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How Climate Change Affects Half of the World’s Photosynthesizers: Phytoplankton

Updated: Jun 26, 2021

You probably don’t think of plankton very often. They’re small, sometimes microscopic, marine organisms — out of sight, out of mind. But these tiny organisms are actually critical for marine ecosystems, and they’re not immune to the effects of climate change.

The category plankton encompasses many different types of marine life, referring to any organism that cannot control its own movement well and is carried by currents. Plankton are either meroplankton or holoplankton. Meroplankton are larvae that will grow into larger organisms, such as jellyfish, crustaceans, sea stars, and most fish species. Holoplankton are probably what you imagine when you think of plankton: They remain small their entire lives, and include copepods, krill, and phytoplankton. However, phytoplankton are distinguishable from all other planktonic organisms, or zooplankton, and their unique characteristics make them crucial for the existence of marine life.

Phytoplankton are photosynthetic, meaning they use sunlight and carbon dioxide to make oxygen, just like terrestrial plants. They include single-celled algae and bacteria, such as cyanobacteria, both of which are holoplankton. All together, phytoplankton produce half of the atmospheric oxygen we breathe. Being at the bottom of the marine food chain, they also have a crucial role in maintaining a healthy ecosystem, providing essential nutrients for organisms from zooplankton to whales. Their impact, however, expands beyond the marine organisms they feed.

Through photosynthesis, phytoplankton convert atmospheric carbon dioxide into organic carbon, playing a significant role in carbon sequestration, or the process of capturing and storing carbon dioxide. When phytoplankton die, carbon that was once in the atmosphere sinks to the ocean floor, reducing excess atmospheric carbon dioxide and helping to mitigate climate change. Phytoplankton, however, are still impacted by climate change. Populations have shown different responses to climate change in recent years. Scientists are trying to understand how to best address both the dangerous increases and decreases in their productivity.

A recent study suggested that phytoplankton populations are fluctuating across different regions in the world. More specifically, it was found that open ocean populations have decreased long-term, while near-shore populations have increased in more recent years. Coastal changes are likely due to runoff, nutrient influx, and warming waters, allowing for large, potentially harmful algal blooms, which produce dangerous toxins.

On the other end of the spectrum, dangerous decreases in phytoplankton productivity have been recorded in open oceans, such as the North Atlantic. Using ice core samples in Greenland, researchers analyzed phytoplankton productivity over hundreds of years, tracking a detectable byproduct of blooms, methanesulfonic acid (MSA). It was found that since the start of the Industrial Revolution in the mid-1800’s, north Atlantic phytoplankton have experienced a 10% drop in productivity. This observation is potentially connected to weaker nutrient upwelling due to warming surface water temperatures.

There’s still much for scientists to learn about phytoplankton and the potential effects climate change may have on their populations. However, it’s certain that marine ecosystems begin with the smallest organisms. The impact of these changes, though not fully understood yet, have the potential to radiate throughout the entire marine food web. To learn more about these important organisms, search for educational resources about phytoplankton and their critical role in marine ecosystems.

Share something you learned about phytoplankton in the comments below!

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