As sea ice decreases dramatically across polar oceans, some scientists see a silver lining: The algal blooms that seem to thrive where ice has recently disappeared could damper climate change by trapping carbon in the deep ocean. Now, a new study of Antarctica’s Scotia Sea is complicating such speculation, revealing how seasonal sea ice may play an important role in supporting carbon-consuming algae and the Arctic food chain—a dynamic that will be lost in permanently open waters.
To measure the productivity of algae blooms, most researchers use satellite images that capture the distinctive green color of chlorophyll in open seas. Therein lies a problem. The satellites cannot peer into or under sea ice, which goes from paper thin in some places to several meters thick, says Katrin Schmidt, a krill ecologist at the University of Plymouth in the United Kingdom who led the study, published this week in Biogeosciences. That means satellites can’t measure algae that thrive in, and under, the ice. “For convenience, it is often assumed there’s no production in sea ice,” Schmidt says. That convenience, however, is misleading.
Measuring algae produced in sea ice is no easy task. First, Schmidt needed a method to distinguish algae growing under the ice from those that thrive in open seas and in the transitional areas in between—a seasonal landscape of frozen floes and melting ice known as the marginal ice zone (MIZ). Another study provided the key: a fatty biomarker called IPSO25 that was only found in a particular species of sea-ice algae, Berkeleya adeliensis. Using this marker and one other—HBI III, found in algae in the MIZ and to a lesser extent in open seas—Schmidt revisited the digestive glands and stomachs of krill from 47 different points in the Scotia Sea, caught 15 years ago in a 2-month campaign. She found that the krill—whose diets consist primarily of algae—caught in the ice-related zones with high concentrations of IPSO25 and HBI III were remarkably healthy compared with their peers.
Schmidt speculates that sea ice acts as a protective lid for the ice-bound algae, allowing it to produce more nutrients through photosynthesis. In the open seas, heavy winds and deep waters lead to a high turnover in the water column, which prevents photosynthetic microorganisms like algae from converting sunlight into energy. These single-celled organisms can’t get enough light to thrive because they are constantly riding an up-and-down roller coaster of deep mixing that limits photosynthesis.
Surprisingly, the sea ice has an equally calming effect even after it is gone. For up to a month after sea ice had melted, krill samples from transitional zones had high concentrations of IPSO25 and HBI III. That’s likely because the water column was still separated into distinct saltwater and mostly freshwater layers—and hadn’t started turning over at high rates, Schmidt says. After just 2 weeks of feeding in the transitional zone, krill with high HBI III content were heavy for their size, easily matching krill that took 16 weeks to reach an equivalent healthy state in an open-seas algal bloom. Melting sea ice, Schmidt says, may also supply an essential nutrient that is particularly sparse in the Southern Ocean: iron. Other research has found that sea ice is a natural reservoir of iron, which is captured by ice crystals as they form in deeper water and float to the surface.
Jørgen Berge, a marine ecologist at The Arctic University of Norway in Tromsø who was not involved in the study, says the “intriguing” findings likely apply to the Arctic’s growing ice-free regions, as well. “As for the Antarctic, satellites only provide part of the answer regarding ice algae and phytoplankton blooms.” While krill flourish and whales feast in the Antarctic’s marginal ice zone, the rapidly consumed algae remain invisible to satellites—but no longer to science.