Carbon is the main building block of life on earth and is present in many non-living things. Plants and animals need carbon to grow throughout their lives.
Despite constant CO2 consumption, the planet never seems to run out of it. This is because carbon circulates through ecosystems, whether they are aquatic or terrestrial. Now let’s focus on the aquatic carbon cycle.
The oceans play a particularly important role in the carbon cycle. Surface waters exchange gases with the atmosphere by absorbing and releasing carbon dioxide, oxygen, and other gases. Plant-like phytoplankton that lives in the ocean convert carbon dioxide into sugars that feed marine ecosystems.
Carbon Cycle Basics
The aquatic carbon cycle has two basic stages: the biotic stage, and the abiotic stage. The biotic stage refers to carbon being part of a living organism, and the abiotic stage means that it is not part of something alive. Any given carbon atom will spend time in both, most typically alternating from one to the other.
Carbon is an element. It has six protons and can form up to four chemical bonds at once, which is an impressive feat for a single atom. Because of this superpower, carbon forms the backbone of some fairly large and complex molecules. Fats, sugars, DNA, and proteins are physically impossible without carbon atoms.
The Ocean carbon cycle has two basic phases: the biotic phase and the abiotic phase. The biotic state refers to carbon being part of a living organism and the abiotic state means it is not part of anything living. Any given carbon atom spends time in both, most typically alternating from one to the other.
The Freshwater Carbon Cycle
Aquatic ecosystems can also take one of two very basic forms, and there will be variations in the carbon cycle in both. In freshwater systems such as lakes and streams, carbon enters the ecosystem as a gas called carbon dioxide.
Plants and algae clinging to the sides of streams or on the sunny shores of lakes use photosynthesis to convert carbon dioxide into sugars for their own growth.
Since gaseous carbon dioxide is not part of any living thing, scientists consider it part of the abiotic phase of the carbon cycle. However, once the plant converts to an organic form like sugar, it becomes biotic.
From there, an animal could feed on the plant and absorb some of its carbon. After that, a few things can happen to a carbon atom. The fish may be eaten by a larger fish, the fish may exhale carbon dioxide gas, the fish may die, or the fish may emit waste.
Further Freshwater Transitions
At this four-pronged fork in the road, carbon can immediately return to the abiotic phase as a gas and be taken up again by plants. This is the shortest and most fundamental loop in the aquatic carbon cycle.
When the fish is eaten by a larger fish, it goes through another identical biotic stage before later facing the same crossroads.
When the fish dies, it can sink to the bottom of the lake or stream. From there, the fish decomposes with the help of scavengers, fungi, and microbes. Again, much of this carbon is converted to gas, while some are incorporated into longer-lived abiotic stages such as soils
The Marine Aquatic Carbon Cycle
In marine systems, the carbon cycle resembles the freshwater carbon cycle in many ways. Photosynthesis, which is most commonly performed by plankton near the ocean’s surface, will pull carbon dioxide from the atmosphere.
This plankton is consumed by animals, which either die or exhale carbon dioxide for plankton to use again.
The big difference here is the enormous depth of the ocean. Most sea creatures live near the surface, but when they die, they sink to the bottom, carrying their carbon with them.
When it sinks to the seafloor, carbon has a really hard time getting back to the surface and can stay on the bottom as sediment for thousands of years.
Even in shallow water, carbon can still be temporarily withdrawn from the cycle. Coral reefs are produced by animals that build skeletons from calcium and carbon, ultimately creating a carbon-based structure that builds up over time.
This ability to hold carbon is why scientists refer to the ocean as a carbon sink. Carbon sinks are important because they help the planet regulate the amount of carbon cycled through the atmosphere and, more broadly, help regulate global climate.
Why is Ocean Carbon cycle important?
Human activities have increased atmospheric carbon dioxide to levels not seen in more than 800,000 years. Carbon dioxide, a greenhouse gas, acts like a blanket, trapping the sun’s heat and keeping it close to the earth’s surface.
As we increase the concentration of this gas in the atmosphere, we have caused the Earth to warm, with far-reaching effects.
The oceans have absorbed much of the carbon dioxide that humans have released into the atmosphere, slowing the rate of warming. But the oceans can’t absorb it indefinitely, and there are signs the rate of absorption may be slowing.
Also, when carbon dioxide reacts with surface water, it produces carbonic acid, which causes ocean water to become more acidic. Such ocean acidification is detrimental to ocean life, particularly corals and other organisms whose shells are damaged by acidic water.
As carbon sinks to the deep sea and some reaches the seafloor, it can be trapped for millennia, ultimately allowing the oceans to play a critical role in combating climate change.
References
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NASA. Carbon cycle. https://science.nasa.gov/earth-science/oceanography/ocean-earth-system/ocean-carbon-cycle. Accessed on February 18. 2021.
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NOAA. What is the carbon cycle? https://oceanservice.noaa.gov/facts/carbon-cycle.html. Accessed on February 18, 2021.
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