The subpolar gyre regulates silicate concentrations in the North Atlantic

The North Atlantic is characterized by diatom-dominated spring blooms that results in significant transfer of carbon to higher trophic levels and the deep ocean. These blooms are terminated by limiting silicate concentrations in summer. Numerous regional studies have demonstrated phytoplankton community shifts to lightly-silicified diatoms and non-silicifying plankton at the onset of silicate limitation. However, to understand basin-scale patterns in ecosystem and climate dynamics, nutrient inventories must be examined over sufficient temporal and spatial scales. Here we show, from a new comprehensive compilation of data from the subpolar Atlantic Ocean, clear evidence of a marked pre-bloom silicate decline of 1.5-2 mu M throughout the winter mixed layer during the last 25 years. This silicate decrease is primarily attributed to natural multi-decadal variability through decreased winter convection depths since the mid-1990s, a weakening and retraction of the subpolar gyre and an associated increased influence of nutrient-poor water of subtropical origin. Reduced Arctic silicate import and the projected hemispheric-scale climate change-induced weakening of vertical mixing may have acted to amplify the recent decline. These marked fluctuations in pre-bloom silicate inventories will likely have important consequences for the spatial and temporal extent of diatom blooms, thus impacting ecosystem productivity and ocean-atmosphere climate dynamics.
This work was initiated through the Nordic Council of Ministers project (AEG-11323). H.H. is supported by the European Union 7th Framework Programme (n. 308299 NACLIM www.naclim.eu), the EU Horizon 2020 Blue-Action project (BG-10-2016, www.blue-action.eu) and the Danish project NAACOS 10-093903/DSF. R.S. is supported by a Marie Curie Clarin Cofund Grant (ACB-1431). S.P. was funded by the Danish government through the research center 'Climate and oceanographic changes in the marine area between Greenland and Faroes - Influences on plankton and fish'. C.J. was funded under the European Union's Horizon 2020 research and innovation under grant agreement No. 678760 (ATLAS). This output reflects only the author's view and the European Union cannot be held responsible for any use that may be made of the information contained therein. Funding information on the model simulations and the observation programs is given in the Supplementary Information 1. The model simulations were supported by the co-operative project "RACE - Regional Atlantic Circulation and Global Change" funded by the German Federal Ministry for Education and Research (BMBF). We thank Solva Jacobsen for providing the Faroe Shelf silicate record.