depends strongly on the degree of confinement, i.e.
the layer thickness and the strength of the viscosity jump. Lastly, we show
that confinement fosters the emergence of large-scale flow structures within
the layer housing the ACs--not previously reported. Our findings shed light on
the complex interplay between confinement and hydrodynamics in floating viscous
film biological systems, providing valuable insights with implications spanning
from ecological conservation to bio-inspired engineering.
