The response of a simple pelagic ecosystem is investigated in a domain with zero, one- and two-dimensional descriptions of the physical processes. Assuming complete mixing, internal recycling and external exchange of nutrients and their biological products are not additive in terms of the rate of primary production. In a one-dimensional system, advection without diffusion leads to low values of primary production. With two horizontal dimensions, cross diffusion at the boundary gives higher values of production. The consequences for higher trophic levels are described.