Trophic flow kinetics in marine ecosystems: Toward a theoretical approach to ecosystem functioning

Based on a theoretical approach, Gascuel showed that trophic flow kinetics is a key characteristic of ecosystems’ functioning, partly determining their response to human disturbances such as fishing pressure. The kinetics quantifies the speed of the trophic flow, i.e., the velocity of biomass transfers from low to upper trophic levels, due to predation and/or ontogeny. In the present paper, we show that the production/biomass ( P/ B) ratio, used in many ecosystem models and particularly in the EwE software, can be considered a measure of this speed of the trophic flow. We propose an empirical model that expresses the P/ B ratio as a generic function of the trophic level ( τ) and the mean water temperature ( θ). The model was fitted to two datasets. The first was extracted from FishBase and includes 162 unexploited fish stocks for which the required parameters were available ( P/ B = M, τ and θ). In that case, the model represents transfer kinetics in fish communities as P/ B = 2.31 τ −1.72 exp(0.053 θ) ( R 2 = 0.37). It is consistent with estimates of natural mortality M based on FishBase data and Pauly's empirical equation ( n = 5258). Additionally, a model explains 68% of the total variance is proposed, which links P/ B ratios to the trophic level and to the von Bertalanffy growth coefficient, K. The second dataset included 1718 groups from 55 published Ecopath models. Here, the model expresses the mean trophic flow kinetics through all groups, from primary producers to top predators, as P/ B = 20.19 τ −3.26 exp(0.041 θ) ( R 2 = 0.54). A negative size effect is shown, with faster transfers occurring in young and less productive ecosystems. Transfers faster than mean are also observed in upwelling ecosystems and for zooplankton, while slower flow characterize benthos, marine mammals and birds. A relationship between trophic level and time ( t) was finally deduced from the P/ B model. It expresses the trophic level reached at time t by a unit of carbon fixed in the food web by primary producers at time t = 0. Considering the P/ B ratio a measure of the trophic flow kinetic provides a new way to look at this parameter. Implications of the model are discussed and its usefulness for building a general theory on ecosystem functioning is argued. We especially discuss how changes in the flow kinetics may contribute to top-down regulations and why fast transfer through the food web is a factor of ecosystems’ resilience.