Abstract. In the North Atlantic Ocean north of 40∘ N, intense
biological productivity occurs to form the base of a highly productive
marine food web. SeaWiFS satellite observations indicate trends of biomass
in this region over 1998–2007. Significant biomass increases occur in the
northwest subpolar gyre and there are simultaneous significant declines to
the east of 30–35∘ W. These short-term changes, attributable to
internal variability, offer an opportunity to explore the mechanisms of the
coupled physical–biogeochemical system. We use a regional biogeochemical
model that captures the observed changes for this exploration. Biomass
increases in the northwest are due to a weakening of the subpolar gyre and
associated shoaling of mixed layers that relieves light limitation. Biomass
declines to the east of 30–35∘ W are due to reduced horizontal
convergence of phosphate. This reduced convergence is attributable to
declines in vertical phosphate supply in the regions of deepest winter
mixing that lie to the west of 30–35∘ W. Over the full time frame of the
model experiment, 1949–2009, variability of both horizontal and vertical
phosphate supply drive variability in biomass on the northeastern flank of
the subtropical gyre. In the northeast subpolar gyre horizontal fluxes drive
biomass variability for both time frames. Though physically driven changes in
nutrient supply or light availability are the ultimate drivers of biomass
changes, clear mechanistic links between biomass and standard physical
variables or climate indices remain largely elusive.
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