Impact of Climate Change on Arctic Sea Ice and Phytoplankton Dynamics
Keywords: Sea ice, sea algae, phytoplankton, climate change, global warming
Abstract: Global warming is rapidly changing the marine ecosystems of the Arctic Ocean. Arctic marine food webs rely on a predictable annual interaction between sea ice and phytoplankton. Climate change is warming ocean water, increasing acidification, and increasing stratification, all of which are affecting the composition of sea ice and the phytoplankton that interact with it. Observational studies and simulation modeling have clarified the impact of climate change on the Arctic Ocean. The last decade has increased the understanding of the dynamic relationship of sea ice and phytoplankton and has allowed scientists to predict how the Arctic Ocean will continue to change as sea ice loss progresses. The progressive retreat of Arctic sea ice and change in water composition are creating earlier, more productive phytoplankton blooms that extend under the sea ice. These earlier blooms are redefining where primary production is occurring and are changing the phytoplankton community over time. The current paper also explores topics such as the speed at which sea ice is being lost, the impact of young ice replacing old ice, and the timing of critical annual events in the Arctic Ocean are also discussed.
Climate change has drastically altered the polar regions of the planet in the past few decades (Constable et al., 2014). Global warming threatens to eventually eliminate the iconic landscapes brought to mind when picturing the Arctic and Antarctica. Sea ice is being lost all over the planet, but the polar regions are where the worst losses have taken place. Ecosystems that revolve around consistent annual ice formation patterns are experiencing changes in timing of critical events and in quantity of production. Oceanographic phenomena that has previously minorly contribute to the Arctic Ocean food web is beginning to take center stage due to shifting conditions of sea ice (Selz et al., 2018). Recent research has embarked to investigate how climate change is affecting Arctic sea ice and the phytoplankton that interact above, within, and below it (Campbell, Mundy, Belzile, Delaforge, & Rysgaard, 2018). By observing and modeling current change, scientists can predict and prepare for future shifts in sea ice dynamics as global warming progresses.
2. Sea Ice Change
Both the Antarctic and the Arctic are losing sea ice at a staggering rate. The Arctic Ocean is experiencing rapid reduction of sea ice extent, which poses important implications for sea ice ecosystems and the animals that rely on them. This drastic reduction of sea ice in the Arctic Ocean, which is progressing at an increasing rate, has propelled multiple environmental stressors, including warming, acidification, and strengthened stratification of global ocean waters (Harada, 2016). All these processes can be correlated and linked to global warming. Geospatial records reveal that over the last 50 years, the minimum annual sea ice extent in the Arctic Ocean has been declining by 10% or more per decade (Perrette, Yool, Quartly, & Popova, 2011). This decrease in sea ice has led to a longer and more widespread open water season in which portions of the Arctic Ocean remain unfrozen and exposed to the air. Accordingly, the annual cycle of sea ice formation has shifted over time. For example, compared to historical records from 1979–1980, sea ice in the Arctic Ocean is retreating two months earlier than previously observed and reforming more than a month later in 2010–2011. The result is a three month increase in the open water season (Lowry et al., 2018).
In addition to the loss of sea ice, there has been a general shift in type of ice in the landscape of the Arctic Ocean. In the past, thicker ice formed from multiple years of snow fall, melt, and refreezing and was the bulk of ice found in sea ice assemblages. Within the last few decades, there has been a shift to a domination of first-year sea ice, which is thinner than older sea ice (Galindo et al., 2014). Global warming has caused the annual cycle of sea ice melting and freezing to intensify, with each year experiencing greater ice melt, which then refreezes as first-year ice. This replaces the thicker multi-year ice that once dominated the sea ice composition (Blais et al., 2017). The changes in the annual timing of melt and refreeze have shifted the extent of sea ice and the duration in which it is present (Lowry et al., 2018). Sea ice is a critical habitat for phytoplankton assemblages, which act as the first link in Arctic food webs. Any shift in annual timing of sea ice dynamics will inevitably affect upper level trophic organisms that rely on phytoplankton during the spring and early summer (Palmer, Saenz, & Arrigo, 2014). The changing patterns of ice type, sea ice extent, and the timing of sea ice dynamics have significant impacts on Arctic Ocean ecosystems as well as significant implications for future trends of global climate change.
