Arctic sea ice as feeding ground for amphipods – food sources and strategies

The amphipod species Gammarus wilkitzkii, Apherusa glacialis, Onisimus nanseni and O. glacialis live permanently associated with the Arctic sea ice. Qualitative and semi-quantitative investigations of gut contents and faeces showed that all four species use detritus as the main food source. Detrital lumps from the underside of sea ice had the same item composition as amphipod gut contents and faeces. Crustacean remains and ice algae were additional food items, but overall they were quantitatively less important. All species are omnivorous; however, differences in gut contents, behavioural observations and functional–morphological studies of the mandibles suggest a differentiation within this feeding strategy. G. wilkitzkii is a detritivorous-carnivorous-necrophagous-suspension-feeding species and shows the most complex feeding strategy. O. nanseni and O. glacialis are predominantly detritivorous-necrophagous, whereas A. glacialis is characterised as a more herbivorous-detritivorous species. By using a variety of the available food sources under Arctic sea ice, the amphipods are well adapted to the under-ice habitat and are less influenced by temporal and spatial variations. Furthermore, the wide food spectrum of all four species reduces the intra- and interspecific competition in a habitat where certain food sources are limited or only seasonally available. Accepted: 30 June 2000     

Abundance, biomass and small-scale distribution of cryopelagic amphipods in the Franz Josef Land area (Arctic)

Arctic sea ice is inhabited by several amphipod species. Abundance, biomass and small-scale distribution of these cryopelagic (=ice associated) amphipods were investigated near Franz Josef Land in summer 1994. The mean abundance of all species was 420 ind./m²; the mean biomass was 10.61 g ww/m². Gammarus wilkitzkii was the dominant species, whereas Apherusa glacialis, Onisimus nanseni and O. glacialis were only scarcely found. Amphipods were concentrated at the edges of ice floes and were less frequent in areas further away under the ice. The relationship between the distribution and ecological/physiological requirements of cryopelagic amphipods, as well as the small-scale morphology of Arctic sea ice, are discussed. Received: 14 January 1998 / Accepted 14 April 1998     

Lipid indicators of the diet of the sympagic amphipod Gammarus wilkitzkii in the Marginal Ice Zone and in open waters of Svalbard (Arctic)

Triacylglycerols were the major lipid and wax esters a minor lipid in the Arctic autochthonous, sympagic amphipod, Gammarus wilkitzkii, from less than 1 year old to 3 years old in the Marginal Ice Zone around Svalbard. The fatty acids of the triacylglycerols, especially in young G. wilkitzkii, were mainly characteristic of diatoms and, to a lesser extent, flagellates. Small amounts of 20:1n-9 and 22:1n-11 fatty acids characteristic of calanoid copepods were also present in the triacylglycerols in young G. wilkitzkii from the Marginal Ice Zone and the amounts of both of these fatty acids increased in the triacylglycerols as the animals matured. G. wilkitzkii in open waters in Kongsfjord had minor amounts of triacylglycerols rich in 20:1n-9 and 22:1n-11 and major amounts of wax esters characteristic of calanoid copepods. We conclude that young G. wilkitzkii in the Marginal Ice Zone feed predominantly on ice algae and that they consume increasing amounts of calanoid copepods as they mature, albeit with ice algae remaining a prominent component of their diet. In open waters, young G. wilkitzkii consume mainly calanoid copepods.     

