Euphausiid assemblages of the oceanographically complex north-west marine bioregion of Australia

The north-west marine bioregion of Australia, which includes the waters adjacent to the Kimberley and Ningaloo coasts, is influenced by both the Indian and Pacific oceans and has high tropical biodiversity, some of which is conserved in a suite of Marine Protected Areas. In the present study, the epipelagic euphausiid assemblages of this bioregion were investigated and related to the physical and biogeochemical properties of the water column, as well as food availability. Twenty-five euphausiid species were identified, including three new records for Australian waters. Pseudeuphausia latifrons was the most abundant species, dominating the shelf waters across both study areas. Stylocheiron carinatum replaced P. latifrons in the deeper waters where species richness was greater. Off Ningaloo, there were higher concentrations of euphausiids, and this may be linked to the bathymetry, the narrowness of the shelf and the resultant effects of these features on oceanography and biogeochemistry. Assemblages were primarily structured by depth, but mean seawater density, dissolved oxygen, fluorescence and mesozooplankton abundance also significantly explained some of the variation in euphausiid assemblages. The present study has confirmed that the physical and biogeochemical properties of the water column and food availability are recurrent factors affecting euphausiid assemblage variation in the eastern Indian Ocean.