3. Current Understanding of Sea Ice and Phytoplankton Dynamics
The landscape of the Arctic Ocean is a dynamic environment that is continuously changing as seasons pass. Each year, extreme seasonal changes in solar irradiance and sea ice extent determine the make-up of the sea ice environment. In between these extremes are transitionary periods where sunlight and sea ice experience a negatively correlated relationship. These factors influence the abundance of phytoplankton, which act as the basis of Arctic Ocean food webs. Typically, the winter months are characterized by low temperatures and long periods of low to no irradiance, which contributes to the creation of large swaths of sea ice (Steele, Dickinson, Zhang, & W. Lindsay, 2015). As winter transitions to spring, irradiance intensifies, and the water begin to warm, which causes the sea ice to retreat. Eventually, in summer, the Arctic displays periods up to 24 hours a day of continuous sunlight. During this time, sea ice extent is minimal and large areas of open water exist. In shallower areas of the Arctic Ocean, such as continental shelf seas, sea ice can be particularly absent. In the fall, solar irradiance begins to decrease, which leads to lower temperatures and less stratification. This allows for sea ice to freeze and advance until eventually, the 24 hour period is dominated by darkness.
Phytoplankton blooms in the spring are limited by the amount of light that can penetrate through ice-covered waters. Early in the season, before any of the top layer of snow has melted into ponds called ‘melt ponds’, this can present a challenge to phytoplankton (Mortenson et al., 2017). Once the top layer melts and light is able to travel through the ice, ice algae are capable of explosive blooms in favorable growing conditions. Approximately 2-30% of seasonal production for both pelagic and benthic ecosystems following sea ice melt is from sea ice algae (Lowry et al., 2018). As the spring season continues and sea ice melts, the underlying water column is stratified and an increased amount of light is available for phytoplankton. These blooms follow the edge of ice and are referred to as ‘ice-edge’ blooms or ‘marginal ice zone’ blooms (Lowry et al., 2018). The nature of ice-edge blooms has traditionally designated them as major contributors to the total primary production of the Arctic ecosystem. As ice algae experience blooms, the quantity of chlorophyll a and of particulate organic carbon increases drastically throughout the Arctic Ocean. These resources are the fuel that allows the Arctic food web to grow, as the phytoplankton are consumed by aquatic grazers, such as krill or cod larvae (Campbell et al., 2018). While the sea ice and phytoplankton dynamics of the spring and summer seasons are well understood, due to the accessibility of the Arctic Ocean when sea ice is melting or absent, the fall and winter seasons remain relatively understudied due to the difficulty of collecting field data as sea ice forms and storms worsen.
As the sea ice extent forms and advances in the fall, it excludes salt precipitates from the seawater as it freezes in a process called ‘brine rejection’ (Lowry et al., 2018). This creates a layer of first-year sea ice. The rejected cold and saline brine is denser than the surface water and begins to sink in the water column, being replaced by deeper seawater due to convection. This process mixes the water column as the brine sinks, bringing nutrient rich deep water to the surface (Lowry et al., 2018). As winter progresses and more sea ice is formed, brine rejection and resultant convective mixing continue in areas where the ice edge interacts with open areas of water, such as polynyas or leads. The combination of winter processes forms a water mass that acts as a major source of nutrients for phytoplankton in the eventual spring and summer blooms. For example, concentrations of nitrate, which act as the limiting resource of bloom growth in the Arctic Ocean, were recently found to 10-fold higher in winter mixed water masses than in water masses adjacent in the water column (Selz et al., 2018).
