What Lies Beneath

The Richness of Herald Shoal and Chukchi Sea

Walrus are attracted to the abundant food supply of the Chukchi Sea’s “hotspots.”

By: Bodil Bluhm

We are approaching the ice edge — that special moment on every Arctic expedition. The wind is still howling, but the waves now get smaller, the boat’s rocking and rolling diminishes; our stomachs can relax. After passing swiftly through small and scarce floes, we slow down as we are now in heavier ice cover, looking for leads where our ice-strengthened vessel may pass. The color of the horizon has changed from deep ocean-blue to white-grayish ice-and-snow color. Those white floes of frozen salt water slowly move around us, pushed by wind and current. The clonking sound they make when bumping into the double-layered metal hull reminds us that we are not invincible. A few first puddle-sized melt ponds are scattered across the frozen seascape and shimmer in shades of tropical-turquoise — summer melt is approaching here between Herald Shoal and Herald Canyon east of Wrangell Island in the northwestern Chukchi Sea.

Many species of tiny microalgae form a brown-greenish layer at the ice-water interface during the spring bloom.

Broken ice floes get turned upside down as our ship moves through the endless white, and they reveal the hidden life at the bottom of the ice. Seabirds come out of nowhere to pick up the amphipod crustaceans (beach hoppers) or small Arctic cod that also get pulled out of their habitat with the floes. Thin, green-brown layers composed of millions of microalgae coat the underside of many floes, reminding us that life is abundant in sea ice, even when there are no obvious signs of it from above. I think about the more than 1,000 species of microalgae and other tiny organisms that it takes to feed — directly or indirectly — the animals, small and large, that live amid the sea ice, in the water column, and even at the seafloor. There would not be that charismatic polar bear chowing down a seal over on that ice floe without all the little things that live here.

“My” Arctic looks different than my fellow researchers’ Arctic. Like inhabitants of a given area on land, we researchers relate closely to the places we work. Through our fieldwork, we experience our study area, smell it, dig around in it, see and feel the animals that live in it, and experience the environmental conditions, the weather. This personal experience over the years, of course in combination with the samples and data we collect in the area, shapes our understanding of how the Arctic or a part of it works. In my view, this ‘place-based learning’ is — just like in K-12 education — a critical part of system understanding.

The Chukchi Sea shelf — where Herald Shoal and Herald Canyon are located — is a special place for me and the research community, and for those that live in and around the Chukchi Sea area (man and biota). It is also important for the world’s climate. This very shallow shelf sea — no more than 50 meters (~150 ft) in most places — is located north of Bering Strait between the Russian Chukotka and Alaskan west coast. To the north, it extends to Wrangell Island in the west, where we were on this particular expedition, and to the Chukchi Borderland and deep Canada Basin. Though Bering Strait is narrow and shallow, the Chukchi shelf and its fauna are greatly influenced by the inflowing water from the Bering Sea, because it brings high nutrient loads and massive amounts of phytoplankton, zooplankton and also larvae of benthic (seafloor) fauna. This is why our colleagues Carmack and Wassmann in 2006 termed the Chukchi Sea an Arctic ‘inflow shelf’ that is tightly linked to what happens in the upstream areas of the Bering Sea. But that is not to say that local processes do not play a role. Indeed they do. The western Chukchi Sea where we are on this expedition is affected by the East Siberian Coastal Current that brings cold water and larval supply from the Siberian shelves, and at the shelf break, shelf-basin interactions prevail.

Sorting the rich benthos in the Chukchi Sea, a group effort.

We begin to sample the seafloor. We are interested in this habitat because it is in a way a time-integrating archive for what happens in the water column. This is both because Arctic benthic fauna often live many years to decades and they eventually depend on food produced in the upper water column. What do we do when we do research at the seafloor? Although a shallow shelf, the Chukchi seafloor is mostly beyond safe SCUBA diving depth. We, therefore, have a suite of tools that we deploy to sample the soft sediments in the Chukchi and here around Herald Shoal and Canyon. The many bivalves, bristle worms (polychaetes) and amphipod crustaceans are best sampled with a van Veen grab, a metal box that ‘bites’ a 0.1 square-meter area out of the seafloor. We then sieve the sediment over a mesh of a known size and count and identify what remains on the sieve, not much different than playing in the sandbox really. For the larger invertebrates such as crabs, snails, brittle stars and sea stars, we use small trawls or camera systems.

Animals from the benthic environment at a Chukchi Sea test location include bivalves, small snow crab, hermit crabs, snails, sea stars, and more.

All of us carefully planning our field work up north are acutely aware of the seasonal cycle and timing of the ice cover in the Chukchi Sea ice. This ice cover is in fact indirectly a reason why there are so many benthic-feeding marine mammals in the Chukchi Sea, some of which as described in other blogs in this series. When the ice starts to break up and melt in the spring when sunlight increases, the melt layer helps form a stable upper water column (because water of lower salt content sits on top of saltier water) where algae can bloom. At that early time, few zooplankton are generally around that could graze on the bloom and, hence, much of the bloom sinks to the seafloor ungrazed. This bloom settles to the seafloor and provides high quality food for large benthic invertebrate standing stocks. In particular, we find small bivalves, polychaetes, amphipods, but also larger brittle stars, sea stars and crabs that form large biomass accumulations at the seafloor. These and other benthic animals attract benthic-feeding marine mammals, first and foremost walrus, bearded seals and gray whales. Nowhere else on the planet are these majestic animals as abundant as in the Chukchi Sea!

About the author: Bodil Bluhm is a marine biologist and oceanographer focused on the ecology of Arctic seafloor and sea ice biological communities. She is interested in studying their biodiversity, food web and how the environment influences their distribution and ecology. Bodil works for The Arctic University of Norway (UiT) and is also affiliated with the University of Alaska Fairbanks.