Mortality and loss functions

The mortality terms for invertebrate consumers and autotrophs are in terms of lost biomass while those for vertebrates refer to the number of individuals lost. Nevertheless the general form of the equations is the same (but the units of the coefficients obviously differ between the vertebrates and other groups). The natural mortality term for group XX is given by

where m_lin,XX is the coefficient of linear mortality for XX, m_quad,XX is the coefficient of quadratic mortality for the group XX, m_O2,XX is the coefficient of oxygen dependent mortality and m_special,XX is the special (additional) loss rate for XX. This rate of “special” mortality is usually set to zero, except in the following cases:

where m_STRESS and m_DIN are the coefficient of mortality due to mechanical stress and fouling by epiphytes, respectively. Lastly:

with m_starve,FX is the threshold ratio of reserve to structural weight at which death due to starvation is likely. The final term of equation (C.61) was adopted from ERSEM I (Bryant et al. 1995) to represent the impact of seabirds and any other top predators not represented explicitly by the dynamic groups in the model. While all the groups in the model had a linear mortality term, some groups (the vertebrates and higher trophic level zooplankton and benthic groups) suffered mortality described by a quadratic term. Only benthic consumers had oxygen dependent mortality, the macrophyte and vertebrate groups had special mortality as shown above and m_top is only applied to the vertebrate groups.

with m_lys,PX the rate of lysis.