  This paper introduces a one-dimensional NPZD-model developed to simulate
biological activity in a turbulent ocean water column. The model consists of a
system of coupled semilinear parabolic equations. An initial-boundary value
problem is formulated and the existence of a unique positive weak solution to
it is proved. The existence result is derived using a variational formulation,
an approximate model and a fixed-point method. It is shown that the qualitative
analysis performed still applies if different parameterizations of several
biological processes found in the biogeochemical modeling literature are used.
