A new application of reflectance spectroscopy enables inferences of lake sediment chlorophyll a concentrations and hence of historical trends in lacustrine primary production. In a survey of six arctic lakes on Baffin Island (Nunavut, Canada), pronounced increases of spectrally-inferred chlorophyll a concentrations are consistently expressed in sediments deposited during the 20th century. Climate warming appears to be increasing both aquatic chlorophyll a production and its sequestration to sediments, as these lakes enter new biological regimes that are largely unique in the context of the late Holocene.