Changing sea-ice affects diet of Arctic sea-birds

Blog written by Mark Mallory. Read the full paper here.

The imposing cliffs of Prince Leopold Island rise sheer from Lancaster Sound and the Northwest Passage in the Canadian high Arctic.  Thick-billed murres (Brunnich’s guillemot in Europe – Uria lomvia) and black-legged kittiwakes (Rissa tridactlya) nest in the tens of thousands along the steep northeast cliffs, with glaucous gulls (Larus hyperboreus) nesting on rock towers and promontories, overlooking their prey. Thousands of northern fulmars (Fulmarus glacialis) nest around the perimeter of the island, typically near the top of the cliffs, while a few thousand black guillemots (Cepphus grylle) nest in scree and crevices, particularly on the southern side of the island. Polar bears (Ursus maritimus) often roam the beaches below the birds, while narwhal (Monodon monoceros), beluga (Delphinapterus leucas), bowhead whale (Balaena mysticetus), walrus (Odobenus rosmarus), harp seal (Phoca groenlandicus), bearded seal (Erignathus barbatus) and ringed seal (Pusa hispida) all move among the landfast ice and pack ice that forms around the deep water edges of the island.

When birds return to the island each year to breed (first the fulmars and gulls, and then later the murres, guillemots and kittiwakes), they are met with differing conditions of sea ice cover in Lancaster Sound. Some years the ice is extensive and solid, effectively blocking access to open water (necessary for feeding) up to 200 km to the east, while in other years, the water is mostly open, or in loose pack ice, through the Sound and as far west as Resolute Bay.

In earlier studies, we showed that the breeding success of the birds varies considerably with annual sea ice conditions: they do well in years with moderate to little ice cover, but both effort and success are lower in late, extensive ice years (Gaston et al. 2005. Ecography 28:331-344). We think that heavy and solid ice cover does two things: a) it delays the pulse of productivity in the ocean, meaning food supplies are late and less abundant; and b) it forces the birds to fly much farther to feed, meaning they spend more energy. Some birds in relatively poor condition may just defer breeding or abandon part way through under those conditions.

In addition to monitoring breeding, we have tracked contamination of the Arctic marine food web since 1975, using eggs from seabirds at this colony (Braune et al. 2019. Science of the Total Environment 646:551-563). Getting these data is no easy feat! After consultations with the nearby community (Resolute Bay), and usually bringing an Inuit field assistant or student trainee, we fly to the island by helicopter or Twin Otter, and if we are working on all 5 species, we set up a camp on top of the cliffs for anywhere from a week to 2 months. If we are just doing the annual monitoring of murres and fulmars, we fly down for a long, jam-packed single day of work. Once at the colony, we set up anchors and descend the 330m cliffs on 11 mm static safety ropes to breeding ledges that don’t offer too much footing. We usually do the collections around the first week of July, taking only 1 egg from selected nests, which means that some birds will lay a new egg to replace the one we’ve taken, although for 15 fulmars, we’ve robbed their breeding efforts that year (hopefully a small sacrifice for the greater good!).

That continuing story has shown that some contaminants have declined markedly since the 1970s (e.g. DDT, PCBs), others increased for a period and now seem to be plateaued (e.g., mercury), and yet others may be increasing now (various new “emerging contaminants”). While studying the contaminants of the birds, we also recorded the trophic level (i.e. where in the food web) they fed at by analyzing stable isotopes of nitrogen and carbon (as a possible correction to changes in contaminant loads). Then it dawned on us: I wonder if the trophic level of the birds differed with ice conditions, or has changed through time? That’s what precipitated our recent study.

Looking back at our data through time, and with the availability of satellite-derived information on ice cover over the years, we linked the dietary information from the isotopes to the sea ice cover conditions during egg-formation for the species. That told us that indicators of diet (i.e., isotopes) changed across the seabird community depending on sea ice conditions. During a year of particularly heavy ice cover, most species tended to forage on a less diverse array of foods, and in general the breadth of what the entire community was feeding on was smaller. We interpreted this to mean that heavy ice reduced what was available to the birds, or perhaps forced birds to eat what was more prevalent around the ice. In contrast, under minimal sea ice cover, species had more different diets from each other, and within species they were more varied. We think this may mean that individual specializations are expressed more under these environmental conditions.

However, the big point we found was that we really only saw substantial changes in seabird diet and environmental conditions when we considered the whole community at the island, and we think that in the future, efforts to determine the effects of stress from a more unpredictable climate will be best assessed by looking at communities and multiple species.

We can tell a lot from a seabird egg, and if timed right, we can get this information without having much negative effect on a colony. Eggs from Arctic seabirds continue to divulge secrets about threats to the Arctic environment, and how seabirds respond to those stressors.

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