  Bacteria and plankton populations living in oceans and lakes reproduce and
die under the in- fluence of turbulent currents. Turbulent transport interacts
in a complex way with the dynamics of populations because the typical
reproduction time of microorganism is within the inertial range of turbulent
time scales. In the present manuscript we quantitatively investigate the effect
of flow compressibility on the dynamics of populations. While a small
compressibility can be induced by several physical mechanisms, like density
mismatch or the finite size of microorganisms with respect to the fluid
turbulence, its effect on the the carrying capacity of the ecosystem can be
dramatic. We report, for the first time, how a small compressibility can
produce a sizeable reduction in the carrying capacity, due to an integrated
effect made possible by the long replication times of the organisms with
respect to turbulent time scales. A full statistical quantification of the
fluctuations of population concentration field leads to data collapse over a
broad range in parameter space.
