This study tested the influence of system complexity on the environmental assessment of a chemical. Marine microcosms were perturbed with a phthalate ester, di(2-ethylhexyl) phthalate (DEHP). Concentrations of 1, 10, and 100 µg of DEHP per litre were added to the water-column subsystem of marine microcosms for 30 days during the summer (18°C) and winter (1°C) seasons. Significant portions of the DEHP losses from the water-column subsystem during the experiments were due to the complete breakdown of the parent compound to carbon dioxide (CO2). In the benthic subsystem, DEHP concentrations in the sediment were increased by more than 50 and 250 times the concentration in the water column during the winter and summer seasons, respectively. Statistically significant (α = 0.05) reductions in the fluxes of ammonia (NH3) from the benthic subsystem were observed in microcosms receiving 100 µg of DEHP per litre during the summer. Concentrations of DEHP in selected bivalves ranged from 174 to 229 000 µg of DEHP per wet kilogram, depending upon the water column exposure concentrations and the season of the year.