Ice amphipod distribution relative to ice density and under-ice topography in the northern Barents Sea

Arctic ice amphipods are part of the sympagic macrofauna in the Marginal Ice Zone of the northern Barents Sea and represent an important link from lower to higher trophic levels in some Arctic marine food chains. The species diversity in this area (1995/1996) consisted of four species: Gammarus wilkitzkii, Apherusa glacialis, Onisimus nanseni and Onisimus glacialis. The larger ice amphipod, G. wilkitzkii, was the most abundant with the highest biomass (>90%), whereas A. glacialis was abundant, but contributed little to the total biomass (<4%). The other two species were found only in small numbers. Both abundance and biomass of ice amphipods decreased along a latitudinal gradient from north to south across the Marginal Ice Zone. Their distribution was also related to the under-ice topography with regard to mesoscale structures (edge, flat area, dome and ridge). Overall, the abundance and biomass on ridges were much higher in comparison to other mesoscale structures, although edges also showed high abundance, but low biomass. The large G. wilkitzkii was consistently abundant on ridges. The small A. glacialis was predominately associated with edges, but also showed high numbers in dome-shaped areas. The Onisimus species were present in low numbers at all structures, and their biomass contributed <10% on any one structure. The reasons for different distribution patterns of the dominant amphipod species under Arctic sea ice are probably related to different requirements of the species, especially for food, shelter and physiological conditions. Accepted: 27 November 1999     

Molecular relationships of gammaridean amphipods from Arctic sea ice

The information on the biology and ecology of the Arctic sea ice-associated amphipods (Apherusa glacialis, Gammarus wilkitzkii, Onisimus glacialis, and O. nanseni) has increased, but their molecular taxonomic information still remains undisclosed. In the present study, we investigated long-range DNA sequences spanning 18S to 28S rDNA of these four sea ice-associated amphipods and analyzed their genetic relationships with other amphipod taxa. Variations of rDNA within the individuals of the same species were not detected. Phylogenetic analyses showed that each ice amphipod was separated, forming clusters with other conspecifics. Pairwise comparisons led to similar phylogenetic results, showing that the molecular taxonomy of the ice amphipods was in accordance with morphological systematics. In addition, these findings suggest that all four amphipods have little genetic variation compared with their morphologically defined conspecifics from temperate regions. Based on DNA taxonomy, G. wilkitzkii was supported as a species in good standing, refuting a recent synonymization with Gammarus duebeni. Considerably low genetic divergences of O. glacialis and O. nanseni in 18S, ITS, and 28S rDNA suggest the presence of population distinctions within species.     

Osmotic responses to hyposmotic stress in the amphipods Gammarus wilkitzkii,Onisimus glacialis and Parathemisto libellula from Arctic waters

Summary The amphipods Gammarus wilkitzkii and Onisimus glacialis, which commonly occur in burrows and pockets or near the bottom of the Arctic sea-ice, are euryhaline osmoregulators, displaying regulation of haemolymph concentrations of sodium and chloride. O. glacialis is the most efficient regulator. The amphipod Parathemisto libellula, which seems restricted to the subice fauna, is a stenohaline osmoconformer with no ability to regulate haemolymph concentrations of sodium and chloride. Thus, the physiological measurements made support the observed micro-distribution of the amphipods in relation to the Arctic sea ice.     

Sympagic amphipods in the Arctic pack ice: redescriptions of <Emphasis Type="Italic">Eusirus holmii</Emphasis> Hansen, 1887 and <Emphasis Type="Italic">Pleusymtes karstensi</Emphasis> (Barnard, 1959)

During an expedition into the Arctic Ocean, in September 2004, six different species of amphipods were collected in the ice above 82°N. All six species (Apherusa glacialis, Gammarus wilkitzkii, Onisimus nanseni, O. glacialis, Pleusymtes karstensi and Eusirus holmii) were observed to be living adjacent to the sea ice or partly within its brine channels. The nature of the association with the ice for the last two species is uncertain, but the finding raises important questions regarding our knowledge of the sympagic fauna. Based on the obtained material, the two species E. holmii and P. karstensi are redescribed, and their association with the sea ice is discussed.     

Size,age and diet of polar cod,Boreogadus saida (Lepechin 1773), in ice covered waters

Summary Polar cod (Boreogadus saida) associated with drifting sea-ice were collected in the western Barents sea and north of Svalbard with dip-nets while SCUBA-diving in 1986 and 1987. Length-frequency measurements and otolith-readings suggested that the specimens were either one or two years old. The diet of fish from the western Barents sea (first-year ice) consisted mainly of copepods (Calanus finmarchicus, Calanus glacialis) and the hyperiid amphipod Parathemisto libellula. Fish collected north of the Svalbard archipelago (multi-year ice) had a more diverse diet, in which P. libellula and the sympagic amphipod Apherusa glacialis contributed more to the total diet biomass than copepods.     

