“Plankton” is something of a catchall term for small organisms that live in the water. Phytoplankton are microscopic plants that feed off the sun, while zooplankton are tiny animals that feed on phytoplankton. Both are integral to the sea’s ecosystem, as they’re a source of food for small fish, crustaceans and even larger organisms, such as humpback and blue whales.
Scientists have been studying the effects of climate change and the decline of Arctic sea ice on this important sea-dwelling population for years. Plankton form the base of the aquatic food chain, and any disruption in this population would have negative effects across marine life.
Previous hypotheses stated that as the seas warmed, these organisms, which are dependent on the sun to survive, would become more numerous. However, a new study published in Nature Geoscience presents evidence that contradicts that widely held belief, making it clear there are other factors that will affect the plankton population as Arctic sea ice melts further.
Why Won’t the Population Increase in Warmer Waters?
It’s easy to understand why scientists believed that this population would grow in number as the Arctic sea ice melts and the oceans warm. Greater light availability, scientists theorized, would mean a higher growth rate for phytoplankton, which feed off the sun hitting the surface of the water. This would lead to a greater bounty for marine life.
However, a team at Princeton University and the Max Planck Institute for Chemistry have a different perspective: These organisms need more to grow than just sunlight. Nitrogen is vital for the population to thrive, and research suggests that available nitrogen will be limited in melting Arctic waters, which will in turn limit the population growth rate.
How the Team Researched Arctic Nitrogen Levels
While phytoplankton thrive off sunlight they obtain from near the ocean’s surface, nitrogen is located at deeper levels of the ocean. The ability for this population to acquire nitrogen depends on the stratification of the ocean, which occurs because it’s the meeting place of the Atlantic and Pacific oceans, where dense Atlantic water flows under lighter Pacific water.
“When the upper ocean is strongly stratified, with very light water floating on top of dense deep water, the supply of nutrients to the sunlit surface is slow,” said Jesse Farmer, the lead author of the study.
To measure the stratification levels of the Arctic Ocean throughout history, the team looked at the nitrogen levels within fossils, tracking how the shifting Atlantic and Pacific oceans have changed over time and affected the nutrient levels these organisms can access.
Nitrogen Levels Have Not Been Consistent
The researchers specifically looked at the last ice age, when the sea levels were low and Asia and North America were connected by the Bering Land Bridge. At that time, the Arctic Ocean consisted of Atlantic water only, which made it colder and much less stratified. There was more nitrogen available to the plankton during this last ice age, which ended over 11,000 years ago.
Stratification levels increased after the land bridge was subsumed and the Bering Strait filled in with water. The scientists then turned to the Holocene Thermal Maximum (a period around 6,000 years ago when temperatures were warmer than they are today) to look at what the future might hold for plankton and nitrogen levels.
What they found contradicted commonly held beliefs: The increase in sunlight reaching Arctic waters due to melting ice cover will not correspond to an increase in organism activity because the waters will become more stratified. Climate change and warm Pacific waters will continue to increase the stratification levels in Arctic waters, which means the organisms will have a more difficult time acquiring nitrogen from the depths of the water.
What Does This Mean?
Scientists had hoped that an increase in plankton activity and growth would be a small silver lining in the ravages of climate change, but now it looks as if that isn’t going to happen.
“A rise in the productivity of the open Arctic basin would likely have been seen as a benefit — for example, increasing fisheries,” said Farmer. “But given our data, a rise in open Arctic productivity seems unlikely.”
The impact of climate change on these tiny organisms will be felt throughout the ocean and will directly affect creatures that rely on the population as food. This is just one more reason for us to put our heads together and focus our resources on slowing climate change to protect our world.
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