Allometric relationships of ecologically important Antarctic and Arctic zooplankton and fish species

Allometric relationships between body properties of animals are useful for a wide variety of purposes, such as estimation of biomass, growth, population structure, bioenergetic modelling and carbon flux studies. This study summarizes allometric relationships of zooplankton and nekton species that play major roles in polar marine food webs. Measurements were performed on 639 individuals of 15 species sampled during three expeditions in the Southern Ocean (winter and summer) and 2374 individuals of 14 species sampled during three expeditions in the Arctic Ocean (spring and summer). The information provided by this study fills current knowledge gaps on relationships between length and wet/dry mass of understudied animals, such as various gelatinous zooplankton, and of animals from understudied seasons and maturity stages, for example, for the krill Thysanoessa macrura and larval Euphausia superba caught in winter. Comparisons show that there is intra-specific variation in length–mass relationships of several species depending on season, e.g. for the amphipod Themisto libellula. To investigate the potential use of generalized regression models, comparisons between sexes, maturity stages or age classes were performed and are discussed, such as for the several krill species and T. libellula. Regression model comparisons on age classes of the fish E. antarctica were inconclusive about their general use. Other allometric measurements performed on carapaces, eyes, heads, telsons, tails and otoliths provided models that proved to be useful for estimating length or mass in, e.g. diet studies. In some cases, the suitability of these models may depend on species or developmental stages.

Epipelagic euphausiids assemblage in the Gerlache Strait (Antarctic Peninsula) during January 2015

The spatial distribution of the epipelagic Euphausiids was investigated during the first Colombian Scientific Expedition to Antarctica “Expedición Caldas” in the Gerlache Strait during the austral summer of 2015. Surface zooplankton was collected at 20 oceanographic stations with a simple conical net of 0.6 m in diameter mouth and 200 µm of mesh net. Four euphausiid species were collected in 70 % of the sampling stations: Euphausia crystallorophias Holt and Tattersall 1906, Euphausia superba Dana 1850, Thysanoessa sp. and Thysanoessa macrura Sars 1883. Euphausia crystallorophias was the most abundant species with maximum density of 818 ind 1000 m-3 associated with the entrance of the Wilhelmina Bay, in the central area of the Gerlache Strait. The northern and southern sectors of the strait had significant differences in surface temperature and salinity associated with significant differences in the composition of the euphausiid species assemblages between both sectors. Although E. crystallorophias was the numerically dominant euphausiid species in the Gerlache Strait, E. superba was mostly distributed in the northern sector while Thysanoessa sp. was only distributed in the southern sector.

In situ growth rate estimates of Southern Ocean krill, Thysanoessa macrura

Growth, which is intrinsically linked to environmental conditions including temperature and food availability are highly variable both temporally and spatially. Estimates of growth rates of the Southern Ocean euphausiid Thysanoessa macrura are currently restricted to limited studies which rely upon repeated sampling and length-frequency analysis to quantify growth rates. The instantaneous growth method (IGR) was used to measure the growth rate of T. macrura successfully in the southern Kerulen Plateau region during summer, providing the first IGR parameters for the Southern Ocean euphausiid species. Results of the four-day IGR incubation indicate a period of low somatic growth for adult T. macrura. Males had a longer intermoult period (IMP) (62 days) than females (42 days), but the sexes exhibited similar daily growth rates of 0.011 mm day−1 and 0.012 mm day−1 respectively. Juveniles exhibited the fastest growth, with an IMP of 13 days and daily growth rate of 0.055 mm day−1 indicating a prolonged growth season, similar to the Antarctic krill E. superba. Consequently, we highlight the usability of the IGR method and strongly encourage its use in developing a comprehensive understanding of spatial and seasonal growth patterns of T. macrura.

Distribution of euphausiid larvae along the coast of East Antarctica in the Dumont d'Urville Sea (139–145°E) during summer 2004

The distribution of euphausiid larvae along the coast of Terre Adélie, Antarctica, was assessed using oblique tows of a double-framed bongo net at 38 stations during summer 2004. Larvae of Euphausia crystallorophias and Thysanoessa macrura were observed. For E. crystallorophias larvae, the calyptopis I stage was dominant along the coast, while the most commonly observed stage of T. macrura was the furcilia. The distribution of E. crystallorophias larvae were correlated with abiotic factors, including depth and sea surface salinity, whereas those of T. macrura larvae were correlated with biotic factors, especially chlorophyll a and nitrate. Developmental stages of both species increased in age from west to east in the survey area, with younger developmental stages (metanauplius and calyptopis I) in the western part of the region and older stages (calyptopis II and III and furcilia I to VI) in the eastern part near the Mertz Glacier Tongue (MGT). It is suggested that these patterns could be linked with the water circulation and wind: near the MGT gyres could concentrate all developmental stages of both species near the coast, while katabatic winds near Dumont d'Urville will promote larval advection seawards, with younger stages near the coast and older stages further offshore.