Operation of light-dark cycles within simple ecosystem models of primary production and the consequences of using phytoplankton models with different abilities to assimilate N in darkness

Abstract

We compare the output of a nitrogen-phytoplankton-zooplankton (NPZ) model and of a hypothetical diatom-flagellate competition scenario operating in continuous light or in a diurnal light-dark cycle of equal daily photon dose. Within these models, phytoplankton were configured with contrasting abilities to assimilate nitrogen in darkness. If only a single phytoplankton group is being considered then it appears unnecessary to describe the diurnal light cycle. However, given the minimal additional processing time required to include the light cycle when running complex biological models, inclusion is recommended where model output is used to provide an insight into the behaviour of the organisms and food webs. This is particularly so for considering temporal variations in the use of ammonium and nitrate (pertaining to the f ratio for new production) by different phytoplankton groups and hence the interaction between these organisms and their zooplankton predators that regenerate ammonium. In that instance it is important to endow the phytoplankton model with a capacity for dark N assimilation commensurate with the type of organism (namely diatom or non-diatom) being considered. Such different capabilities likely affect competition and succession of these groups in nature and do affect these interactions in simulations.