Biomarkers of polycyclic aromatic hydrocarbon (PAH) toxicity to aquatic plants were developed using the wood preservative creosote. We tested physiological indicators of photosynthetic performance in cultures of the floating aquatic macrophyte Lemna gibba (G3). Creosote was applied at concentrations ranging from 1–300 ppm, and plants were grown under laboratory lighting that mimics the relative levels of UV radiation found in natural sunlight (simulated solar radiation; SSR). Population growth bioassays demonstrated that similar to individual PAHs, creosote exhibited UV-enhanced phototoxicity. Chlorophyll content and chlorophyll fluorescence induction parameters were also diminished by creosote, and closely corresponded to functional responses of population growth by the end of each experiment. Fluorescence induction thus is a validated biomarker assay that is closely and functionally related to population growth inhibition in aquatic plants.