Vertical structuring of epipelagic euphausiid assemblages across a thermohaline front in the south-east Indian Ocean

Abstract

Oceanographic fronts are strong discontinuities in water mass properties and can result in differences in biota over short horizontal distances. The vertical structuring of epipelagic euphausiids was investigated across an ephemeral thermohaline front between Leeuwin Current Water (LCW) and Sub Tropical Surface Water (STSW) in the southeast Indian Ocean. Vertical ontogenetic layering was evident and euphausiid larvae were most abundant in the 0-50 m stratum in both water masses. Differences in assemblages between water masses were mostly driven by the numerical abundance of shared species; most species were in greater abundance in LCW. LCW was characterized by a uniform water column and no significant structuring of assemblages with depth. Structuring was, however, evident in STSW and analyses indicated this was a result of stratification of temperature, salinity and dissolved oxygen. Higher chlorophyll a concentrations and zooplankton settled volumes were evident in LCW, as well as the upper 100m of STSW. Decreases in temperature and salinity were correlated with the variation in assemblages at >100m depth in STSW. This is the first study to examine vertical structuring and environmental correlations of euphausiid life stages and assemblages across a thermohaline front in the south-east Indian Ocean.