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Biological oceanography

Millennial algae are not as productive: lazy, or less sea ice opportunities?

Article: Neeley, A. R., Harris, L. A., & Frey, K. E. (2018). Unraveling phytoplankton community dynamics in the northern Chukchi Sea under sea-ice-covered and sea-ice-free conditions. Geophysical Research Letters, 45, 7663–7671. https://doi.org/10.1029/2018GL077684

Arctic sea ice has been on the decline since 1979, and that decline is expected to continue as the atmosphere warms. We’re already seeing how sea ice loss can affect our world, from weather patterns to large-scale ocean circulation to marine life in the Arctic. Despite the growing body of research describing our current predicament, it’s not so easy to predict what will happen when all of the sea ice inevitably disappears from the region.

The importance of life in the Arctic

Many studies focus on the impacts of sea ice loss on the tiniest algae in the marine food web – phytoplankton. These are extremely complex and diverse microscopic ocean plants that use sunlight and nutrients in the water to produce oxygen and energy for larger marine organisms, helping to regulate atmospheric carbon dioxide (the greenhouse gas that enhances global warming). They reside at the base of the marine food web, meaning they feed the tiny critters that feed the fish that feed the larger sea creatures; this makes them one of the most essential parts of marine ecosystems all over the world.

Figure 1: Map showing the locations of 81 sampling sites during the 2011 Climate on EcoSystems and Chemistry of the Arctic Pacific Environment expedition. Colors show the timing of sea ice break-up.

Because there is so much diversity among phytoplankton, specific groups respond differently to environmental conditions. In the Arctic, sea ice provides a cap to the upper ocean that protects it from sunlight and wind – that is, there is no way for the air and sea to interact with each other. This makes for calm, cold, high-nutrient, low-light conditions. When the ice begins to melt into fresh (non-salty) water, the surface becomes stratified – that is, layered by density – from fresh, light water on top to dense, salty water below, which inhibits vertical movement of nutrients. Sunlight at the surface promotes growth of some phytoplankton, but this depletes the surface of nutrients. Stratification prevents nutrients from reaching the surface to replenish their food source. These changes could affect what types of phytoplankton dominate the  ocean surface, and thus how the marine food web operates.

Linking phytoplankton “demographics” to ice conditions

Figure 2: Top – Phytoplankton are differentiated by ice conditions (red: icy, green: low-ice, yellow: no-ice, black: rare types). Closeness to an arrow of an environmental variable – such as light, ice presence, temperature, salinity, and nutrients (silica Si, dissolved inorganic nitrogen DIN, phosphorus P) – indicates how much each type is affected by it. Bottom – Phytoplankton are differentiated by depth shallower or deeper than 25 meters, or 82 feet (red/blue squares: shallower/deeper under ice, red/blue circles: shallower/deeper with no ice).

Dr. Aimee Neeley of the NASA Goddard Space Flight Center led a study that aims to understand how changes in environmental conditions due to sea ice loss affect phytoplankton populations. A research expedition to the Chukchi Sea in 2011 (north of Alaska – see Figure 1) provided data used to address the question of how phytoplankton might respond to an Arctic with little to no sea ice. Dr. Neeley’s team linked observed phytoplankton “demographics” to environmental conditions that may be controlling them, such as water movement, sunlight, nutrients, temperature, and salt content.

They measured environmental conditions at each of the 81 stations, and overlaid them with satellite data of sea ice cover to examine possible relationships. Bottles of water samples were examined under a microscope to identify the types of phytoplankton present at each site and at discrete depths down to 200 meters (about 656 feet). By doing this, they were able to separate communities of phytoplankton based on the conditions in which they thrived (see Figure 2). They further differentiated the phytoplankton into three groups based on the amount of ice present – ice cover, fragmented melting ice, and ice-free. Phytoplankton who thrive in icy conditions possessed similar characteristics, particularly a high carbon biomass – basically, the amount of carbon phytoplankton consume to turn into fuel for larger organisms. Low-and-no-ice phytoplankton tended to be smaller and have low carbon biomass. For organisms that rely on phytoplankton for nutrition, this shift from ice cover to ice-free is akin to visiting the grocery store to buy nutritious food but only finding candy. Although candy is delicious, how long could you really live off of it?

Using observations to predict the future

Dr. Neeley’s team contributed essential observations to the body of work needed to help predict phytoplankton response to an Arctic system with little to no sea ice – an inevitable outcome of a warmer climate. By differentiating these creatures based on their presence under different sea ice conditions, ice cover could be used by computer models to help predict their demographics, and from them, their effects on marine food webs and carbon in the atmosphere. Looks like the millennial phytoplankton problem is more than just plain laziness!

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