4. How Phytoplankton Blooms Have Shifted
The future of Arctic Ocean ecosystems will be determined by how sea ice phytoplankton communities respond to the rapid changes in the sea ice environment. Phytoplankton are the cornerstone of Arctic food webs and allow for the recycling for nutrients and transfer of energy throughout the Arctic. Ice algae and phytoplankton have historically accounted for the majority of primary production during the spring (Perrette et al., 2011). They have typically represented from between 3 to 60% of the total primary production in the Arctic Ocean (Campbell et al., 2018). Historically, large blooms begin in the early spring, with a second series of blooms in the late summer and early fall. These blooms facilitated food webs throughout the Arctic Ocean. However, in recent years, the quantity and timing of bloom production has shifted. Overall, enhanced primary production has been observed throughout the Arctic Ocean, although it is occurring at different times than historically observed. Modern field, satellite, and modeling work have revealed that peak primary production in areas of high nutrient availability, such as the continental shelf regions of the Arctic Ocean, are shifting to initiate earlier in the year (Blais et al., 2017). Open water phytoplankton primary production increased by 42% in the Chukchi Sea from 1998 to 2012, according to satellite estimates (Palmer et al., 2014). Recent studies have attributed this shift to earlier ice retreat and longer open water seasons, along with the growing presence of under-ice phytoplankton blooms. These observations of phytoplankton blooms occurring in areas and at times previously associated with low amounts of primary production has initiated a series of investigations to determine how phytoplankton and sea ice dynamics will continue to change as climate change continues to alter the Arctic landscape.
Historically, sea ice phytoplankton blooms and thicker, older sea ice shaped the ecological processes of the Arctic Ocean. In recent years, observational and modeling work have also helped to identify how the reduction of sea ice is affecting the dynamics of different types of phytoplankton blooms. For example, under-ice blooms, which previously were thought to weakly contribute to the overall primary production of Arctic food webs, are becoming increasingly important as multi-year sea ice is replaced by younger and thinner sea ice (Lowry et al., 2018). Recently, large and unprecedented under-ice blooms have been observed in the Arctic where previously only small under-ice blooms were observed, such as the Chukchi Sea (Lowry et al., 2018). The combination of first-year sea ice and melt ponds, formed from the upper layers of sea ice and snow exposed to the sun, may be the cause for the rising significance of under-ice blooms. Melt ponds can transmit up to 55% of the incident irradiance to the water column under the sea ice, which is easily penetrated due to the composition of first-year sea ice (Selz et al., 2018). The penetrating light coupled with the nutrient availability in the underlying water column serves as the fuel needed to initiate under-ice phytoplankton blooms. The growing abundance of under-ice blooms are exciting opportunities for scientists to investigate how climate change is directly changing Arctic food webs.
Traditional paradigms of primary production and phytoplankton blooms are being redefined as global warming shifts the importance of different types of blooms to total primary productivity. It was historically observed that under-ice blooms were not major contributors to the Arctic Ocean’s total production and that ice algae blooms were the most important contributors (Palmer et al., 2014). However, climate change has changed the dynamics of sea ice and phytoplankton interactions causing under-ice blooms to be the more significant contributor over ice algae blooms in modern years(Lowry et al., 2018). As under-ice blooms terminate and decay, they release nutrients back into the water column, possibly seeding the water for future blooms in the same season at the surface. Estimates of primary production in the future will need to accommodate these newly discovered phytoplankton and nutrient dynamics.
5. Concluding Remarks
Modern investigations have highlighted shifts in sea ice dynamics throughout the Arctic Ocean. The Arctic that exists today is drastically different than the Arctic from even half a century ago. As global warming progresses, scientists will continue to investigate how the Arctic Ocean food web and sea ice is redefined. The sea ice of today is younger and thinner than sea ice historically observed and with each passing year, there is less and less. The open water season has increased by 3 months since the late 1970’s. It is possible that in the coming decades, stretches of the Arctic Ocean remain open throughout the year. The change in sea ice has already shifted the composition of ice algae and phytoplankton communities. With these shifts have come a change in the timing and intensity of primary production that takes place in the Arctic. The upcoming decades will be a dynamic time for scientists to continue to monitor and investigate the Arctic ocean. There has never been such an exciting time to be a polar scientist.
6. Further Readings
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