The subpolar gyre regulates silicate concentrations in the North Atlantic

Abstract

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 µM 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.

Temporal Variability in the Nutrient Biogeochemistry of the Surface North Atlantic: 15 Years of Ship of Opportunity Data

“…Regionally, changes are likely dominated by variability in convective mixing. A similar decreasing trend in silicate concentrations was observed in the subpolar North Atlantic (Hátún et al, ). This change was attributed to decreased winter convection, a weaker subpolar gyre, and a higher influx of subtropical origin water.…”

On the Recent Ambiguity of the North Atlantic Subpolar Gyre Index

“…The PC1 presented by Foukal and Lozier (2017, their Figure 2b), resembles Figure 1b, and thus exhibits a more linear trend than any of the previously published gyre index versions (Berx & Payne, 2017;Häkkinen et al, 2013;Häkkinen & Rhines, 2004;Hátún et al, 2005;Hátún, Azetsu-Scott, et al, 2017;Hátún, Olsen, et al, 2017;Larsen et al, 2012). A comparison of some of the previous gyre index versions is provided by Berx and Payne (2017, their Figure 7).…”

“…The general aim of EOF analysis is to efficiently extract the most dominant modes of variability from large data sets by decomposing the associated space-time field into spatial patterns and time indices, or principal components (Wilks, 2006). This analysis has been demonstrated to be skillful in capturing the essential dynamics of the subpolar gyre (SPG), which is now recognized to have significant implications for a wide range of climatic (Lohmann et al, 2009a), and ecological (Hátún, Payne, Beaugrand, et al, 2009;Hátún, Payne, & Jacobsen, 2009;Hátún et al, 2016;Hátún, Azetsu-Scott, et al, 2017;Hátún, Olsen, et al, 2017;Hovland et al, 2013;Solmundsson et al, 2010) aspects of the North Atlantic Ocean.…”

Response to reviewer 2

“…Buoyancy and turbulent fluxes also impact mixed layer depths and influence bloom timing and strength . Consistent with this expectation, links between physical changes in the subpolar gyre and in situ observed changes in nutrients and ecosystems at several subpolar time series sites have been suggested (Johnson et al, 2013;Hátún et al, , 2017.…”