The Marine Carbon Cycle

Carbon is the most important element to life on earth. But we are changing the carbon cycle at an unprecedented rate.

In the Arctic Ocean there are changes occurring to the carbon cycle, which will have impacts on the Arctic food web.

Carbon cycle

In the sunlit surface ocean, carbon which is dissolved in seawater can be converted into organic matter by the photosynthesis reaction. This is done by tiny floating plants, called algae. These carbon-rich plants are then eaten by many animals in the ocean. Every living thing is made up of carbon-rich organic matter; the plants, fish, seals, whales.

The carbon which is available to be photosynthesised by algae, can get into the ocean via different pathways:

Atmospheric sources The amount of carbon in the atmosphere can balance with the upper ocean. If we add more carbon to the atmosphere we can increase the amount of carbon held in the ocean. Natural processes add carbon to the atmosphere such as volcanoes, but humans are currently increasing the amount of carbon through industrialisation. This CO2 comes from oil and gas reserves that have accumulated in the earth over millions of years. Carbon is pumped into the atmosphere, mostly in the form of CO2 and can dissolve into the ocean through air-sea gas exchange.

Sources from land Carbon does not only come from the atmosphere, but from coasts and rivers. The carbon from land and the atmosphere have different chemical signatures, so you can analyse where the different carbon has come from.

The re-cycling of carbon in the marine system When animals respire they breathe out carbon dioxide, just like humans. This transfers CO2 from the animals back into “marine CO2” which can stay in the water column and potentially be released back into the atmosphere.

These processes all provide a food source for algae, and can vary in quantity over time and location. This is why we want to measure and understand the relative importance of these sources to plants and animals in the Arctic.

When animals and plants die, they release carbon back into the ocean and can also sink to the seafloor (organic matter). This gets buried with the sediments and the carbon is locked away.

Is it beneficial to marine life that the ocean takes up extra carbon? Although carbon in the ocean provides an essential food source to all marine life, too much can be harmful to certain species. Some of the carbon which we pump into the atmosphere is added to the ocean and changes the chemistry of the seawater. Not all animals can easily adapt to these fast changing conditions. Some creatures have shells which are sensitive to changes in the carbon concentrations in the water, and this can make it difficult for them to form their shells. This is having a bigger impact in polar regions as colder waters take up more carbon.

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