Modelling the influence of food C:N ratio, and respiration on growth and nitrogen excretion in marine zooplankton and bacteria

Abstract

Zooplankton and bacteria use food to synthesize new biomass and meet respiratory demands. A steady-state bioenergetic model is presented which explores the effects of the quality of assimilated food and respiration on growth and nitrogen excretion in marine heterotrophs. Processes of grazing and assimilation are not considered. Assimilated food is divided into two groups: nitrogenous (proteins) and non-nitrogenous (carbohydrates, lipids), and assumptions used to determine how each group is utilized for growth and respiration. Zooplankton growth is predicted to be carbon-limited, in contrast to existing experimental evidence which suggests nitrogen limitation. Excretion of nitrogen increases with the nitrogen content of food, and quantities of food consumed. A dissolved organic matter carbon to nitrogen ratio of 10.2:1 was predicted below which bacteria remineralize nitrogen, and above which ammonium is taken up from the environment and used as a growth substrate. This compares well with experimental studies. The model provides valuable insight into the use of simpler nutrient-balancing models: critical assumptions about organisms' carbon net growth efficiency must be made in order to apply them successfully. In general, the study highlighted the importance of having a proper understanding of respiration in marine organisms and how it is related to their biomass and growth.