Diel feeding behaviour of krill in the Gerlache Strait, Antarctica

Abstract

Abundance, gut fluorescence and gut content of juvenile krill Euphausia superba (15 to 20 mm) were measured during a diel cycle in the Gerlache Strait (Antarctic Peninsula). Krill remained in the upper layers (0 to 100 m) during the day and migrated downward below this depth during the night, coinciding with the vertical ascent of the copepod Metridia gerlachei to shallower layers. Krill fed on phytoplankton during the day (as deduced from gut fluorescence measurements), whereas they switched to carnivory during the night (as deduced from gut contents). The vertical migration and the feeding behaviour of krill agree with different observations in the literature and gives an additional explanation to the observed inverse relationship between krill and non-krill zooplankton. The fact that krill is able to prey on mesozooplankton suggests that euphausiids can exert a top-down effect which structures the plankton community of Antarctic waters.