Internal wave turbulence at a biologically rich Mid-Atlantic seamount

The turbulence regime near the crest of a biologically rich seamount of the Mid-AtlanticRidge southwest of the Azores was registered in high spatial and temporal resolution. Internaltides and their higher harmonics dominate the internal wave motions, producing considerableshear-induced turbulent mixing in layers of 10±50 m thickness. This interior mixing ofabout 100 times open-ocean interior values is observed both at a high-resolution temperaturesensor mooring-site at the crest, 770 m water depth being nearly 400 m below the top ofthe seamount, and a CTD-yoyo site at the slope off the crest 400 m horizontally away, 880m water depth. Only at the mooring site, additionally two times higher turbulence is observednear the bottom, associated with highly non-linear wave breaking. The highest abundanceof epifauna, notably sponges, are observed just below the crest and 100 m down the easternslope (700±800 m) in a cross-ridge video-camera transect. This sponge belt is located in awater layer of depressed oxygen levels (saturation 63±2%) with a local minimum centeredaround 700 m. Turbulent mixing supplies oxygen to this region from above and below and isexpected to mix nutrients away from this biodegraded layer towards the depth of highestabundance of macrofauna