Article: Kohlbach, Doreen, et al. “The importance of ice algae‐produced carbon in the central Arctic Ocean ecosystem: Food web relationships revealed by lipid and stable isotope analyses.” Limnology and Oceanography 61.6 (2016): 2027-2044.
A top-down look of the Arctic doesn’t exactly stir images of a thriving and diverse habitat. This bird’s eye view makes the Arctic look like a vast, cold, snow and ice covered desert. But underneath that ice lies an impressive ecosystem, teeming with benthic (or sea floor dwelling) invertebrates, large fish, and many marine mammals (Fig. 1). Like other cold water systems, much of the support for the Arctic ecosystem comes from phytoplankton, microscopic algae that harness sunlight and carbon dioxide to create usable energy. But there is a potentially unheralded source of energy for this ecosystem coming from ice algae.
Ice algae is composed of diatoms (small primary producers) in addition to phytoplankton (Fig. 2). Diatoms and phytoplankton would be found in the water columns in warmer climates, but due to the fact that the Arctic is ice covered, some of the algae get trapped within the ice. The algae then grows in the pockets and channels found in the ice (Fig. 3). Harnessing the light that penetrates the ice, allows the algae to grow and flourish.
Zooplankton (small marine animals; Fig. 4) eat ice algae in high quantities and are in turn eaten by larger organisms like fish, spreading the energy through the food web. When Arctic waters get warmer in the spring and some of the ice begins to melt, ice algae is released into the water where other creatures can start eating it, too. While it is known that ice algae is important in polar ecosystems, its role as a food source is even more important to understand since climate change and global warming are resulting in the decline of ice cover. Therefore, researchers in Germany recently set out to determine just how big of a role ice algae has in supporting the Arctic ecosystem.
What makes this study possible is the fact that growing in ice results in ice algae having a different make-up than algae in the water. Being isolated in ice means these algae live in a carbon-limited environment, which results in different isotopic ratios and different fatty acids (FAs). When small zooplankton eat algae, one can go back and determine their diet by looking at isotopes and FAs. By testing the chemical make-up of Arctic zooplankton, researchers aimed to find out which food sources were most important, and if ice algae played a major role in the diets.
Researchers sampled 10 sites across an area of the Arctic Ocean seasonally covered by sea ice (Fig. 5). At each site scientists took ice cores in order to determine what species of ice algae were present along with their isotope and FA composition. Water samples were taken to determine what species of free-floating or swimming algae (pelagic algae) were present and what their isotope and FA make-ups were. Finally, tows were done at each site to sample the zooplankton community and find out what they were eating!
After running the algae through isotopic analysis, done with some fancy machinery, researchers were able to flag which FA differed between ice algae and the pelagic algae (Fig. 6). Running similar analysis on the captured zooplankton would reveal what these animals were eating. It was found that dominant copepods, Calanus species (Fig. 7), exhibited a mixed diet of pelagic and ice algae, whereas amphipods (Fig. 8) relied on ice algae for the majority (60-90% based on the species) of their food. It was unsurprising that these amphipods received that much carbon from ice algae as they live on and in the ice. But what was surprising was that other zooplankton that live at greater depths and away from the ice, more closely resembled the copepods by getting 20-50% of their carbon from the ice algae.
By determining where zooplankton’s primary carbon sources, researchers were able to better understand the role and importance of ice algae in supporting Arctic ecosystems. They found that not only did ice algae allow ice-associated creatures to thrive, but that it contributed up to 50% of diets for creatures found elsewhere, so the impact of ice algae is widespread. But as the planet, and especially the Arctic, warms up, ice will disappear. No ice, no ice algae. Given this ecosystem houses commercially important fisheries and a diversity of marine mammals, the importance of ice algae hits closer to home, doesn’t it? It will be important to monitor how this ecosystem shifts through warming, and hopefully by paying attention to the species at the base of the food web, we’ll be better prepared as the ice disappears.