Program

For several years, a persistent phytoplankton bloom has been observed off the Kerguelen Plateau in the Indian Ocean.  It was hypothesized that hydrothermal vents supplied iron to the biological community and increased biological productivity.  The HEOBI (Heard Earth-Ocean-Biological Investigation) project was designed to find hydrothermal vents, measure the iron in the water column, observe biological activity, and investigate the role of the ocean circulation and mixing over the plateau in distribution of iron supplied by the vents.  In January and February of 2016, the RV Investigator went to MacDonald and Heard Islands on the plateau to confirm this hypothesis.  Although no hydrothermal vents were located and there was no evidence of geothermal vents or geothermal influence on temperatures, the water column was enriched with iron and many acoustic plumes with bubbles were observed in the water column.  The strongest upward vertical velocities were observed at the sites of these acoustic/bubble plumes.  Generally, tides dominated the ocean circulation with a general northeastward drift across the plateau.  The water column was well mixed most of the time, with typical temperature differences between the surface and the bottom, 100-200 m, less than 0.2^oC and often less than 0.1^oC.  Surface stratification in the upper ocean developed through insolation.  Deep stratification was established through a combination of upwelling from canyons and over the edge of the plateau and tidal advection.  The stratification was primarily removed by a combination of wind and tidal mixing.  Persistent winds of 30 knots mixed the upper 20-50 m.  Strong wind events, 40-60 knots, mixed the water column to 100-200 m depth, essentially the entire water column over the plateau.  The homogenized the water column resulted in the surface often indicating prior upwelling and/or advective events.  Between the strong wind events, benthic tidal friction mixed the bottom 30-50 m.  Although the water column was essentially unstratified at two intensively investigated plume sites, tidal velocities were baroclinic, probably due to topographic controls.