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Acid and phytoplankton in the ocean’s food chain

Research into the demise of dinosaurs at the end of the Cretaceous Period also points to another massive loss of life at the microscopic level of the spectrum. Calcareous plankton in the ancient oceans experienced catastrophic population declines and species loss. With the disappearance of these tiny creatures, whole ecosystems collapsed because phytoplankton were the first link in the food chain.

The calcareous plankton fell prey to a phenomenon called ocean acidification. The famous asteroid strike that ended the reign of the dinosaurs also released large amounts of sulfur and nitrous oxide into the atmosphere. These compounds are believed to have made their way into the oceans, where they reacted with the water to increase ocean acidity. 

Unfortunately for the plankton, their fragile shells are very sensitive to acid. Their shells weakened and then dissolved in acidic waters. Many calcareous plankton species became part of the mass extinction occurring at the end of the Cretaceous. 

The past is the key to the present, and we find ourselves observing ocean acidification today. Acidification this time is from the absorption of CO2 into the oceans and the subsequent formation of carbonic acid.

The bottom rung

Food chains are nature’s way of transferring energy between different species. The food chain is organized into trophic levels, and the base trophic level, or bottom rung of this system, contains organisms that make their own food. 

In the oceans, phytoplankton and algae form the lowest trophic level. Small fish, crustaceans, and zooplankton are the next level up, and they feed on the phytoplankton. The chain continues upward with larger fish and ocean mammals eating the smaller creatures, and eventually land mammals, including humans, consuming a wide range of the ocean’s bounty. 

The phytoplankton and algae photosynthesize their food, using sunlight to build carbohydrates. Phytoplankton enjoy floating around in the upper levels of the ocean, because this is where they can bask in the sun and do their photosynthesis trick. The phytoplankton are divided into two groups, dinoflagellates, and diatoms. 

The dinoflagellates live within tiny shells made out of calcium carbonate, while the diatoms extract silica from the water and use this biogenic silica to build their shells. 

Acid and shells

Without their shells, the phytoplankton can’t exist, and without the phytoplankton, the food chain collapses. Current research on the acidity tolerance of phytoplankton shells is limited, but the geologic record is clear that at some point, as oceans acidify, the phytoplankton decline or disappear.

It is normal that, as humans, we tend to focus on the land-based symptoms of climate change. Rising sea levels, increased wildfire frequency, and more severe weather events all affect our homes and our livelihoods. So for many of us, the oceans feel a bit distant, and ocean acidification seems a more remote concern. 

However, seafood provides nearly 3 billion people on our planet with a significant part of their daily protein. The food chain that provides this protein starts at the lowest trophic level – phytoplankton. Hence, a threat to the phytoplankton is an existential threat to a large portion of humanity.  


Ocean Acidification (Source: ArcheanWeb) – Also:

A crab crisis from coastal water acidification (Source: ArcheanWeb) – Also:

    Heat affects the ocean’s food chain1    January 30, 2020Heat affects the ocean’s food chain (Source: ArcheanWeb) – Also:


How the dinosaur-killing asteroid primed Earth for modern life (By TIM VERNIMMEN – National Geographic) – Also:

Ocean Food Chain (By Amanda Robb – – Also:

Aquatic food webs (NOAA – National Oceanic and Atmospheric Administration) – Also:

What are phytoplankton? (NOAA – National Oceanic and Atmospheric Administration) – Also:


OCEAN ACIDIFICATION (by The Ocean Portal Team – Smithsonian) – Also:

Ocean acidification (NOAA – National Oceanic and Atmospheric Administration) – Also:

Understanding the Science of Ocean and Coastal Acidification (EPA) – Also:

Feature Image: Phytoplankton – the foundation of the oceanic food chain (NOAA) (Modified) – This image is in the public domain because it contains materials that originally came from the U.S. National Oceanic and Atmospheric Administration, taken or made as part of an employee’s official duties.

William House
William is an earth scientist and writer with an interest in providing the science "backstory" for breaking environmental, earth science, and climate change news.