In this study we combine for the first time silicon (Si) isotope compositions of small mixed diatom species (d 30 Si bSiO2) and of large handpicked mono-generic (i.e. genus = Coscinodiscus) diatom samples (d 30 Si Coscino) with diatom assemblages extracted from marine sediments in the Peruvian upwelling region in order to constrain present and past silicate utilisation. The extension of a previous core-top data set from the Peruvian shelf demonstrates that d 30 Si Coscino values record near-complete Si utilisation, as these are similar to the isotopic composition of the subsurface source waters feeding the upwelling. In contrast, the d 30 Si bSiO2 of small mixed diatom species increase southward along the shelf as well as towards the shore. We attribute highest d 30 Si bSiO2 values partly to transient iron limitation but primarily to the gradual increase of Si isotope frac-tionation within the seasonal diatom succession, which are mainly recorded by small diatom species during intense bloom events. In contrast, lower d 30 Si bSiO2 values are related to initial Si isotope utilisation during periods of weak upwelling, when low Si(OH) 4 concentrations do not permit intense blooms and small diatom species record substantially lower d 30 Si signatures. As such, we propose that the intensity of the upwelling can be deduced from the offset between d 30 Si bSiO2 and d 30 Si Coscino (D 30 Si coscino-bSiO2), which is low for strong upwelling conditions and high for prevailing weak upwelling. We apply the information extracted from surface sediments to generate a record of the present-day main upwelling region covering the past 17,700 years and find that this location has also been characterized by a persistent offset (D 30 Si coscino-bSiO2). By comparison with the diatom assemblages we show that the coastal upwelling system changed markedly between weak and strong upwelling conditions. In addition, our model calculations to quantify species-specific Si isotope fractionation effects based on the diatom assemblages indicate an overall minor influence that cannot explain the high amplitude in the measured d 30 Si bSiO2 record.Read less <