  It has been proposed that biologically generated turbulence plays an
important role in material transport and ocean mixing. Both experimental and
numerical studies have reported evidence of the non-negligible mixing by
moderate Reynolds number swimmers in quiescent water, such as zooplankton,
especially at aggregation scales. However, the interaction between biologically
generated agitation and the background flow as a key factor in biologically
generated turbulence that could reshape our previous knowledge of biologically
generated turbulence, has long been ignored. Here we show that the geometry
between the biologically generated agitation and the background hydrodynamic
shear can determine both the intensity and direction of biologically generated
turbulent energy cascade. Measuring the migration of a centimeter-scale
swimmer-as represented by the brine shrimp \textit{Artemia salina}-in a shear
flow and verifying through an analogue experiment with an artificial jet
revealed that different geometries between the biologically generated agitation
and the background shear can result in spectral energy transferring toward
larger or smaller scales, which consequently intensifies or attenuates the
large scale hydrodynamic shear. Our results suggest that the long ignored
geometry between the biologically generated agitation and the background flow
field is an important factor that should be taken into consideration in future
studies of biologically generated turbulence.
