The changing Arctic Ocean

All of the energy that fuels the highly productive marine ecosystems of the Arctic Ocean originates in the activity of microscopic algae that utilize the energy of the sun to make organic matter during photosynthesis. These algae live both in the sea ice and in the water column (phytoplankton). As the sun rises higher in the sky during the Arctic spring, enough light energy passes through the ice to allow algae in the ice to grow, generally attached to the bottom where nutrients are plentiful (Arrigo, Lizotte and Mock, 2010). Sea ice algae can attain population levels sufficient to color the ice with their brown pigments. Eventually, the ice warms and algae are released into the water column, where most of them sink. On the shallow continental shelves, ice algae provide some of the food early in the spring for a rich fauna of benthic invertebrates, and in turn, the animals that depend on the benthos for food, including diving ducks, walruses, and gray whales (Dunton et al., 2005; Leong et al., 2005; Grebmeier et al., 2006).

After the ice melts, phytoplankton take over, growing rapidly in the surface ocean until they consume all of the available nutrients (Hill and Cota, 2005). The amount of phytoplankton usually far exceeds that of the ice algae, particularly on the nutrient-rich continental shelves (Arrigo, Lizotte and Mock, 2010). While a significant fraction of this phytoplankton biomass sinks to the sea floor, more is eaten in the water column by zooplankton such as krill and copepods, which are themselves fed upon by fish, birds, and ultimately seals and whales (Leong et al., 2005; Tremblay et al., 2012).

This regular pattern of production and consumption in Arctic waters is now being altered by the earlier retreat and overall loss of sea ice. Ice algae cannot begin to grow until the sun is high enough in the sky and sufficient radiation can penetrate the ice, so earlier sea ice retreat means a shorter growing season. Although a shorter growing season does not necessarily translate to a reduction in overall ice-algal production (nutrient supply may ultimately determine that level despite length of growing season; Gradinger, 2009; Pineault et al., 2013), where it does, the potential delivery of food to the benthos will be less. A reduction in sea ice algae may not have immediately obvious ecological implications, however, because as the window for sea ice algal growth is closing, the window for phytoplankton is opening. The amount of primary production by phytoplankton in Arctic waters is closely tied to sea ice extent and particularly the length of the open water season (Arrigo, van Dijken and Pabi, 2008; Arrigo and van Dijken, 2011). As sea ice has declined in recent decades, phytoplankton productivity has accelerated dramatically. Between 1998 and 2012, the average amount of open water area in the Arctic Ocean and total annual primary production both increased by approximately 30% as the length of the open water season increased by 45%. The largest increases in annual production (30–112%) have been in marginal seas along the Pacific side of the Arctic where the loss of sea ice and the increase in the length of the growing season is greatest (Arrigo et al., submitted; Bélanger, Babin and Tremblay, 2013).

Whether this increase in productivity will continue as sea ice declines further is not known. Models disagree as to the consequences of Arctic sea ice loss. Some predict lower production as ice melt intensifies ocean stratification, which will make it more difficult for nutrients to mix into surface waters (Walsh et al., 2005). Other models predict that reduced ice cover will expose more ocean surface to winds, thereby diminishing stratification and bringing more nutrients to the surface (Zhang et al., 2010). Observational work has confirmed that such upwelling of nutrients does stimulate primary production, yet apparently only in coastal regions of the Arctic (Tremblay et al., 2011). What the remote sensing data clearly show is that, in the short term, the loss of sea ice has resulted in a dramatic increase in Arctic productivity — a major perturbation to the Arctic marine ecosystem.

One consequence of the replacement of old, thick multiyear ice by first year ice is that conditions are now more favorable for phytoplankton to bloom beneath the ice pack (Fortier et al., 2002). Historically, thick, snow-covered sea ice transmitted too little light to the water column below and phytoplankton bloomed predominantly in open water along the retreating ice edge (Alexander and Niebauer, 1981). Now, the younger, thinner ice pack is more uniform and supports more numerous melt ponds (Polashenski Perovich and Courville, 2012), which transmit far more light than bare ice (Frey, Perovich and Light, 2011; Arrigo et al., 2012), making under-ice phytoplankton blooms possible. One example of a particularly large and well-documented under-ice phytoplankton bloom was observed in the Chukchi Sea in 2011. This bloom stretched for over 130 km beneath a continuous cover of pack ice that ranged in thickness from 0.8 to 1.3 m with a melt pond fraction of 25–50% (Arrigo et al., 2012). Phytoplankton biomass was extremely high from the sea surface down to approximately 30 m and growth rates were double those measured in nearby open water. Productivity under the ice exceeded 80 g C m⁻² yr⁻¹, more than 10 times the value attained in the same area after the sea ice had retreated. Phytoplankton blooms beneath the ice have the potential to dramatically alter the timing and magnitude of production on the vast continental shelves of the Arctic Ocean. However, they remain virtually unexplored and an accounting of their contributions to regional energy and carbon budgets is currently missing.

