The observed size difference within developmental stages suggests that the growth rate varied between AC0 Antarctic krill cohorts. The study of larval and juvenile krill indicated that sea-ice derived carbon sources are very important for krill populations residing in ice-covered waters during winter (Chapter 3).
Not only consisted most of the krill stomach contents of food items closely associated with sea ice, stable isotopes and fatty acids confirmed that sea-ice associated carbon sources are an important food source over a larger temporal scale (Chapter 3). Although exact turnover times of fatty acids are largely unknown, the proportion of sea-ice algae-derived carbon was approximately 2/3 in storage fatty acids, which have a shorter turnover time, while it was lower in the membrane fatty acids, which have a longer turn over time (Kohlbach et al. 2017), suggesting that the utilization of sea-ice derived food sources increased with seasonal progression. In addition, the AC0 krill showed low lipid levels suggesting that their reserves were low, also indicating that sea-ice derived food sources may not support high growth rates during wintertime, but are potentially critical for their survival in the under-ice habitat (Chapter 3). Variation in the stomach contents of AC0 krill in the northern Weddell Sea mirrored variation found in the zooplankton community structure (David et al. 2017), suggesting that feeding occurred opportunistically.
In a study from the same expedition, AC0 krill caught below the ice-water interface (~11m depth) showed increased feeding on zooplankton and detritus during the night (Halbach 2015), providing further evidence for opportunistic feeding behaviour. Although the AC0 krill from our study were caught at different times of day, no differences in the average diet composition were found between day and night (unpublished data),
confirming that variation was a result of differences in food availability rather than sampling time (Chapters 2 & 3). In addition, Halbach (2015) found that, in the pack-ice region, increased feeding activity was related to increased food availability in the sea ice. It is clear that food items released or associated with sea ice provide the main food source in ice-covered waters during winter (Chapter 3; Halbach 2015; Kolhbach et al. 2017; Meyer et al 2017). AC0 E. superba caught with a Rectangular Midwater Trawl (RMT) in the South Georgia region during mid-winter, had full stomachs which was related to high phytoplankton biomass in this region (Meyer et al. 2017). This indicated that the young krill residing in open water were able to find ample food (Halbach 2015). Daly (2004) found lower krill abundances in the open waters of this area compared to ice-covered waters and, furthermore, found that young krill continued feeding at the under-ice surface even when phytoplankton concentration increased with the melting sea under-ice. This suggest that AC0 krill prefer to reside underneath the ice for more reasons than solely food abundance (Marshall 1988;
Meyer et al. 2009). This is supported by the high recruitment of Antarctic krill found after years with a high sea-ice extent (Kawaguchi & Satake 1994; Siegel & Loeb 1995). Lower energy expenditure due to protection from currents has been proposed to be an advantage of residing between sea-ice structures (Meyer et al.
2017), although passive sinking, resulting in DVM, has also been suggested to be energy saving behaviour (Youngbluth 1975). The multiple advantages of sea ice for young Antarctic krill raises the question on the suitability of open water as a habitat during winter.
Further support for the importance of sea-ice resources arises from differences found in fatty acids and
Figure 7.1: Backward-projected drift trajectories of sea-ice areas from Chapters 2 & 3. The specific ice area is tracked backwards until the ice reaches a position next to a coastline, or the ice concentration at a specific location reaches a threshold value of <40% when ice parcels are considered lost (Krumpen et al. 2016). Stations with a distance <60 km to each other were represented as one dot. Triangular symbols mark the approximate position of the two ice camps discussed in Chapter 3. Dashed circles, grouping stations by krill cohort, are distinguished by different colours. Dots in corresponding colours mark the back-tracked origin of sea-ice drift trajectories from these station groups. Colour code of drift trajectories represent the monthly sea-ice position (from Kohlbach et al. 2017).
stable isotopes between cohorts. Findings indicated that a lower availability of sea-ice resources over a larger time-scale can negatively impact the condition of krill larvae residing in ice-covered waters (Chapter 3). The results suggest that spawning time and location have a marked influence on the development of larval krill during advection due to the encountered food availability, which thus likely impacts their survival rate. This emphasizes the importance of multiple spawning batches in a reproductive season. The impact of potential environmental changes on food availability and quality is variable between regions and should be evaluated at the spawning areas and the subsequent nursery and feeding grounds at a population level. Modelling the advection pathway of AC0 krill from the winter Weddell Sea indicated that they originated from the north-western Weddell Sea, from April onwards (Meyer et al. 2017). The sea-ice found at the different sampling locations of the cohorts showed different sea-ice drift histories, parts of which originated near the coast in the southern part of the Weddell Sea (Fig. 7.1; Kohlbach et al. 2017). Together with differences in the isotopic signatures of the ice-algae from different ice floes sampled during the study (Kohlbach et al. 2017), this suggests that there is indeed a variation in sea-ice algae and other in-ice fauna assemblages found in different ice floes depending on timing and origin of sea-ice formation.
