A nonlinear model for biological and physical dynamical interactions in a laminar upwelling flow field in parts I and II of this study is extended to turbulent flow. In the previous studies, a prescription for obtaining quadrature solutions to the fundamental biodynamical equations was developed. In this study, we use a probability density function approach on these solutions to obtain statistics of the biodynamical state variables and their self-interaction for the case of turbulent advection. To illustrate the theory, a simple nutrient (
N
), phytoplankton (
P
) problem is considered, that of upwelling into a surface turbulent layer. Biological interaction is modelled as bilinear, representing the uptake of
N
by
P
in a uniform light euphotic zone. A random walk model is used to obtain the appropriate probability density function for the advective turbulent field. The mean quantities,
,
, as well as the biological interaction term
are calculated. The term
has two contributions,
, and the turbulence-induced interaction term,
. It is shown that the often neglected turbulence-induced coupling term
is of the order
and opposite in sign. This results in, over a wide range of Peclet numbers, the mean interaction term
being significantly smaller than either of its constituent terms,
and
.