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Abstract

Seamounts, often rising hundreds of metres above surrounding seafloor,
obstruct the flow of deep-ocean water. While the retention of deep-water
by seamounts is predicted from ocean circulation models, its empirical
validation has been hampered by large scale and slow rate of the
interaction. To overcome these limitations we use the growth of
planktonic bacteria to assess the retention time of deep-ocean water by
a seamount. The selected Tropic Seamount in the North-Eastern Atlantic
is representative for the majority of isolated seamounts, which do not
affect the surface ocean waters. We prove deep-water is retained by the
seamount by measuring 2.4* higher bacterial concentrations in the
seamount-associated or 'sheath'-water than in deep-ocean water
unaffected by seamounts. Genomic analyses of flow-sorted, dominant
sheath-water bacteria confirm their planktonic origin, whilst proteomic
analyses of the sheath-water bacteria, isotopically labelled in situ,
indicate their slow growth. According to our radiotracer experiments, it
takes the sheath-water bacterioplankton 1.5 years to double their
concentration. Therefore, the seamount should retain the deep-ocean
water for 1.8 years for the deep-ocean bacterioplankton to grow to the
2.4* higher concentration in the sheath-water. We propose that turbulent
mixing of the seamount sheath-water stimulates bacterioplankton growth
by increasing cell encounter rate with ambient dissolved organic
molecules.