Early spawning of Antarctic krill in the Scotia Sea is fuelled by "superfluous" feeding on non-ice associated phytoplankton blooms

• Published in: Deep Sea Research Part II: Topical Studies in Oceanography
• Main Authors: Schmidt, Katrin, Atkinson, Angus, Venables, Hugh J., Pond, David W.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2012
• Subjects: Antarctic; Scotia Sea; Antarc*; Antarctic Krill; Euphausia superba; ice algae; Sea ice
• Online Access: https://pure.uhi.ac.uk/en/publications/3c9b4094-f196-4cde-b30a-2b15605ed832; https://doi.org/10.1016/j.dsr2.2011.05.002

The spawning success of Antarctic krill (Euphausia superba) is generally assumed to depend on substantial winter sea ice extent, as ice biota can serve as a food source during winter/spring and the seasonal ice melt conditions the upper water column for extensive phytoplankton blooms. However, direct observations during spring are rare. Here we studied krill body condition and maturity stage in relation to feeding (i.e. stomach fullness, diet, absorption of individual fatty acids and defecation rate) across the Scotia Sea in November 2006. The phytoplankton concentrations were low at the marginal ice zone (MIZ) in the southern Scotia Sea (Stn. 1, 2, and 3), high in open waters of the Southern Antarctic Circumpolar Current Front (SACCF) in the central Scotia Sea (Stn. 5), and moderate further north (Stn. 6 and 7). Krill had low lipid reserves (similar to 6.5% of dry mass. DM), low mass:length ratios (similar to 1.7 mg DM mm(-1)), and small digestive glands (similar to 7% of total DM) near the ice edge. The stomachs contained lithogenic particles, diatom debris, and bacterial fatty acids, but low proportions of diatom-indicating fatty acids, which suggest that these krill were feeding on detritus rather than on fresh ice algae. In the SACCF, krill had higher lipid reserves (similar to 10% of DM), high mass:length ratios (similar to 2.2 mg DM mm(-1)), and large digestive glands (similar to 16% of total DM). Stomach content and tissue composition indicate feeding on diatoms. In the north, moderate food concentrations co-occurred with low lipid reserves in krill, and moderate mass:length ratios and digestive gland sizes. Only in the phytoplankton bloom in the SACCF had the mating season already started and some females were about to spawn. Based on the way krill processed their food at the different stations, we indicate two mechanisms that can lead to fast regeneration of body reserves and oocyte maturation in E. superba. One is ``superfluous'' feeding at high food concentrations, which maximises the overall ...