  We investigate marine ecosystem models of N-DOP type with regard to
nontrivial periodic solutions. The elements of this important, widely-used
model class typically consist of two coupled advection-diffusion-reaction
equations. The corresponding reaction terms are divided into a linear part,
describing the transformation of one model variable into the other, and a
bounded nonlinear part. Additionally, the model equations conserve the mass
contained in the system, i.e. the masses of both variables add up to a constant
total mass. In particular, the trivial function is a periodic solution. In this
paper, we prove that there is at least one periodic solution for every
prescribed total mass. The proof makes use of the typical properties of N-DOP
type models by combining results from monotone operator theory and a fixed
point argument. In the end, we apply the theorem to the PO4-DOP model, an N-DOP
type model which is well-known and often used.
