Towards an adaptive model for simulating growth of marine mesozooplankton: A macromolecular perspective

Ultimately, the structure and functioning of marine ecosystems is defined by the transfer of autotrophic production to higher trophic levels and selective consumption of these autotrophs by predators. Hence, feeding regulation via modification of grazing and food incorporation by predators is critical for understanding and predicting the dynamics of ecosystems. In marine ecosystem and biogeochemical models, feeding regulation by consumers is assumed to be mainly dictated by food quality (Q), which is determined using food quality modules (FQMs) that mimic a consumers’ ability to anticipate fitness consequences for feeding on specific prey items. Current FQMs are based on frameworks that a priori identify specific food components, usually nitrogen (N), and/or phosphorus, as limiting. This negates the importance of consumer physiology, and ignores biochemical constrains on the limiting role of chemical elements in animal production. To help address these problems, we propose a new adaptive approach that bases Q on consumers’ capacity for food uptake and metabolic physiology. Uniquely, it (i) has separate pathways for the utilisation of carbon (C) associated with proteins, lipids and carbohydrates, (ii) considers stage-specific structural biochemical requirement of animals, and (iii) does not treat consumers’ structural demand for carbon as a “unitary requirement” but discriminates among the required biochemical forms of carbon. The approach is applicable to all heterotrophs. In the example given here the model has been configured to represent the calanoid copepod Acartia tonsa. Consistent with experimental observation, but unlike previous models, our model predicts the relationship between Q and food C:N to be unimodal with a maximum Q only at the threshold C:N for biomass production. Results suggest that prey C:N ratios may be irrelevant for food quality due to macromolecular biochemical constrains on the utilisation of chemical elements. This result emphasizes the importance of biochemical substances in animal nutrition and production as well as the necessity of developing food quality models able to adapt to the biochemical needs of the consumer.