Large flocks of seabirds have a striking tendency to gather where glaciers meet the ocean. PhD student Philip Bertrand has worked in some of the kittiwake colonies monitored by SEAPOP on Svalbard to find out how these birds use glacier fronts when searching for prey. He found evidence that both supports and undermines some of his hypotheses.

Brings food to the surface

Tidewater glacier fronts have for a long time been viewed as important feeding areas for Arctic predators. Meltwater from the glaciers forms rivers that run towards the sea under the ice and enters the ocean below the surface. Being less dense than the surrounding seawater, the meltwater creates an upwelling at the foot of the glacier. This plume may entrain plankton and other small organisms from the deeper water masses, making them available as prey for surface-feeding animals such as seabirds. However, recent studies show that the importance of glacier fronts as foraging habitats may vary between years, although the drivers behind this variation remain unknown.

Different hypotheses

The ongoing melting of ice in the Arctic underlines the importance of uncovering the mechanisms that determine the use and profitability of glacier fronts as foraging areas in order to understand how important these spatially restricted habitats are for the ecosystem. This is why Philip Bertrand at the Norwegian Polar Institute chose this subject for his PhD. In cooperation with, among others, researchers from SEAPOP, he studied the interannual variation in the use of glacier fronts in Kongsfjorden, Svalbard, by black-legged kittiwakes (Rissa tridactyla) by combining GPS tracking with data on zooplankton biomass and estimations of discharge from the glaciers. It was predicted that the fronts would be used to a greater extent in years of higher discharge and in years with high prey biomass. Another important part of the study was to test the hypothesis that glacier fronts may influence the foraging patterns of kittiwakes and drive spatial segregation among colonies.

Surprising results

The analyses revealed that the use of glacier fronts by kittiwakes did indeed vary between years, as Bertrand and his partners had expected. They were, however, surprised to find that, contrary to their assumptions, the variations were negatively correlated to both glacier discharge and zooplankton abundance. A suggestion was made that when there is a high amount of plankton in the fjord, it may in fact be equally profitable to collect prey further out in the fjord as at the glacier fronts. The results of the study likely reflect complex interactions between local and regional environmental factors that affect the relative profitability of glacier fronts as foraging areas for seabirds.

Impacts seabird movement patterns

Using GPS tracking data from tags deployed on kittiwakes, the researchers gained detailed insights into where birds from five colonies went in search for food. The kittiwakes mostly visited glacier fronts close to their colonies, and they rarely went to forage at glacier fronts located further away than 18 km. These results support the hypothesis that spatially predictable foraging patches like glacier fronts can have strong structuring effects on seabird colonies when it comes to the birds’ choice of foraging area, even when the colonies are located relatively close to each other.

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This figure, taken from Philip Bertrand’s PhD, illustrates how meltwater discharge from under the glacier is pushed towards the surface when it meets the salty Atlantic water, creating a plume that brings nutrients to the surface from the lower depths.
Illustration: Philip Bertrand
Black-legged kittiwake (Rissa tridactyla) in a breeding colony in Kongsfjorden, Svalbard. Photo © Sébastien Descamps
Black-legged kittiwakes mostly visit glacier fronts located close to their colonies when searching for food.
Photo © Sébastien Descamps

Contact person: Sébastien Descamps, Norwegian Polar Institute