Krill along the 110°E meridian: Oceanographic influences on assemblages in the eastern Indian Ocean

In pelagic ecosystems in all oceans, euphausiids (krill) are important components of the zooplankton. In May–June 2019, euphausiids were investigated from temperate to tropical waters along the 110°E meridian, originally surveyed in 1962–1963 during the first International Indian Ocean Expedition (IIOE). A total of 28 species of tropical, subtropical and temperate euphausiids were identified from day and night vertical hauls using an Indian Ocean Standard Net (IOSN) through the upper 200 m of the water column between 11.5°S and 39.5°S. Abundances of larvae, juveniles and adults were generally higher in warmer tropical waters, as were the number of species, albeit the highest abundance recorded (4240 inds·1000 m−3), comprising of Thysanoessa gregaria, occurred in the cooler waters south of the Subtropical Front. Species from the genera Stylocheiron and Euphausia dominated along most of the transect line. The different water masses together with the influence of ocean currents appeared to affect the euphausiid assemblages with, for example, the occurrence of the Pacific Ocean species, Euphausia pacifica/nana, likely reaching the study region via the Indonesian Throughflow, and the occurrence of the typically neritic Pseudeuphausia latifrons, likely a result of the westward propagation of Leeuwin Current eddies. Redundancy analysis showed that the properties of water masses had a stronger influence on euphausiid assemblage variability than food availability (chlorophyll a), which was relatively low across most of the transect. In comparison to the first IIOE, which used the same IOSN collection method, a similar number of species was identified from the more temporally extensive sampling period in 1962–1963, though the occurrences of rare species were different. At comparable latitudes during May–June in 1963 and 2019, total euphausiid abundances and proportions of life stages were similar. This rare opportunity to repeat an open ocean transect provides an updated baseline on euphausiid species occurrence, distribution and assemblages in the eastern Indian Ocean.