The aim of this thesis is to acquire knowledge on the association of key species with the sea-ice habitat by looking at several aspects of their life cycle and biology. Furthermore, the aim is to increase knowledge on the functioning of polar food webs. The final objective is to estimate the level at which polar ecosystems and their key inhabitants are affected by changing sea-ice habitats by investigating the distribution, population structure, diet and energy density of trophic key species in the under-ice habitat.
Specific goals are to:
• Assess the abundance and distribution of trophic key species in the under-ice habitat;
• Estimate the importance of sea ice-derived carbon sources (ice algae, sea-ice microfauna) for trophic key species.
• Increase knowledge on the functioning of polar food webs.
Although Antarctic krill (Euphausia superba, hereafter krill) has been studied extensively, many questions remain unanswered. This is in part caused by their large flexibility in behaviour resulting in annual, regional and seasonal differences, which also varies with ontogeny (Atkinson et al. 2008; Flores et al. 2012b). Particular knowledge gaps addressed in this thesis are regarding larval and juvenile krill during winter time, when young Antarctic krill are known to reside in the under-ice surface layer. As they hatch in the preceding summer, larval and juvenile krill have to deal with the harsh conditions of their first winter, which is regarded a critical period for krill survival. The role of sea ice herein is expected to be significant, but particularly large scale observations are scarce. Especially for sustainable fisheries management it is important to ensure that new recruits survive to reproductive age, hence the need to gain insight into the critical winter period. Therefore, it was investigated how young, age class 0 Antarctic krill use the sea ice during winter. This was done, firstly, by looking at their population dynamics and composition in the ice-water interface and comparing that to deeper water layers (Chapter 2). Secondly, the diet, and the contribution of sea-ice associated food sources of young krill was studied. By looking at a large population and its structure as investigated in Chapter 2, it was possible to draw conclusions on drivers of spatial variability in the diet (Chapter 3). By using different methods to examine the diet, temporal variability and effects on body condition could be studied.
Although krill is a key species, it is not the only species in the diet of top predators. The top predator’s diet depends on season and region, and variability can have an effect on e.g. the growth of offspring. Furthermore, diel and seasonal shifts in vertical distribution alter the availability of prey for top predators between seasons and/or different times of day (Ainley et al. 1991). To make a good estimate of the value of a species as a food source as well as for making energy flux and food web models, a good estimate of the energetic
value of different prey species is necessary. Therefore, the energetic density of a variety of zooplankton and nekton species caught in the Southern Ocean was measured. Measurements were compared with values from literature, resulting in a review summarizing what is currently known about the energetic value of species, and the source and degree of variation of energetic values between and within species (Chapter 4).
Furthermore, the review gives a good overview of available data enabling the identification of knowledge gaps.
In the Arctic, young, one- or two-year-old, polar cod have been found to dwell in the under-ice surface layer. Although this was known, David et al. (2016) conducted the first large scale investigation of the abundance and distribution of polar cod in the ice-water interface layer using SUIT. The study resulted in the hypotheses that young polar cod use the sea ice as a transport mechanism and suggested that the fish found in the central Arctic Ocean originated from spawning grounds in the shelf regions of the Kara and Laptev Seas. In order to investigate the importance of sea ice as a food source for polar cod, a diet study, using a similar multiple method approach as to Antarctic krill, was conducted (Chapter 5). Stomach content analysis gave insights in the species composition of the diet, while fatty acid and stable isotope analyses revealed the proportional contribution of ice-algal produced carbon in different tissues of the fish.
Apart from polar cod, amphipods constitute a major part of the zooplankton community associated with the sea-ice food web. The amphipod Apherusa glacialis is a highly abundant, ice-obligate species and an important link in between sea-ice and the pelagic food webs. Studies have shown that it feeds on sea-ice resources at least during summer. But how the sea-ice structures the A. glacialis population and consequently how this changes with a changing sea-ice habitat is largely unknown. Small scale studies, often performed by SCUBA diving, yield variable results. The relationship of this amphipod with general properties of the Arctic environment would be helpful to gain insight in preferred habitats and large-scale consequences of a reduction in sea ice. Therefore, the effect of sea ice and other environmental parameters on the abundance and distribution of this species has been examined. The results of a research expedition conducted north of Svalbard were compared with results from a another research expedition (David et al. 2015), which was conducted in the Eurasian basin of the Arctic Ocean (Chapter 6).
In the final chapter (Chapter 7) the findings of this thesis are discussed. The order of the chapters is based on geography for clarity, starting with the Southern Ocean chapters followed by Arctic Ocean chapters. The general discussion will, however, be structured per topic based on the above mentioned aims.
Recommendations for further research are given.