Lipids and life strategies of Calanus finmarchicus, Calanus glacialis and Calanus hyperboreus in late autumn, Kongsfjorden, Svalbard

Stage IV and V copepodites were the dominant forms of Calanus finmarchicus, C. glacialis and C. hyperboreus in Kongsfjorden in late September 1997. Stage IV and V copepodites of C. glacialis and C. hyperboreus were rich in lipid, largely wax esters, and were well fitted to overwinter. Stage IV copepodites of C. finmarchicus were also rich in wax esters, but stage V copepodites of C. finmarchicus were less wax ester-rich. Large size increments between stage IV and V copepodites and between stage V copepodites and females were noted in C. finmarchicus. A very large increment between stage IV and V copepodites was noted for C. glacialis but the size difference between stage V copepodites and females was very small in this species. Particularly large increments were noted between stage IV and V copepodites of C. hyperboreus and also between stage V copepodites and females of this species. The very large, wax ester-rich C. hyperboreus is well adapted to survive the most extreme variations in the Arctic, in Arctic basin waters, whereas the smaller, wax ester-rich C. glacialis is adapted to survive less extreme Arctic variations, as in Arctic shelf waters. The smallest of the three, C. finmarchicus, is best adapted to survive the more predictable waters of the North Atlantic and the Barents Sea. Accepted: 3 January 2000     

Feeding, respiration and life history of the hyperiid amphipod Themisto libellula in the Arctic marginal ice zone of the Greenland Sea

Daily ingestion rates of the pelagic hyperiid amphipod Themisto libellula were studied in the marginal ice zone of the Arctic Fram Strait by feeding experiments, respiration measurements and an allometric approach based on body mass. Amphipods were collected by stratified multiple opening/closing net hauls and Rectangular Midwater Trawl (RMT 8) in August 2000 during the expedition ARK XVI/2 of R/V “Polarstern”. T. libellula occurred with abundances of 0.043 and 0.015 ind. m⁻³ in the upper 30 m of the water column at two RMT 8 stations. Based on respiration data, the daily ingestion necessary to cover metabolic energy demands measured 1.9±0.6% of body carbon per day. Actual prey consumption during feeding experiments with Calanus copepodids as prey was very similar and accounted for 1.9±1.5% day⁻¹, indicating that feeding on Calanus can meet the energy demands of T. libellula. In general, experimental results were slightly lower than the maximum potential ingestion (2% day⁻¹ for an individual of median body dry mass of 32 mg) estimated by an allometric equation based on body mass, but feeding experiments showed a strong variability. Reduced metabolism and low ingestion rates of T. libellula are consistent with low ambient temperature, large body size, slow growth and long life span of this polar species. The effect of the active pelagic life style of T. libellula on metabolism and ingestion rate is discussed in comparison to the sympagic (i.e. ice-associated) amphipod Gammarus wilkitzkii of similar body size living in the same environment. In relation to the mesozooplankton biomass in the investigation area, the predation impact by T. libellula was low. However, high-Arctic conditions also limit the secondary production of principal prey species, such as Calanus glacialis and Calanus hyperboreus, so that even low predation rates may affect the growth of prey populations.     

