Zooplankton distribution relative to the shallow hydrothermal plume of Rumble III: an acoustic survey using at 75 kHz ADCP and vertical plankton net tows.
Some scientists believe hydrothermal vents to be the most biologically dynamic regions of the deep sea. Hydrothermal vents release hot and harsh chemicals and metals, such as sulfur, iron and manganese that rise in the water column due to density differences with the surrounding water. While venting is typically observed at depths of thousands of meters, the effects of the nutrients still manage to influence the entire water column, even to the surface. Zooplankton have been found to form aggregations around the rising vent effluent, and surface abundances of zooplankton have also been known to increase as a result of the venting. These observations have been conducted for deep hydrothermal vents; however the discovery of Rumble III, a shallow hydrothermally active seamount in the Kermadec Arc (northeast of New Zealand) gives scientists an excellent opportunity to see how shallow hydrothermal plumes affect zooplankton aggregations. Scientists track zooplankton using an Acoustic Doppler Current Profiler (ADCP) that measures the Doppler shift of reflected acoustic signals that have bounced off of particles suspended in the water column. The intensity of the resulting backscatter signal provides information about the abundance of scatterers within these aggregations, and these backscatter signals can be compared with the signal return over background water. Comparisons of backscatter intensities over Rumble III reveal that zooplankton aggregate both below and above the plume, and that they avoid the main core of the plume itself (as described in previous studies). It is believed that zooplankton dwell around the plume as a response to the physical processes associated with seamounts and in order to use the enhanced food source provided by the venting.