Using records from the historical eider surveys, we selected candidate islands prior to conducting our work to attempt to minimize variation in their physical criteria as much as possible (e.g. distance from coastal shoreline, area, elevation—see Appendix 1), although this was not always possible. For instance, it was challenging to find large islands that had few nesting common eiders, or small islands with large colonies. We were also not always able to visit all candidate islands due to logistical and environmental conditions, such as local sea ice or inclement weather. We also attempted to maximize the spatial extent of our sampling as much as possible within the logistical constraints of our sampling method.

Islands were then divided into two treatment groups based on recent eider use (see Table Table1).1). Islands with large numbers of recent active eider nests and clear evidence of an eider colony were included in the ‘high eider’ group (n = 17), whereas islands with very few nests and no evidence of a colony were placed in the ‘low eider’ group (n = 5). We also included a third group in our analyses, which we called ‘reference’ (n = 14, but sample size varies across moss, soil, and sediments, see Table Table1).1). These eider-free sites were located on the nearest large terrestrial landmass adjacent to the islands that we chose in the two groups above. Reference sites were chosen to provide a control group where we expected the nutrient sources to be of terrestrial origin, but that were also nearby to our study locations. These sites were located along the shorelines of each region where islands were being sampled and were taken from a similar distance from shore as island samples. We treated Digges Island sites as reference sites because of the island’s large area (~ 92 km2) and lack of eider nesting activity.

List of islands and reference sites sampled for this study

Islands and reference sites were placed into groups based on the amount of common eider nesting activity. High Eider sites were islands with large, obviously utilized common eider colonies present. Low Eider sites were islands that had little evidence of eider nesting activity. Reference sites were included to provide comparisons for alternative sources of nutrients. Data on nesting activity were taken from historical surveys of the region from the period 1956–2012

aPond(s) present, but sediments not collected

bNo pond present at this site. No sediments collected or analysed

cOnly pond sediments collected at this site, no soil or moss collected or analysed

We grouped sites in the previously described manner as we were primarily interested in the difference between islands with a large eider influence (high eider) and islands without (low eider) but wanted a third group that was independent of any eider influence for comparison (reference). Ideally we would have sampled from islands with no eider nests for our ‘low eider’ group, but to do so would have required sampling islands too small to be comparable to ‘high eider’ islands as any island with zero nests on it was usually too small or low in elevation that it would have been awash at high tide or during winter storms. Despite this, the ‘low eider’ islands were clearly different than islands from the ‘high eider’ group as the ‘high eider’ islands had large, established colonies where the density of nest sites was extremely high, while the ‘low eider’ islands had no evidence of a colony and a few lone nests scattered across the island. We avoided sampling islands that historically had large colonies that have disappeared in modern times, as these islands would have been difficult to place into either of our treatment groups.

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