Iris Werner

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

Abstract 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.

The sub-ice fauna of the Laptev Sea and the adjacent Arctic Ocean in summer 1995

Abstract The sub-ice habitat and fauna in the Laptev Sea and the adjacent Arctic Ocean were investigated during the “Polarstern” cruise ARK XI/1 in summer 1995. At the ice-water interface a thin thermo- and halocline developed at many stations due to melting processes. In the lower centi- to decimetres of the ice, an accumulation of organic matter was found (particulate organic carbon: 1.9 mg l−1, chl a: 3.3 μg l−1) that may have provided a food source for the fauna. The water layer directly beneath the ice was inhabited by high numbers of various nauplii (130–23911 ind. m−3), and two ecological groups, the pelagic sub-ice fauna that originates from the surface water plankton, and the sympagic sub-ice fauna that migrates into this boundary layer from the ice interior. The pelagic fauna dominated the sub-ice community both in terms of species number and abundance. Both groups mainly comprised small copepods (e.g. Oithona similis, Oncaea borealis, Pseudocalanus spp., Halectinosoma spp., Tisbe spp.), but foraminifers and pteropods, for example, also occurred regularly. Diversity was generally low. Factors influencing the composition and abundance of the sub-ice fauna were most likely water depth, salinity and sea-ice sediments.

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

Abstract 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.

Environmental conditions and overwintering strategies of planktonic metazoans in and below coastal fast ice in the Gulf of Finland (Baltic Sea)

A field study was conducted in Santala Bay with weekly samplings during February and March 2000. Ice thickness was 20–28 cm, snow cover 0–1 cm. The under‐ice water column was stratified with a cold (−0.3–0.2°C) and less saline (S = 2.1–4.9) interface layer. Concentrations of particulate organic carbon (0.5–5.8 mg POC l ) and algal pigments (0.3–18.2 μg chlorophyll a l ) were higher in the ice than in the water (0.2–0.5 mg POC l , 1.6–7.1 μg chlorophyll a l ) and peaked mostly in the bottom part of the ice. The thin ice and almost lacking snow cover had favoured an early ice‐algal and phytoplankton bloom. The diversity of metazoans was low, with six species in the ice and eight species in the under‐ice water. The rotifer Synchaeta cf. littoralis dominated both in ice and water, with maximum abundances of 230 individuals l in the bottom part of the ice. Rotifer eggs were also observed in the ice. Baltic sea ice seems to be a suitable habitat for rotifers. Nauplii and copepodids of the calanoid Acartia longiremis in the under‐ice water showed some herbivorous feeding (<0.1–0.23 ng gut pigment individual ), but analysis of fatty acids, fatty alcohols and biomarker ratios indicated a more omnivorous/carnivorous diet. Despite low temperatures, this copepod showed growth and development below the ice, doubling in numbers (mainly CI, CII) from 118 to 230 individuals m during the third week of March.

Biogenic Particle Sources and Vertical Flux Patterns in the Seasonally Ice-Covered Greenland Sea

Pelagic and ice-associated particle sources have been investigated to determine their contribution to vertical fluxes from upper ocean layers. Process studies were conducted from 1988 to 1997 during various seasons between 72° N and 82° N in the Marginal Ice Zone (MIZ) and open waters of the Greenland Sea. Ice-bound (in-ice and under-ice) particle production begins as early as April, prior to pelagic production, and provides material which may be set free in the course of melting or originate from food-web processes in the under-ice habitat (namely grazing by sympagic amphipods). These particles may be deposited from surface waters, degraded within pelagic food webs or, in the case of autotrophic components, may serve as a seeding population for pelagic production. Findings on transects from the open water into the pack ice stress the overall importance of the MIZ for particle export. The MIZ is characterized by highly variable physical and biological conditions which foster local phytoplankton blooms. Particle exports from the MIZ are very variable but generally high, with prominent autotrophic diatom contributions (up to 60 mg poc m−2 d−1, 30 mg Opal-Si m−2 d−1 and 107 diatoms m−2 d−1). Analyses of algal pigments and their degradation products, combined with microscopical inventories, permit the differentiation of sources of particle export. Freshly produced material from the MIZ can rapidly sediment to great depths, feeding the benthos and affecting sediment geochemistry.

Observations on Calanus glacialis eggs under the spring sea ice in the Barents Sea

Abstract During a spring cruise to the ice-covered Barents Sea in May 1997, high concentrations of Calanus glacialis eggs were found below the pack ice. Maximum abundance was 5,900 eggs m–3 directly at the ice-water interface. Vertical distribution in the water column showed a decrease with depth, indicating positive buoyancy of the eggs. The possible role of the under-ice habitat as a nursery ground for C. glacialis is suggested.