Seamounts, often rising hundreds of metres above surrounding seafloor, obstruct the flow of deepocean 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 sheathwater. We propose that turbulent mixing of the seamount sheath-water stimulates bacterioplankton growth by increasing cell encounter rate with ambient dissolved organic molecules.The 1,000-year-long global thermohaline circulation 1 connects the bulk deep water of the modern World Ocean, irrespective of barriers erected by continents, islands and thousands of seamounts 2-4 . While continents and islands shape the circulation, seamounts affect this deep-water flow by creating enclosed circulation cells 5 , thereby reducing exchange between the so-called 'sheath-water' retained by seamounts and the surrounding deep water.This does not, however, mean that the sheath-water is stagnant. The interaction of seamounts with deep water currents and waves (internal and tidal) causes complex sheath-water dynamics 6 , specified by the unique geometry of individual seamounts 5 . The complexity arises from interactions of parallel, rapid, turbulent mixing at centimetre-scales on seamount slopes 7,8 and much slower flowing circulations (including Taylor columns) at the seamount-scale 9 . The complex sheath-water dynamics shapes seamount habitats for resident benthos and plankton 5,10,11 , causes erosion, controls sedimentation and affects ferromanganese crust formation on seamount slopes 12 .A number of seamounts peak close to the ocean surface and mix nutrient-rich deep water with the nutrient-poor surface water enhancing local phytoplankton growth 13 and causing a surface seamount effect 5,13,14 ; retention of the produced organic matter in the seamount proximity raises productivity and enriches diversity of the entire seamount-associated ecosystem 9,10,[15][16][17][18][19] . The majority of seamounts do not cause the surface seamount effect because their summits are ...