The high ingestion of pennate diatoms by AC0 Antarctic krill and the slightly higher surface water chlorophyll a at the northernmost stations was likely a result of the ice starting to melt. However, due to the low abundance of zooplankton grazers at these stations, reduced competition could also be a factor. The reduced competition could again be a result of a shift in vertical distribution of e.g. copepods, due to changes in depth allocation of resources and/or ocean floor depth (David et al 2017). In contrast, Meyer et al. (2017) did not find an increase in diatoms in the stomach contents of krill from the north-east area, which could be a result from the krill being caught at deeper water layers as they were sampled with RMT and Bongo nets that do not sample the surface very well and require open water to be handled in. Results indicate that a combination of studies on different spatial scales can be beneficial to obtain a complete view of a species biology and ecology, as studies on a small scale can obtain a biased picture due to the sampling of particular features and a lack of coverage of certain habitat types, while studies on a larger scale may obtain a more general picture, but can overlook certain particular important features of a habitat due to a lack of detail. In addition, it again marks the importance of sampling the ice-water interface layer, as certain analyses on krill from other sampling sites can yield different results.
In the Arctic, the one- and two-year old polar cod were also found feeding on species that were closely associated with sea ice such as A. glacialis and the copepod Tisbe spp. in summer (Chapter 5). This importance of sea-ice associated food sources was again supported by fatty acid and stable isotope analyses. The fish were, furthermore, in good condition as suggested by the high total lipid content of the liver (Chapter 5) and high energy density (David et al. 2016). Compared to Southern Ocean species, the energy density of the polar cod (David et al. 2016) was similar to that of myctophid fish, or the highest values found in notothenoid fish, which represented the highest energy densities of all taxa investigated (Chapter 4). This indicates that polar cod provide a high quality food source for top predators residing in the ice-covered region.
The differences in the estimated proportion of ice-algal produced carbon between tissues suggests that the polar cod had been feeding more on ice-associated food sources before the time of sampling than during sampling, although exact turnover of carbon rates are also unknown (Chapter 5). Other studies estimating the proportion of ice-algal produced carbon in the tissue of polar cod show ranges between negligible and high importance (Budge et al. 2008; Christiansen et al. 2012; Graham et al. 2014). This could be influenced by the fish’s food, the size of which tends to increase with increasing fish size (Renaud et al. 2012) or which can have a different composition when the fish reside in open water and/or shelf regions (Graham et al.
2014; Budge et al. 2008).
Other species that are regarded as less ice-associated, such as Calanus spp. and Themisto libellula, were found to be part of the diet of polar cod (Chapter 5). Whereas the input of ice-algal produced carbon in these less ice-associated species is still considerable (Kohlbach et al. 2016), this could potentially lower the sea-ice algal isotopic signal of the fish. However, Calanus spp and T. libellula have a wide depth distribution and are regularly found in the ice-water interface (David et al. 2015). As young polar cod are not known to occupy deeper water layers in the oceanic part of the central Arctic Ocean, the ice-water interface can be regarded as the sole feeding ground of young polar cod in this region during the summer season. Estimates of daily consumption compared to food availability have indicated that sufficient food is available in the under-ice surface to support polar cod growth (Chapter 5). Future investigations on the diet of polar cod and the abundance and distribution of its prey can give further insight in feeding behaviour, and potential seasonal and regional variation. The use of a combination of methods that all deliver specific information has already been suggested to be an effective way of studying a species feeding habits (Schmidt et al. 2006).
Studying the energy density and proximate composition of species does not give direct information on the diet, but can help to gain information on feeding activity, trophodynamics and life cycle strategy (Chapter 4). For example, decreasing lipid reserves, and concomitant energy density, during winter can be found in species that rely on energy reserves during this season (Chapter 4 and references therein). Seasonal and regional differences between a size/weight/energy density relationship within species gives information on variability in the availability and quality of food for these species. In addition, the relationship between size and energy density gives information on energy allocation in individuals of varying age or developmental stage (Chapter 4 and references therein). For many species in the Southern Ocean, sufficient seasonal and regional coverage for assessing such life cycle strategies and relationships is currently lacking (Chapter 4).
Findings show that, for species inhabiting the ice-water interface layer, sea ice can provide an important direct or indirect food source. The presence of this food source, and the consequences it has on the distribution of species, has major implications for the structure of polar food webs.