Fieldwork took place during the summer of 2013, 2014, 2015 and 2016. In 2013, the sediments of one lake in the community of Ivujivik and nine lakes on the northern side of Digges Island East were sampled by helicopter (sites with DI prefix—see Fig. 1). For this study, these lakes have been grouped with the reference sites since Digges Island is substantially larger than the study islands and has no nesting eiders present. In 2014, 2015 and 2016, coastal islands in Digges Sound, Québec (sites with D prefix—see Fig. 1) and east of Cape Dorset, Baffin Island (sites with A prefix—see Fig. 1) were accessed by boat with the aid of local Inuit guides.

Study site and sampling locations. The field work for this project took place in 2013, 2014, 2015 and 2016. Surveys were completed by boat with the assistance of local guides from Cape Dorset, Nunavut and Ivujivik, Quebec. Samples were also collected from the East Bay island migratory bird sanctuary and from lakes and ponds on Digges Island

On our island surveys, a team of 3 to 5 people landed by boat at each study island to complete bio-physical surveys. On islands with colonies, we sampled the main pond that typically occurs at the centre of the colony on each island. We identified this pond by surveying the entire island and then selecting the pond surrounded by the highest density of eider nests. Most islands only had one or two ponds on them, so it was usually quite easy to determine which pond to sample. To standardize choice of ponds on islands with very little or no nesting activity, the largest pond furthest from the ocean was selected for sampling. Once a selection was made, a 10 cm × 10 cm sample of moss and soil was excavated using a trowel from a random location selected ad hoc within 2 m of the main pond for isotope analysis. On islands with large eider colonies the main pond was always surrounded by a large fringe of vegetation, which provided a large area from which to sample. On islands with few eider nests, this was not usually the case, and we were often forced to sample whatever vegetation we could find within 2 m of the pond. Coastal reference sites were sampled opportunistically during the field seasons where safe landings could be made, though effort was made to ensure they were spread out over the study area as best as possible (see Fig. 1). Samples were stored in sealed plastic bags and frozen upon returning from the field, until analysis. Pond sediments were collected using a sediment push corer and the cores were sectioned into 0.5 cm intervals at site (Glew 1988; Glew and Smol 2016). Sediment for stable isotope analysis was taken from the surface samples (0–0.5 cm) of each core.

Goose droppings were commonly observed on islands with large eider colonies and were collected opportunistically from several islands for isotopic comparison to eider guano to rule out goose droppings as a major alternative source of nutrients. No other animals or their droppings (e.g. gulls, terns, bears) were observed in any large numbers on islands with large eider colonies. Eider droppings were collected in 2015 at the long-term research site at the East Bay Migratory Bird sanctuary, as they do not persist on the landscape as readily as goose droppings.

Additional moss and soil collections were also completed on a subset of 5 ‘high eider’ islands on the coast of Baffin Island (A-045, A-048, A-056, A-101, and A-135). These collections were completed in the summer of 2016, other than A-135, which was collected during the 2015 field season. These islands were selected to represent a sample of large eider colony islands across the geographic range of the survey area in the Baffin Island region. On each island, 30 m transects were laid out in each of the 4 cardinal directions beginning at the edge of the main pond. Samples of moss and soil were collected every 5 m along the transects to examine the relationship between stable nitrogen isotope values and nutrient levels in relation to patterns of eider habitat use across the islands. Samples of soil and moss were not always available within each quadrat; however, in general, all islands and distances were represented (sample breakdown by distance—0 m: n = 10, 5 m: n = 7, 10 m: n = 6, 15 m: n = 7, 20 m: n = 7, 25 m: n = 7, 30 m: n = 6).

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