Sampling Zooplankton: Correction

We have learned that there is an internal inconsistency in the zooplankton dataset used in our report "Climatic warming and the decline of zooplankton in the California Current" (3 Mar., p. 1324) (1). The data file we used, written from the archive maintained by the National Marine Fisheries Service, was found to change at the end of 1977, without notation, from values of zooplankton normalized by sample volume to raw values of zooplankton volume. Because the series was treated uniformly, pre-1978 data were incorrectly normalized a second time, while post-1978 data were handled correctly. This boosted pre-1978 values by up to a factor of 2. Correction of this error does not change the conclusions of our report. There has been a large decrease in zooplankton biomass during the past 43 years, which is likely related to the concurrent warming of the upper 100 meters. However, the correction does reduce the magnitude of the observed downward trend in zooplankton.