Zooplankton boom and ice amphipod bust below melting sea ice in the Amundsen Gulf, Arctic Canada

Early summer in the Arctic with extensive ice melt and break-up represents a dramatic change for sympagic–pelagic fauna below seasonal sea ice. As part of the International Polar Year-Circumpolar Flaw Lead system study (IPY-CFL), this investigation quantified zooplankton in the meltwater layer below landfast ice and remaining ice fauna below melting ice during June (2008) in Franklin Bay and Darnley Bay, Amundsen Gulf, Canada. The ice was in a state of advanced melt, with fully developed melt ponds. Intense melting resulted in a 0.3- to 0.5-m-thick meltwater layer below the ice, with a strong halocline to the Arctic water below. Zooplankton under the ice, in and below the meltwater layer, was sampled by SCUBA divers. Dense concentrations (max. 1,400 ind. m⁻³) of Calanus glacialis were associated with the meltwater layer, with dominant copepodid stages CIV and CV and high abundance of nauplii. Less abundant species included Pseudocalanus spp., Oithona similis and C. hyperboreus. The copepods were likely feeding on phytoplankton (0.5–2.3 mg Chl-a m⁻³) in the meltwater layer. Ice amphipods were present at low abundance (<10 ind. m⁻²) and wet biomass (<0.2 g m⁻²). Onisimus glacialis and Apherusa glacialis made up 64 and 51% of the total ice faunal abundance in Darnley Bay and Franklin Bay, respectively. During early summer, the autochthonous ice fauna becomes gradually replaced by allochthonous zooplankton, with an abundance boom near the meltwater layer. The ice amphipod bust occurs during late stages of melting and break-up, when their sympagic habitat is diminished then lost.     

Chick diet of dovekies Alle alle in Northwest Greenland

This paper reports the results of an investigation of dovekies (Alle alle) breeding at Hakluyt Island in the Thule District, Northwest Greenland. The paper presents a comprehensive examination of the chick diet of dovekies in the species' most important breeding area. Food delivered to nestlings consisted primarily of copepods Calanus hyperboreus and C. glacialis in the copepodite stages IV and V. Themisto libellula, other amphipods, decapods and fish larvae in small numbers were found in the chick diet. The zooplankton in the chick diet is presented both in terms of numbers and dry weight. The characteristic copepod of high-arctic waters, C. hyperboreus, was found to be the single most important species in the chick diet, contributing approximately 54% of the dry food weight. The food samples revealed low diversity, and the three species C. hyperboreus, C. glacialis and T. libellula made up 91.5% of the total zooplankton dry weight. The results from this paper are compared with previous dovekie-chick diet studies. Accepted: 14 June 2000     

Breaking the ice: large-scale distribution of mesozooplankton after a decade of Arctic and transpolar cruises

Mesozooplankton collected during five summer expeditions to the Arctic Ocean between 1987 and 1991 was analysed for regional patterns in biomass and species distribution, distinguishing between an epipelagic (0–100 m) and a deeper (0–500 m) layer. A total of 58 stations was sampled mainly in the Nansen, Amundsen and Makarov Basins of the central Arctic Ocean and in areas of the Greenland Sea, West Spitsbergen Current and Barents Sea. Results from the different expeditions were combined to create a transect extending from the Fram Strait across the Eurasian Basin into the Makarov Basin. Mesozooplankton dry mass in the upper 500 m decreased from 8.4 g m⁻² in the West Spitsbergen Current to less than 2 g m⁻² in the high-Arctic deep-sea basins. In the central Arctic Ocean, biomass was concentrated in the upper 100 m and was dominated by the large copepods Calanus hyperboreus and C. glacialis. In contrast, the mesozooplankton in the West Spitsbergen Current was more evenly distributed throughout the upper 500 m, with C. finmarchicus as the prevailing species. The distribution of abundant mesopelagic species reflected the hydrographic regime: the calanoid copepod Gaetanus tenuispinus and the hyperiid amphipod Themisto abyssorum were most abundant in the Atlantic inflow, while Scaphocalanus magnus was a typical component of the high-Arctic fauna. The relatively high mesozooplankton biomass and the occurrence of boreal-Atlantic species in the central Arctic Ocean are indicators for the import of organic material from allochthonous sources, especially from the northern North Atlantic. Hence, in spite of its enclosure by land masses, the Arctic Ocean is characterized by an exchange of water masses and organisms with the North Atlantic, and advection processes strongly influence the distribution of plankton species in this high-latitude ecosystem. Received: 18 December 1997 / Accepted: 11 April 1998     

