A significant change in phenanthrene toxicity to P. subcapitata was only seen in the presence of one type of CNT (SWCNT) when considering total phenanthrene concentration in the system. However, based on the measured concentrations of phenanthrene in the water phase, an increase in toxicity in the presence of CNTs was observed relative to phenanthrene only. This indicates that not only the phenanthrene remaining in the water, but also the phenanthrene adsorbed to CNTs, is available to the algae. In the case of D. magna, adsorption to CNTs reduces overall bioavailability of phenanthrene for some CNT types. However, a significant proportion of adsorbed phenanthrene remains bioavailable. Microscopy images show significant ingestion of CNTs by D. magna, which may offer an alternative route for phenanthrene uptake and toxicity.