  We examine the interaction of transcritical and saddle-node bifurcations in a
predator-prey-nutrient system that is stressed by the presence of a toxicant
affecting the prey. This model, formulated by Kooi et al. ({\sl Ecol. Model.}
{\bf 212}(2008), 304--318), has a two-dimensional invariant sub system with
zero predator density. In the sub system, a pair of prey-nutrient equilibria is
created in a saddle-node bifurcation, while predator invasion in modelled by a
transcritical bifurcation of one of this pair. Interactions of these
bifurcations at codimension-two points give rise to bistable, periodic and
heteroclinic predator-prey-nutrient dynamics. We explain why the the
codimension-two points are numerically detected as cusp and Bogdanov-Takens
points when using standard test functions and propose a new test function for
systems with codimension one trancritical curves.