In light of these changes, it might be easy to conclude that a more productive Arctic marine ecosystem is a beneficial consequence of climate change. However, the story is not so simple. More primary production does not necessarily mean more food for the marine ecosystem. Some or all of the increase in production measured between 1998 and 2012 may simply be recycled in the upper ocean. More research is needed to determine how much of this new production is being exported to other components of the ecosystem. Most likely the elevated primary production on the Arctic shelves will increase the flux of organic matter to the sediments, thereby increasing rates of denitrification (Ward et al., 2008; Chang and Devol, 2009; Altabet et al., 2012; Kalvelage et al., 2013), a process that converts fixed nitrogen to relatively unusable nitrogen gas. This loss of fixed nitrogen would act as a negative feedback, limiting further increases in new production in the Arctic.

Finally, while rates of Arctic productivity are increasing rapidly now, there are limits on how much production the Arctic Ocean can support. New sources of nitrogen are required to fuel additional new production, yet potential sources for these nutrients are not clear. The deep Arctic basins contain moderately high nitrate concentrations (Codispoti et al., 2013), but the strong halocline currently prevents their mixing into surface waters, except under episodic conditions of strong atmospheric forcing (Tremblay and Gagnon, 2009). Water flow from the Bering Sea through Bering Strait has increased by 50% in the last decade (Woodgate, Weingartner and Lindsay, 2012), but whether or not this increase is associated with an increased flux of nutrients is not known. In short, we lack a sufficient understanding of the ongoing physical and chemical responses to altered Arctic climate to know what the dramatic changes observed over the last few decades portend for the future.

Sea ice in the Arctic is retreating 2.4 days earlier each year (Arrigo and Van Dijken, 2011), accelerating the blooms of open water phytoplankton (Kahru et al., 2011). Moreover, under-ice phytoplankton blooms develop even earlier and in much colder water (Arrigo et al., 2012). Together, these changes could intensify the mismatch that sometimes occurs between the life cycles of phytoplankton and their zooplankton grazers (Conover and Huntley, 1991), ultimately decreasing the food available to fish, birds, and mammals (Loeng et al., 2005), many of which time their migrations and reproduction cycle to coincide with peak Arctic productivity (Soreide et al., 2010; Wassmann et al., 2010; Wassmann, 2011). Thus, although primary production may continue to increase, its timing may shift such that an increasing fraction will become unavailable to feed the pelagic marine ecosystem, resulting in a further shift to a benthic-dominated ecosystem, in contrast to recent predictions (Grebmeier et al., 2006).

Linked to the same rise in atmospheric carbon dioxide that has resulted in Arctic warming is the acidification of the Arctic Ocean, which is likely to intensify as ice continues to disappear (Yamamoto et al., 2012). Decreasing ocean pH has reduced carbonate ion concentrations in surface waters, making it more difficult for calcifying organisms to make their shells and other supporting structures, particularly on shallow shelves (Yamamoto-Kawai et al., 2009; Bates, Mathis and Cooper, 2009). Ocean acidification in the Arctic will negatively affect pelagic organisms like pteropods (Lischka and Riebesell, 2012), but could be especially detrimental to shallow-water benthic mollusks and crustaceans (Bates, Mathis and Cooper, 2009) on which large marine mammals feed. So, while phytoplankton could fare well in a changing Arctic, the combined effects of a reduced sea ice cover and acidifying surface waters could spell disaster for both pelagic and benthic ecosystems, on which many indigenous human populations rely.

The Arctic Ocean is indeed a bellwether of change — and the Arctic marine ecosystem is changing at an alarming rate. Sea ice is declining rapidly as temperatures rise faster than anywhere else on Earth. The balance between sea ice primary production and water column production is likely shifting in favor of the phytoplankton. Over most of the Arctic Ocean, annual rates of phytoplankton production are increasing dramatically. The timing of production is shifting to earlier in the year in many areas, and in places like the Chukchi Sea, phytoplankton productivity is greatest when the ocean is still covered in ice. And the ocean is acidifying.

There is no geo-engineering solution to both increasing Arctic temperatures and decreasing ocean pH. So the only way to stabilize the Arctic Ocean is to reduce atmospheric levels of carbon dioxide and other greenhouse gases. The big questions are “does the global community have the political will to do it?” and “is it already too late?”

© 2013 Arrigo. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.