The importance of Antarctic krill in biogeochemical cycles

Antarctic krill (Euphausia superba) are swarming, oceanic crustaceans, up to two inches long, and best known as prey for whales and penguins – but they have another important role. With their large size, high biomass and daily vertical migrations they transport and transform essential nutrients, stimulate primary productivity and influence the carbon sink. Antarctic krill are also fished by the Southern Ocean’s largest fishery. Yet how krill fishing impacts nutrient fertilisation and the carbon sink in the Southern Ocean is poorly understood. Our synthesis shows fishery management should consider the influential biogeochemical role of both adult and larval Antarctic krill

The trophodynamics of Southern Ocean Electrona (Myctophidae) in the Scotia Sea

The Scotia Sea is one of the most productive regions of the Southern Ocean, but its surface waters are experiencing a rapid increase in temperature, which may be changing the behaviour and distribution of many myctophids and their prey species. Electrona antarctica and Electrona carlsbergi are two of the most abundant myctophids in the region, but their ecology is poorly understood and their response to ongoing environmental change is difficult to determine. This study investigated spatial and temporal patterns in their abundance, population structure and diets using mid-water trawl nets deployed across the Scotia Sea during spring, summer and autumn. E. antarctica was the most numerically abundant species (0.09–0.21 ind. 1,000 m−3), with greatest concentrations occurring in the sea-ice sectors. E. carlsbergi occurred in more northern regions, comprising densities of 0.02–0.11 ind. 1,000 m−3. There was evidence of seasonal variation in depth distribution, size-related sexual dimorphism and size-specific vertical stratification for both species. Latitudinal trends in sex ratio and female body size were apparent for E. antarctica. Its diet varied between regions, seasons and size classes, but overall, Euphausia superba, Metridia spp. and Themisto gaudichaudii were the dominant prey items. E. carlsbergi appeared not to recruit in the Scotia Sea. Its diet was dominated by copepods, particularly Rhincalanus gigas and Metridia spp., but regional, seasonal and ontogenetic variations were evident. This study contributes to our understanding of how mid-water food webs are structured in the Southern Ocean and their sensitivity to ongoing environmental change

Comparing Bongo net and N70 mesozooplankton catches: using a reconstruction of an original net to quantify historical plankton catch data

If Southern Ocean plankton communities are changing in response to climate, biases in various nets need to be evaluated to help understand regional and temporal differences between historical and contemporary sample collections. A comparison of the catching ability of a Bongo net (mesh aperture 200 μm) and a reconstructed version of an N70 net (upper mesh aperture 445 μm, lower mesh aperture 195 μm) as used by the Discovery Investigations was therefore undertaken. Forty Bongo and forty N70 samples were obtained from 10 stations in the Southern Ocean during December 2009. Bongo net catch abundance was ~3 times greater than the N70 and ~4 times greater when only copepod instars <0.5 mm body length were considered. The Bongo net captured more plankton at all chlorophyll a (Chl a) concentrations encountered, although the difference was less at high Chl a when the N70 appeared to filter all size classes of copepod more efficiently, as well as retaining a relatively greater proportion of non-copepod plankton. Application of these findings was made to a previous study in which N70 samples from 1926/1927 were compared to a series of Bongo net hauls made post-1995. By ‘correcting’ abundances for net bias among a common set of 45 taxa, N70 data were within the range of variability seen in the Bongo net samples. Making such inter-net comparisons allows us to use historical collections and better judge the nature and magnitude of change in these plankton communities

Comparison of five methods for estimating growth of Calanus helgolandicus later developmental stages (CV�CVI)

The activity of the enzymes aminoacyl-tRNA synthetases (AARS) in adult females and males and copepodites stage V of Calanus helgolandicus was studied at the L4 time-series station in the English Channel from June 2002 to December 2003. AARS activity was explored as an index of somatic growth in the laboratory as well as in the field by comparison with other methods of measuring growth: (1) the direct method (Heinle in Chesapeake Sci 7:59�74, 1966), (2) the weight increment (WI) method, (3) the Hirst and Bunker (HB) equation (Limnol Oceanogr 48(5):1988�2010, 2003) and (4) the egg production (EPR) method. AARS activity showed a significant correlation with the direct measurement of growth in the laboratory (R 2=0.55). However, the correlation was lower for growth assessed either with the WI or the HB approaches in the field (R 2=0.05�0.17). Female AARS activity showed a positive correlation with specific EPR during the reproductive season (R 2=0.40) but no relationship was found during the non-reproductive period