ABSTRACT: The effects of single additions of pyrene (low, medium or high) on a natural marine plankton community are reported in this study. Direct and indirect effects on the function and structure of bacteria, phytoplankton and zooplankton communities were investigated using a food-web approach in a mesocosm. Phytoplankton communities suffered from direct effects of the exposure to pyrene and, after a lag-period of 2 d, so did the bacterial communities exposed to medium and high pyrene concentrations. Effects on the zooplankton community function were not as evident, mainly due to high variation in the measured prey uptake. Abundance of phytoplankton decreased, and the community composition changed in the exposed communities. The total phytoplankton community activity remained stable, resulting in high specific activity (activity per unit chlorophyll a), which could be interpreted as functional redundancy. However, we suggest that differences in nutrient availability between exposed and unexposed communities actually caused the indicated functional redundancy, as phytoplankton growth in control communities was nutrient limited during the experiment, whereas nutrient regeneration from the directly affected algae provided the exposed communities with better growth conditions. The system function, which integrates the functions of the 3 trophic levels, was affected at all 3 exposures during the first 5 d of the experiment. The change in system function suggests that PAHs (polyaromatic hydrocarbons) might be an important stress factor for pelagic systems, as a one-time exposure of a single compound changes the development of a pelagic community.