On the distribution of sympagic macro-fauna in the seasonally ice covered Barents Sea

Summary The sympagic fauna (= sea ice fauna) of the Barents Sea was investigated on nine (2–5 weeks) cruises in the period 1983–1988. The amphipods Apherusa glacialis, Onisimus sp. and Gammarus wilkitzkii were the most conspicuous and abundant invertebrate species associated with the ice. Mean biomass-values ranged from 0 to about 2 g/m², and were less than 0.001 g/m² in five of the nine cruises. Apherusa glacialis, Onisimus sp. and Gammarus wilkitzkii are regarded as autochthonous sympagic species, and as such dependent upon a permanent ice cover. Allochthonous sympagic species were few and without significant importance. The variations in species abundance and biomass were believed to be closely connected to the age and drift-history of the ice. The density of the sympagic fauna increases as a result of increasing influx of ice into the Barents Sea from the Arctic Ocean. Density of the sympagic fauna were believed to increase with decreasing distance to the multiyear/first-year ice boundary. A. glacialis was found further away from the spreading centers and in higher numbers than the other sympagic species due to its greater motility. No day-night variations in the occurrences of the sympagic fauna were recorded.     

In-situ observations on the distribution and behavior of amphipods and Arctic cod (Boreogadus saida) under the sea ice of the High Arctic Canada Basin

The occurrence and behavior of sympagic amphipods and Arctic cod (Boreogadus saida) were studied in the High Arctic Canada Basin by diving under the ice at seven stations in summer 2002. Still images of video-transects were used to obtain animal abundances and information on the structure of the ice environment. Mean amphipod abundances for the stations varied between 1 and 23 individuals m^–2, with an increase towards the western part of the basin. The standard deviation within the 31–51 images analyzed per station was small (<1 individual m^–2). Gammarus wilkitzkii was found in low abundances, often hiding in small ice gaps. Small amphipods (Onisimus spp., Apherusa glacialis, and juveniles of all species) tended to move freely along the bottom of the floes. B. saida occurred in narrow wedges of seawater along the edges of melting ice floes at three stations in water depths of 10–50 cm and was never found under the ice. The fish occurred in schools of 1–28 per wedge. Fish were inactive and did not escape the approaching diver. Resting in the wedges may be a strategy to reduce energetic requirements and avoid predators.We dedicate this publication to Professor Dr. J. Lenz (Kiel University).     

Winter–spring feeding and metabolism of Arctic copepods: insights from faecal pellet production and respiration measurements in the southeastern Beaufort Sea

Faecal pellet production (FPP) and respiration rates of Calanus glacialis, C. hyperboreus and Metridia longa were measured under land-fast ice in the southeastern Beaufort Sea during the winter–spring transition (March–May 2004) prior to the phytoplankton spring bloom. Despite different overwintering and life cycle strategies and remaining low concentrations of suspended chlorophyll a and particulate organic matter, all species showed increasing FPP rates in spring. A corresponding increase in respiration was only observed in C. glacialis, while respiration remained constant in C. hyperboreus and M. longa. In C. glacialis and C. hyperboreus calculated ingestion covered respiratory expenditures. The constancy of the oil sac volume in M. longa suggests that the animals fed during winter-spring. Pre-bloom grazing as shown here seems to acclimate the copepod populations physiologically for the upcoming high feeding season, so that they are able to resume maximum grazing and reproduction as soon as the phytoplankton bloom is initiated.     

