Relative production of Calanoides acutus (Copepoda: Calanoida) and Euphausia superba (Antarctic krill) at South Georgia, and its implications at wider scales

Antarctic krill are often described as the major species in Southern Ocean food webs, but there have been no direct site-specific comparisons between their production and that of copepods that dominate mesozooplankton biomass. Here, we compare biomass, growth and production of Euphausia superba (Antarctic krill) and the copepod Calanoides acutus at South Georgia, in the Atlantic sector of the Southern Ocean. Average acoustically derived krill biomass across the 5 study years was 2.6 g C m–2, 4 to 5 times that of Stages CIV and CV of C. acutus. Mean length of krill was 41 ± 4 mm, and the average growth rate was 0.10 mm d–1, giving an average mass-specific growth rate of 0.0084 d–1. This equated to a daily gross production of krill of 0.022 g C m–2 d–1. Gross production of Stages CIV and CV of C. acutus was similar, around 0.026 g C m–2 d–1. These stages of C. acutus represent about 25% of the total copepod biomass around South Georgia, so total copepod production here may exceed that of krill by a factor of 4 during the productive summer months. Biomass of copepods and krill in other parts of the Southern Ocean differ by similar orders of magnitude, whilst growth rates are likely to remain within the range measured in this study. Therefore, our finding has relevance outside the local South Georgia system. All of the krill production is likely to be rapidly consumed by higher predators within the surface layers, whilst it is estimated that around 17% of copepod production may be sequestered at depth during winter diapause.

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