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 subs trate element composition on the N:P ratios of heterotrophic excretion (tru e excretion, i.e. metabolic by-products) and release (excretion + feces pro duction) products. The model uses units of C, N, and P, and depicts three t ypes of heterotrophs: the copepods, cladocerans, and bacteria. Most paramet ers of the model are estimated from experimental data sets representative o f 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 th e simulated N:P ratio for excretion is more representative of the experimen tal measurements than that for release, especially when the growth substrat e N:P ratio is high. Thus the model assumption of adjustable excretion coul d explain stoichiometric regulation of growth substrates by heterotrophs. O ur model gives better fits to observations than two other similar models, m ainly because of its ability to simulate the excretion N:P ratio. The C con tent of growth substrates did not influence stoichiometric excretion and re lease of N and P for mesozooplankton, but it was important for bacteria. St oichiometric regeneration of nutrients by heterotrophs affects phytoplankto n growth, with large organism-dominated ecosystems accentuating N limitatio n, whereas small organism-dominated ecosystems favor P limitation. (C) 2001 Elsevier Science B.V. All rights reserved.