Faecal pellet production by Arctic under-ice amphipods – transfer of organic matter through the ice/water interface

The underside of Arctic sea ice is inhabited by several autochthonous amphipod species (Apherusa glacialis, Onisimus spp., Gammarus wilkitzkii). The amphipods graze on ice-bound organic matter, such as ice algae, detritus and ice fauna, and release faecal pellets into the underlying water column, thus forming a direct link between the sea ice and the pelagic ecosystems. Experiments on faecal pellet production rates showed species-specific differences, which were related to size of the animals. The smallest species, A. glacialis, produced the highest mean number of pellets (15.4 pellets ind.^-1 d^-1), followed by Onisimus spp. (2.7 pellets ind.^-1 d^-1) and the largest species, G. wilkitzkii (1.1 pellets ind.^-1 d^-1). Relative carbon content of the pellets was very similar in all species (21.2–22.6% dry mass). Juvenile amphipods (Onisimus spp., G. wilkitzkii) produced more pellets with less POC than adults. Based on field determinations of the POC concentration in the lowermost 2 cm of the sea ice (mean: 36.4 mg C m^-2) and mean amphipod abundances (A. glacialis: 33.8 ind. m^-2, Onisimus spp.: 0.5 ind. m^-2, G. wilkitzkii: 9.4 ind. m^-2) in the Greenland Sea in summer 1994, the amount of POC transferred from the ice to the water by faecal pellet production was estimated (0.7 mg C m^-2 d^-1 or almost 2% of ice-bound carbon). Since this process probably takes place in all ice-covered Arctic regions as well as during all seasons, grazing and pellet production by under-ice amphipods contributes significantly to matter flux across the ice/water interface.     

Pelagic occurrence of the sympagic amphipod Gammarus wilkitzkii in ice-free waters of the Greenland Sea – dead end or part of life-cycle?

The sympagic (=ice-associated) amphipod Gammarus wilkitzkii usually lives attached to the underside of Arctic sea ice. During an expedition to the Greenland Sea in May/June 1997, high numbers of this species were found in pelagic Rectangular Midwater Trawl catches (0–500 m water depth) in an ice-free area, 35–42 km away from the ice edge. The amphipods seemed to have maintained position in the water column for at least 4 days. Mean biomass data (length: 2.9 cm, organic content: 73% dry mass), gut fullness (>50% in 85% of specimens) and sex ratio (females:males = 1:1.5) of these amphipods were very similar to values for under-ice populations. Due to their relatively high body density (mean: 1.134 g cm⁻³), the energy demand for swimming was assumed to be high. Measurements of oxygen consumption of swimming and resting amphipods (8.8 and 4.0 J g wet mass⁻¹ day⁻¹, respectively) suggested that, from an energetic point of view, G. wilkitzkii would maintain position in an ice-free water column for the time period. Accepted: 11 January 1999     

Contrasting the early life histories of sympatric Arctic gadids <Emphasis Type="Italic">Boreogadus saida</Emphasis> and <Emphasis Type="Italic">Arctogadus glacialis</Emphasis> in the Canadian Beaufort Sea

The early life stages of Boreogadus saida and Arctogadus glacialis are morphologically similar, making it difficult to assess differences in their ecological niche. The present study documented for the first time the early life stage ecology of A. glacialis, compared it to that of B. saida, and identified the factors separating the niches of the two sympatric species. The 10,565 larval gadids collected in the Beaufort Sea from April to August of 2004 and 2008 were identified to species either directly by genetics and/or otolith nucleus size, or indirectly with a redistribution procedure. Between 8.0 and 8.7 % of all gadids were assigned to A. glacialis. Larvae of A. glacialis were longer at hatch and experienced lower mortality rates than those of B. saida. The two species shared similar spatiotemporal and vertical distributions, hatching season, and growth rate. Under the ice, feeding incidence of B. saida was low (14 %) relative to A. glacialis (88 %). At lengths <15 mm, both species specialized on different prey. The diet of fish >15 mm overlapped (Schoener’s index = 0.7), with Calanus glacialis and C. hyperboreus providing >50 % of the carbon intake of both species. The higher mortality in B. saida may be explained by the smaller size at age from hatching to metamorphosis and a lower under-ice feeding incidence. The early larval stage appears to be the key period of niche divergence between the two species.