Ecotoxicological dynamics subject to stoichiometric constraints

Ecotoxicological models developed over the last couple of decades has contributed significantly to help interpret how contaminants cycle through food webs and impact organisms. However, increasing evidence that organisms experience interactive effects of nutrient food conditions and contaminant stressors has been observed. Stoichiometric Ecotoxicology modeling efforts shed light on chemical contaminant and nutrient cycling and has the potential to help improve toxicological risk assessment protocols. We developed and analyzed a system of non-smooth Ordinary Differential Equations (ODEs) producer-grazer population model, which is the first to incorporate the effects of excess nutrients, as well as nutrient limitations on grazer exposed to toxicants. We use analytical, numerical, and bifurcation analysis to reduce and explore model parameterized for an aquatic system of algae and zooplankton exposed to methyl mercury under varying phosphorus conditions. Under certain environmental conditions, our models predict higher toxicity than previous models that neglect the consequences of excess nutrient conditions can have on grazer populations. Then we extended the model for explicitly tracking the environmental toxicant as well as the toxicant in the populations under stoichiometric constraints. In some cases, our extended model predicts higher individual toxicity on grazer (body burden) compared to the previous model, which increases the effectiveness of risk assessment protocols.