Modeling environmental effects on the size-structured energy flow through marine ecosystems. Part 1: The model

This paper presents an original size-structured mathematical model of the energy flow through marine ecosystems, based on established ecological and physiological processes and mass conservation principles. The model is based on a nonlocal partial differential equation which represents the transfer of energy in both time and body weight (size) in marine ecosystems. The processes taken into account include size-based opportunistic trophic interactions, competition for food, allocation of energy between growth and reproduction, somatic and maturity maintenance, predatory and starvation mortality. All the physiological rates are temperature-dependent. The physiological bases of the model are derived from the dynamic energy budget theory. The model outputs the dynamic size-spectrum of marine ecosystems in term of energy content per weight class as well as many other size-dependent diagnostic variables such as growth rate, egg production or predation mortality.In stable environmental conditions and using a reference set of parameters derived from empirical studies, the model converges toward a stationary linear log–log size-spectrum with a slope equal to −1.06, which is consistent with the values reported in empirical studies. In some cases, the distribution of the largest sizes departs from the stationary linear solution and is slightly curved downward. A sensitivity analysis to the parameters is conducted systematically. It shows that the stationary size-spectrum is not very sensitive to the parameters of the model. Numerical simulations of the effects of temperature and primary production variability on marine ecosystems size-spectra are provided in a companion paper [Maury, O., Shin, Y.-J., Faugeras, B., Ben Ari, T., Marsac, F., 2007. Modeling environmental effects on the size-structured energy flow through marine ecosystems. Part 2: simulations. Progress in Oceanography, doi:10.1016/j.pocean.2007.05.001].     

Honey, I cooled the cods: Modelling the effect of temperature on the structure of Boreal/Arctic fish ecosystems

Historically colder regions of the North Atlantic had fisheries dominated by only a few fish species; principally cod and capelin. Possible population dynamic mechanisms that lead to such dominance are investigated by considering how a charmingly simple published multispecies model of the North Sea would react if the system operated at a lower temperature. The existing model equations were modified to describe temperature effects on growth, fecundity and recruitment and the model was rerun based on typical temperatures for the North Sea and a colder system. The results suggest that total fish biomass in the colder system increases but the community is more vulnerable to a given rate of fishing mortality. In the colder system, within species density dependence is reduced but relative predation rates are higher. Consequently, intermediate-sized species are vulnerable to relatively high levels of predation throughout their life history and tend to be excluded, leading to a system dominated by small and large species. The model helps to explain how temperature may govern coexistence and competitive exclusion in fish communities and accounts for the observed dominance of small and large species in Boreal/Arctic ecosystems.