Dynamics of Calanus Copepodite Structure during Little Auks’ Breeding Seasons in Two Different Svalbard Locations

Mine or my neighbours' offspring: an experimental study on parental discrimination of offspring in a colonial seabird, the little auk Alle alle

Individual recognition (IR) abilities may result from various ecological and naturally selected features of a species. Complex IR mechanisms should develop when the risk of misidentification of a chick is high. For colonial seabirds, the ability to identify their own brood is crucial to ensure parental fitness. Vocalizations seem to be a key component of most parent-offspring interactions, although few studies have assessed the interindividual differences in seabird chick calls and their potential usage in IR. The little auk (Alle alle), which breeds in dense colonies, constitutes a perfect model for testing IR. In this study, we (1) examined chick calls at different stages of the nesting period, and (2) cross-fostered chicks to examine the rate of acceptance/nonacceptance of chicks by parents. We found significant interindividual differences in chick begging and fledging calls. Surprisingly, all cross-fostered chicks in our experiments were accepted by their foster parents, and male parents were as equally likely to accept cross-fostered chicks as females, even though the sexes would be expected to differ in offspring recognition due to different postfledging interactions with the chick. The revealed individuality of chick calls suggests the potential for chick vocal recognition in the studied species, but parent birds may disregard the individual characteristics enabling chick discrimination. This may take place as long as the chick is found in the nest because of the high likelihood that the chick present there is the focal one. However, IR during and after fledging requires further study. Studying the complexity of IR mechanisms is important for better understanding various avian social relationships and interactions.

Gone with the wind - Wind speed affects prey accessibility for a High Arctic zooplanktivorous seabird, the little auk Alle alle

Foraging ecology of chick rearing seabirds is affected mainly by the food availability on feeding grounds, but it can be also modulated by environmental conditions during the foraging trip, in that wind force. Considering predicted strengthening of surface winds over the Arctic Ocean, this factor may have a growing impact on the foraging performance of Arctic seabirds. Here, we studied how wind speed could affect prey accessibility for the High Arctic zooplanktivorous seabird, the little auk Alle alle breeding in Svalbard in 2015-2019. First, we estimated availability of its preferred prey, a cold water copepod Calanus glacialis, based on wider-scale mesozooplankton biomass model and environmental conditions. Then we estimated prey accessibility by including wind speed, the factor affecting the flapping flight performance of little auks commuting from/to the colony. Finally, we compared reproductive performance of the little auks (chick diet, growth rate and survival and duration of foraging flights of adults) between the studied years differing in wind and food availability conditions. We found that wind speed could affect significantly food accessibility for a zooplanktivorous seabird. Despite high spatial and temporal variability in prey availability and accessibility in shelf waters of SW Spitsbergen, interannual differences in duration of foraging flights and chick growth rate, little auks were able to sustain high breeding success confirming their capacity to buffer suboptimal foraging conditions. Our multidisciplinary work, combining multi-year remote sensing of oceanographic conditions, zooplankton availability and accessibility modelling, little auks diet composition and chick growth and survival emphasizes the importance of including wind conditions in the studies of foraging ecology of seabirds.

Phenology of Oithona similis demonstrates that ecological flexibility may be a winning trait in the warming Arctic

Rapidly warming Arctic is facing significant shifts in the zooplankton size-spectra manifested as increasing numbers of the small-sized copepod Oithona similis. Here we present a unique continuous data set covering 22 months, on its copepodite structure along with environmental drivers in the Atlantic-influenced high Arctic fjord Isfjorden (Spitsbergen). Abundance maxima of O. similis were observed in September when the highest seawater temperature was recorded. A high concentration of the indicator species of Atlantification Oithona atlantica was also observed at that time. The clear dominance of O. similis in the zooplankton community during the dark, theoretically unproductive season emphasizes its substantial role in sustaining a continuous carbon flow, when most of the large herbivorous copepods fall into sleeping state. The high sex ratio observed twice in both years during periods of high primary production suggests two main reproductive events per year. O. similis reproduced even in very low temperatures (< 0 °C) previously thought to limit their fecundity, which proves its unique thermal tolerance. Our study provides a new insight on ecology of this key copepod of marine ecosystems across the globe, and thus confirm the Climatic Variability Hypothesis assuming that natural selection favour species with such flexible adaptive traits as O. similis.

In a comfort zone and beyond-Ecological plasticity of key marine mediators

Copepods of the genus Calanus are the key components of zooplankton. Understanding their response to a changing climate is crucial to predict the functioning of future warmer high-latitude ecosystems. Although specific Calanus species are morphologically very similar, they have different life strategies and roles in ecosystems. In this study, C. finmarchicus and C. glacialis were thoroughly studied with regard to their plasticity in morphology and ecology both in their preferred original water mass (Atlantic vs. Arctic side of the Polar Front) and in suboptimal conditions (due to, e.g., temperature, turbidity, and competition in Hornsund fjord). Our observations show that "at the same place and time," both species can reach different sizes, take on different pigmentation, be in different states of population development, utilize different reproductive versus lipid accumulation strategies, and thrive on different foods. Size was proven to be a very mutable morphological trait, especially with regard to reduced length of C. glacialis. Both species exhibited pronounced red pigmentation when inhabiting their preferred water mass. In other domains, C. finmarchicus individuals tended to be paler than C. glacialis individuals. Gonad maturation and population development indicated mixed reproductive strategies, although a surprisingly similar population age structure of the two co-occurring species in the fjord was observed. Lipid accumulation was high and not species-specific, and its variability was due to diet differences of the populations. According to the stable isotope composition, both species had a more herbivorous diatom-based diet in their original water masses. While the diet of C. glacialis was rather consistent among the domains studied, C. finmarchicus exhibited much higher variability in its feeding history (based on lipid composition). Our results show that the plasticity of both Calanus species is indeed impressive and may be regulated differently, depending on whether they live in their "comfort zone" or beyond it.