As Johns Hopkins Glacier Advances, Fewer Icebergs for Glacier Bay's Harbor Seals

Johns Hopkins Glacier, Glacier Bay National Park and Preserve. NPS/Sean Tevebaugh.

Icebergs calved from tidewater glaciers are prime habitat for harbor seals. They give seals a place to haul out away from predators while they molt, and a place for pups to get out of the cold water to nurse. In Johns Hopkins Inlet, in Glacier Bay National Park, up to 2,000 harbor seals — one of the largest seasonal gatherings in Southeast Alaska — depend on the ice that Johns Hopkins Glacier sheds.

A glacier that is advancing

Mountain glaciers are mostly shrinking as the climate warms. Tidewater glaciers are different: they follow their own cycle of advance and retreat, driven by that cycle as well as by climate.

Johns Hopkins Glacier has been advancing for the past century. Like other advancing tidewater glaciers, it has done so with the help of a submarine end moraine — a ridge of sediment at its underwater front that keeps growing and being pushed around. The moraine resists the glacier's flow, which lets the ice thicken, and it limits how much ice the glacier loses to calving and to melting underwater.

What the study looked at

The researchers compared iceberg production over time with the glacier's characteristics: the depth and shape of the underwater moraine, the advance of the glacier's front and its thickening, how fast the ice flows, and changes in iceberg habitat. Their evidence included a 15-year record of aerial surveys of the inlet.

What they found

• Fewer icebergs. Iceberg concentrations declined as the moraine grew.
• A steady size pattern. Iceberg sizes followed an approximately power-law distribution, and its exponent varied little and showed no clear trend — even though the amount of ice the glacier delivered swung widely between seasons and years.

Taken together, the authors suggest that a tidewater glacier that keeps advancing should usually produce fewer of the large icebergs seals can use. For harbor seals that rely on icebergs for pupping, molting and resting, that could matter.

Sources

Rewritten, in hubnx's own words, from the National Park Service summary "Impacts of tidewater glacier advance on iceberg habitat" (Glacier Bay National Park and Preserve) and the abstract of the paper it describes: Kaluzienski, L., J. Amundson, J. Womble, A. K. Bliss and L. Pearson, 2023, "Impacts of tidewater glacier advance on iceberg habitat," Annals of Glaciology.