Using a multidisciplinary approach combining stable isotope analysis, biotelemetry and energetic modelling, this study presents indirect evidence that during winter daylight periods copepods are still numerous in surface waters less than 50 m off southwest Greenland. Little auks are known to feed almost exclusively on copepods in summer (e.g. Karnovsky et al. 2008
), and blood δ15
N values found here are in agreement with birds preying upon copepods during this period (Karnovsky et al. 2008
). Isotopic δ15
N signatures are also similar for summer and winter copepods (Sato et al. 2002
; ), as well as for summer and winter adult blood samples (). This strongly suggests that little auks feed predominantly on copepods in winter. During this period, birds may also consume other prey, but in a minor proportion since amphipods, euphausiids or fish larvae have higher isotopic signatures than copepods (Tamelander et al. 2008
) and would thus lead to higher blood δ15
N values. Winter blood values slightly higher than those measured in summer might reflect such minor amphipod/euphausiid/fish consumption, as well as the general food web enrichment from summer to winter (Rolff 2000
Further, we infer from little auk diving behaviour that copepods remain abundant in winter surface waters, even during the daylight period when they are supposed to perform vertical migration to greater depths (; Fortier et al. 2001
; Berge et al. 2009
). Even though our dive depth results are consistent with summer recordings (Harding et al. 2009
), further studies are now needed to confirm the first winter dive data for little auks. Their performance is impressive, with hundreds of dives conducted daily, over 1 minute and to depths of up to 50 metres (see electronic supplementary material S2). However, this maximum dive depth is extremely shallow when compared with the depths greater than 500 metres at which copepods are assumed to occur in winter.
Copepods are classically thought to enter diapause in the autumn and to spend all winter in deeper water (Falk-Petersen et al. 2009
). During early spring, and mostly synchronized with the phytoplankton spring bloom, they are supposed to migrate back to surface waters (Falk-Petersen et al. 2009
). However, a recent acoustic study has shown that even in winter and during the high-latitude polar night, some copepods perform a synchronized diel vertical migration in the epipelagic layer (Berge et al. 2009
), whereby they migrate to the food-rich surface layers during darkness and move to deeper water during daylight, where they are out of reach from predators (Fortier et al. 2001
). Our study strongly suggests that during winter, swarms of copepods are still present in surface waters (sensu Berge et al. 2009
), where they are targeted by little auks both during the daylight and the dark period.
Using a bioenergetic model, we estimated that the amount of copepods available to little auks wintering off southwest Greenland is substantial, since the daily intake of the little auk population wintering off southwest Greenland is estimated to amount to 3600–7200 tonnes. This result is based on a diet composed only of copepods and it might be slightly overestimated since little auks probably also consume a few other prey items such as amphipods and euphausiids. The standing stock of copepods is currently unknown and it is impossible to estimate the proportion of the population caught per day by wintering birds. However, a range of 3600–7200 tonnes of copepods caught per day during the winter off southwest Greenland is larger than the daily amount of copepods consumed during summer by the breeding population of the North Water Polynya (i.e. 3450–6900 tonnes of wet food consumed per day, including 2340–4680 tonnes of copepods; Karnovsky & Hunt 2002
). Therefore, this suggests that the daily presence of copepods in the upper layer of the water column throughout the winter is much more important than usually assumed, with far-ranging consequences for regional food webs and energy flow.