A warmer Arctic will have a large and complex impact to algal production


The warming of the Arctic Ocean could generally increase the primary production provided by algae living in sea ice, but the impact varies greatly between latitudes, claims a study published in the renowned scientific journal Science Advances. Since the Arctic marine food web is short, poorly diverse and seasonally driven by limited pulses of energy, the expected changes in primary production can have relevant impacts to the rest of the food web up to humans.

Sea-ice algae live within the ice, in the limited space of brine channels and pockets. Though sea-ice algae and phytoplankton are microscopic, they are vital to all life in the oceans and on Earth, since they are responsible for fixing about 50% of the oxygen on this planet. © Seija Hällfors, SYKE.

The Arctic Ocean covers only about 3% of the global ocean areas but it is responsible for up to 14% of the global oceanic carbon uptake. This makes it an important sink for atmospheric carbon dioxide. As sea ice is rapidly declining in the Arctic Ocean, the ecological consequences of its decline, including changes in the carbon cycle, are still largely unknown.

In a study published in the renowned scientific journal Science Advances, Senior Researcher Letizia Tedesco, from the Marine Research Centre of the Finnish Environment Institute, together with Professor Marcello Vichi, from the Department of Oceanography at the University of Cape Town (South Africa), and Enrico Scoccimarro, Senior Researcher at the Euro-Mediterranean Centre on Climate Change in Bologna (Italy), investigated how the anticipated snow and sea ice changes would impact sea-ice algae production in the Arctic Ocean. They combined a mathematical model of sea-ice algae developed by Dr. Tedesco in the past decade with an ensemble of climate model results from the IPCC´s (Intergovernmental Panel on Climate Change) “business as usual” climate change scenario (RCP 8.5), which is the one assuming no mitigation measures.

Sea-ice algae play a primary role in the food web of polar oceans

Sea-ice algae are types of marine algae living in seasonal and perennial sea ice. Sea ice is made of a solid matrix of pure ice and a liquid fraction of salty brines. The limited space of the brine channels and pockets within the ice is where sea-ice algae live. Like all marine algae, sea-ice algae are primary producers that transform carbon dioxide into oxygen. They are the very base of the marine food web: the main source of food for zooplankton, which in turn are the main diet for larger zooplankton, sea birds, fish, up to seals, whales, and polar bears.

Sea-ice algae play a primary role in the marine food web of polar oceans. “The sea-ice algae are highly adapted to their extreme environment. They grow earlier in the Arctic spring than phytoplankton, which means they extend the productive season in polar oceans. During the time when there is not enough light for phytoplankton to grow under ice, the sea-ice algae provide the only food source available to the rest of the food chain in the Arctic Ocean”, explains Dr. Letizia Tedesco.

The impact of warming varies greatly between latitudes

By using model outputs at the highest available temporal resolution, the authors found almost linear physical changes along latitudes, such as snow and ice thinning and ice season shortening, when comparing the time period 1961–2005 with 2061–2100. By the end of the century the authors found that melt onsets would occur approximately 30 days earlier than in the historical period at all latitudes. They also found a worrying decrease of seasonal sea ice below 70˚N and a striking increase at the expense of multi-year ice above 70˚N. They then investigated whether an advance in sea ice melt onsets would be associated to a similar advance in algal bloom onsets, but they found a complex response of the ecosystem to the predicted physical changes.

Overall, their model projections suggest a relative increase of 52% in sea-ice algae primary production on a pan-Arctic scale along this century, but with contrasting latitudinal patterns. Below 66˚N there is expected to be so little snow that sea-ice algae are predicted to grow much earlier than the onset of sea ice melt. At 66–74˚N latitudes, the algal bloom onset is predicted to also happen earlier but changes in algal production would be small, since earlier ice melting would set an upper limit to the accumulation of biomass. Finally, above 74˚N, the model indicates that the algal bloom period would shift into a more light-favorable time of the year, i.e. from fall to summer. This would cause the largest relative increase in primary production, up to more than 2000%.

These diverse latitudinal responses indicate that the impact of declining sea ice on Arctic to sea-ice algae production is anticipated to be both large and complex. Since the Arctic marine food web is short, poorly diverse and seasonally driven by limited pulses of energy, the changes can have serious impacts on the rest of the food web

“We expect that it might be challenging for the other organisms in the food web to adjust to the projected changes in algal timing, especially in the lowest latitudes. If grazers would not be able to take advantage of the earlier availability of high‐quality ice algal food, they might not be able to reproduce. On the other hand, if grazers would be able to adjust their timing of growth to that of sea-ice algae, our study shows that there might have more food available, especially at the highest latitudes, than previously thought. In both cases there will be consequences on fish species feeding on these grazers, and thus on fish stocks available for human consumption, as well as for for whales, seals and polar bears. If there won’t be enough preys available at the right time, the survival of some sea-ice dependent top predators could be at risk”, says Dr. Tedesco.

More information (in English and Italian)