A stoichiometric model relating growth substrate quality (C:N:P ratios) to N:P ratios in the products of heterotrophic release and excretion

A stoichiometric model is developed to analyze the influence of growth substrate element composition on the N:P ratios of heterotrophic excretion (true excretion, i.e. metabolic by-products) and release (excretion+feces production) products. The model uses units of C, N, and P, and depicts three types of heterotrophs: the copepods, cladocerans, and bacteria. Most parameters of the model are estimated from experimental data sets representative of these heterotrophs. Net growth efficiencies for N and P vary according to the element composition of growth substrates. The simulated N:P ratios for release and excretion products of copepods and cladocerans are compared to experimental data for heterotrophic regeneration. Results indicate that the simulated N:P ratio for excretion is more representative of the experimental measurements than that for release, especially when the growth substrate N:P ratio is high. Thus the model assumption of adjustable excretion could explain stoichiometric regulation of growth substrates by heterotrophs. Our model gives better fits to observations than two other similar models, mainly because of its ability to simulate the excretion N:P ratio. The C content of growth substrates did not influence stoichiometric excretion and release of N and P for mesozooplankton, but it was important for bacteria. Stoichiometric regeneration of nutrients by heterotrophs affects phytoplankton growth, with large organism-dominated ecosystems accentuating N limitation, whereas small organism-dominated ecosystems favor P limitation.