  We analyze the joint effect of contaminants and nutrient loading on
population dynamics of marine food chains by means of bifurcation analysis.
Contaminant toxicity is assumed to alter mortality of some species with a
sigmoidal dose-response relationship. A generic effect of pollutants is to
delay transitions to complex dynamical states towards higher nutrient load
values, but more counterintuitive consequences arising from indirect effects
are described. In particular, the top predator seems to be the species more
affected by pollutants, even when contaminant is toxic only to lower trophic
levels.
