Arising from D. G. Boyce, M. R. Lewis & B. Worm , 591–596 (2010)10.1038/nature09268 ; Boyce et al. reply Identifying major changes in global ecosystem properties is essential to improve our understanding of biological responses to climate forcing and exploitation. Recently, Boyce et al.1 reported an alarming, century-long decline in marine phytoplankton biomass of 1% per year, which would imply major changes in ocean circulation, ecosystem processes and biogeochemical cycling over the period and have significant implications for management of marine fisheries. Closer examination reveals that time-dependent changes in sampling methodology combined with a consistent bias in the relationship between in situ and transparency-derived chlorophyll (Chl) measurements generate a spurious trend in the synthesis of phytoplankton estimates used by Boyce et al.1. Our results indicate that much, if not all, of the century-long decline reported by Boyce et al.1 is attributable to this temporal sampling bias and not to a global decrease in phytoplankton biomass.