Circulation and temperature effects on the development and distribution of the eggs and larvae of the Antarctic krill, Euphausia superba : a modeling study

Abstract

An existing three-dimensional time-dependent circulation model was adapted to investigate the circulation in the South Shetland Islands - Bransfield Strait region. The simulated circulation distributions show seasonal variability in the upper 150 m. North of the South Shetland Islands there is a westward current along the shelf-slope which is not continuous above 600 m beyond Livingston Island, but is continuous below 1750 m. Northwards flow along the western side of the Weddell Sea enters the eastern Bransfield Strait and continues towards Drake Passage, with a branch of this flow deflected eastward along the southern side of the South Shetland Islands. A second, time- and temperature-dependent model was developed to simulate the descent-ascent behavior of the eggs and larvae of Euphausia superba. Constant temperature simulations (-1 to 2°C) illustrate the importance of temperature in determining the depth to which the krill egg sinks prior to hatching. Warm Circumpolar Deep Water at depth results in the eggs hatching at about 600 m. Larva ascent in the warm water is high which results in the larva reaching the surface just before or just as it develops into the first feeding stage, which has sufficient carbon reserves to drift several days before needing to find food. The two models were combined in a Lagrangian particle tracing model to simulate the trajectories of the krill eggs and larvae. The Lagrangian trajectories show that most of the horizontal transport of the egg-larva particle occurs after the larva has reached the surface. Trajectories of eggs released north of the South Shetland Islands and east of Livingston Island Me westward. Eggs released north of Snow Island are transported into Drake Passage. Krill larvae are transported into the Bransfield Strait from the Bellingshausen and Weddell Seas. The Lagrangian trajectories show that: eggs released north of the South Shetland Islands complete their development faster than eggs released in Bransfield Strait or east of the Antarctic Peninsula, the surface flow is the primary factor influencing the egg-larva trajectories, and the timing of the krill spawning has a significant effect on the eventual position of the feeding larvae.