Continuous moulting by Antarctic krill drives major pulses of carbon export in the north Scotia Sea, Southern Ocean.

Antarctic krill play an important role in biogeochemical cycles and can potentially generate high-particulate organic carbon (POC) fluxes to the deep ocean. They also have an unusual trait of moulting continuously throughout their life-cycle. We determine the krill seasonal contribution to POC flux in terms of faecal pellets (FP), exuviae and carcasses from sediment trap samples collected in the Southern Ocean. We found that krill moulting generated an exuviae flux of similar order to that of FP, together accounting for 87% of an annual POC flux (22.8 g m-2 y-1). Using an inverse modelling approach, we determined the krill population size necessary to generate this flux peaked at 261 g m-2. This study shows the important role of krill exuviae as a vector for POC flux. Since krill moulting cycle depends on temperature, our results highlight the sensitivity of POC flux to rapid regional environmental change.

Threatened species drive the strength of the carbonate pump in the northern Scotia Sea.

The efficiency of deep-ocean CO2 sequestration is regulated by the relative balance between inorganic and organic carbon export respectively acting through the biological carbon pump (BCP) and the carbonate counter pump (CCP). The composition and abundance of calcifying species in the prevailing oceanic plankton community plays a major role in driving the CCP. Here we assess the role of these calcifying organisms in regulating the strength of the CCP in a Southern Ocean region (northern Scotia Sea) known to be a major hotspot for the drawdown of atmospheric CO2. We show that, when shelled pteropods dominate the calcifying community, the total annual reduction of CO2 transferred to the deep ocean doubles (17%) compared to when other plankton calcifiers dominate (3-9%). Furthermore, predation enhances their contribution through the removal of organic soft tissue. Pteropods are threatened in polar regions by ocean warming and acidification. We determine that their potential decline would have major implications to the comparative strengths of the BCP and CCP.

Trophodynamics of Protomyctophum (Myctophidae) in the Scotia Sea (Southern Ocean).

This study investigated spatial and temporal patterns in distribution, population structure and diet of Bolin's lanternfish Protomyctophum bolini, Tenison's lanternfish Protomyctophum tenisoni and gaptooth lanternfish Protomyctophum choriodon in the Scotia Sea using data collected by midwater trawl during spring, summer and autumn. Protomyctophum bolini was the most abundant species of the genus encountered throughout the Scotia Sea with the greatest concentrations occurring around the Antarctic Polar Front (APF). This species had a life cycle of 2+ years, but spatial differences in population structure were apparent as the I-group was absent from all regions south of the APF, suggesting that the species does not recruit in the Scotia Sea. Protomyctophum tenisoni occurred mostly in waters characteristic of the APF and was absent from the southern Scotia Sea. It had a limited size range, but there was clear size-related sexual dimorphism with males significantly larger than females. The species had a life cycle of c. 2 years, but the I-group (c. 1 year old, 1 November to 31 October the next year) occurred only in regions close to the APF suggesting that recruitment is restricted to these waters. A seasonal southward migration for P. choriodon is likely as the species occurred mostly to the south-west of South Georgia in summer, but extended to the sea-ice sectors in autumn. Protomyctophum choriodon had a life cycle of 4+ years in the Scotia Sea and the population was dominated by age classes >3 years old. Larval stages were absent during the surveys for all species. Diurnal variations in vertical distribution were apparent for all three species. Interspecific variations in diet were evident, but all species were primarily copepod feeders, with Metridia spp., Rhincalanus gigas and Calanus simillimus generally dominating their diet. Small euphausiids, principally Thysanoessa spp., were also an important component of their diets, particularly for P. choriodon which had the largest body size. The spatial and temporal variations in diet for both P. bolini and P. tenisoni were broadly consistent with underlying abundance patterns within the mesozooplankton community.

Geographic, seasonal and ontogenetic variation in cadmium and mercury concentrations in squid (Cephalopoda: Teuthoidea) from UK waters.

Cadmium (Cd) and mercury (Hg) levels were measured in the tissue samples of two loliginid (Alloteuthis sp. and Loligo forbesi) and two ommastrephid (Todarodes sagittatus and Todaropsis eblanae) squid species collected from research cruise and fishery (market) samples in UK waters during 2004-05. Concentrations of Cd were generally higher in the ommastrephids, in all tissues except muscle. Hg concentrations were higher in T. sagittatus than in the loliginids. In L. forbesi, metal concentrations differed between tissues and also varied in relation to body size, geographic origin, and season. Cd levels decreased with increasing body size. This may be related to a shift in the diet with growth, since small L. forbesi feed on benthic invertebrates that have relatively high Cd concentrations, whereas larger individuals prey mainly on fish that have low Cd concentrations. Hg levels increased with body size, indicating its retention, and they were highest at the end of the spawning season and in squid from the English Channel and the Scottish West Coast. It is likely that the ambient concentration of Hg in seawater plays an important part in its accumulation in squid tissues. As it is a short-lived species, L. forbesi may therefore function as a bioindicator species for Hg contamination of the marine environment. Our results indicate that there is no significant danger to humans from consuming squid from UK waters.