Phytoplankton was measured fluorometrically at four sites along the Fly River, PNG, one site in the Strickland River and in nine floodplain off-river waterbodies, to evaluate the impact of the Ok Tedi mine. Although previous investigations had proposed that the copper-rich mine wastes would depress algal standing crop, our data, the first direct measurements of phytoplankton standing crop, indicate that the mine wastes increase algal standing crop. The likely cause of the increase is the runoff of nitrogenous compounds from explosives. No mine impact was detected in chlorophyll in the floodplain Off-River Water Bodies. Chlorophyll was often concentrated in a prominent sub-surface maximum, and compositional differences made it clear that the potamoplankton is a discrete assemblage and not primarily washout from the lentic water bodies. Most investigations of mine discharge impacts on waterways have primarily focussed on toxic metals, few have considered the potential impacts of other chemicals such as explosives.