Salp/krill interactions in the eastern Atlantic sector of the Southern Ocean

Spatial distribution and feeding ecophysiology of the Antarctic krill, Euphausia superba, and the tunicate, Salpa thompsoni, were investigated along the 6°E meridian between 49 and 65°S. Krill and salps displayed pronounced spatial segregation along the transect. Salps clearly dominated north of the 56°S parallel, while krill dominated south of it. Overlap in krill/salp distribution occurred only at ≈56°S. This region coincided with the northernmost expansion of the subsurface cold water layer and with the Winter Ice Edge (WIE) region suggesting a clear biotopical separation of these key-stone species in the Antarctic pelagic food web. Krill and salp grazing rates determined using the in situ gut fluorescence technique were low (<1% of daily primary production) to moderate (<25%) in the areas south and north of the WIE region. However, in the region of their overlap, krill and salps alone consumed up to 86% of daily primary production. Assuming that all faecal pellets produced sink, krill and salp would be able to re-package and remove from surface layers <2%, up to 37% and 3–12% of primary production per day in the regions south, within and north of the WIE, respectively. In real terms, krill/salp mediated in situ carbon flux range from 0.01 to 3.7 and from 9.6 to 23.8 mg C m−2d−1 south and north of the WIE region, respectively, and would be as high as 48 mg C m−2d−1 in the WIE region.