Using sediment data as pre-industrial baseline for marine zooplankton biogeography: planktonic foraminifera communities now systematically different from before the onset of the Anthropocene

Marine ecosystems are changing in a direction that is consistent with global change. However, attribution of the observed changes in biogeography to Anthropogenic forcing requires comparison with a baseline state before marked human influence. Very few observational time series on marine species extend back into the pre-industrial era, rendering attribution of the ongoing changes and evaluation of their magnitude uncertain. Planktonic foraminifera – ubiquitous marine zooplankton – have a rich sedimentary record that can provide a baseline to put present-day changes in a long-term perspective. Here we use a large data set (n = 3,774) of sediment-derived species assemblages of pre-industrial age and compare these with modern (1978-2013; 33 sites; 87 observation years) assemblages from moored sediment traps. We find that the difference between modern and pre-industrial species communities at each sediment trap site scales with historical temperature change. Using the location of the most similar pre-industrial species assemblages we have assessed the (thermal) direction of the change in planktonic foraminifera biogeography. In 85 % of the cases the change in the species community is consistent with the change in temperature, irrespective of whether conditions cooled or warmed. The comparison between the modern and pre-industrial species assemblages indicates that species communities have shifted their latitudinal distribution by ~600 km since the onset of the industrial era (median value), consistent with estimates of modern zooplankton displacement rates. Our results not only confirm that the biogeography of marine plankton is changing consistently with anthropogenic global change, but also demonstrate that the community shifts in planktonic foraminifera have been large enough for modern communities to now differ systematically from their pre-industrial state. Our results thus suggest that marine ecosystems have now entered the Anthropocene.