Krill in the Leeuwin Current system: Influence of oceanography and contribution to Indian Ocean zoogeography

Alicia Sutton

Euphausiids (krill) are important in the food webs of pelagic ecosystems and constitute the prey of a range of species including fishes, whales and seabirds. However, knowledge about euphausiid diversity, distribution and abundance in the south-east Indian Ocean off Western Australia is limited. The influence of the Leeuwin Current system on horizontal and vertical structuring of euphausiid assemblages was investigated from a series of plankton surveys with concurrent oceanographic sampling along the west and north-west coasts of Australia. In total, 34 species of euphausiids were identified and assemblages were significantly structured across shelf, shelf break and oceanic waters, as well as alongshore. Further, vertical structuring of assemblages was evident from depth-stratified sampling across a thermohaline front between Leeuwin Current Water and Sub Tropical Surface Water. For all surveys, assemblages were related to a suite of environmental variables, and were significantly influenced by the physio-chemical properties of the prevailing water masses, as well as indicators of food availability, such as chlorophyll a. To ascertain the environmental variables driving euphausiid zoogeography in the Indian Ocean (20°N - 40°S and 20°E - 122°E), data on euphausiids from the Leeuwin Current system were combined with distribution records from the literature (spanning the past 50 years), to generate measures of species richness and taxonomic distinctness across the basin. Salinity and dissolved oxygen were found to be important drivers of euphausiid zoogeography, and were particularly significant with respect to the low numbers of species in the northern Indian Ocean. Finally, to understand the role of euphausiids in marine food webs off Western Australia, fatty acids and stable isotopes placed common euphausiid species from the Perth Canyon (32°S) as omnivorous but feeding primarily on dinfolagellates. Overall, this thesis expands on the knowledge of euphausiid diversity, distribution and abundance in the south-east Indian Ocean and confirms the influence of boundary currents on euphausiid distributions.

Krill in the Leeuwin Current system: Influence of oceanography and contribution to Indian Ocean zoogeography

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