4. Carbon Cycle or Marine Biological Pump


Marine Ecosystem

1. Introduction
2. Phytoplankton
3. Carbon in the Sea
4. Carbon Cycle
5. Conditions for Life
6. Limiting Nutrients
7. Nutrient Distribution
8. Life in the Sea
9. Food Webs

The chemical element, carbon, which is essential for life on this planet, cycles through the land, air and sea in a variety of forms. The exchange and storage of carbon in these different areas has important implications for life as well as global climate.

The exchange of carbon and its storage and release from the ocean in the form of CO2 is critical to the climate system of our planet.

Carbon Cycle


A cartoon of the global carbon cycle. Reservoirs of carbon (in black) are gigatons (1Gt = 1x109 Tons) of carbon, and fluxes (in purple) are Gt carbon per year.

Illustration courtesy NASA Earth Science Enterprise.

Carbon is absorbed by the ocean as a dissolved gas (CO2), which  algae (phytoplankton) incorporate through photosynthesis in the surface waters where light is available (remember the photic zone).

Organic carbon is passed through the marine food web, beginning with phytoplankton before incorporated into comsumers (organisms that eat primary producers, like phytoplankton).  These organisms are consumed by predators, but ultimately a significant percentage of the carbon is discarded in the remains of dead organisms and sinks to the seafloor to be buried in deep sea sediments.

Upon reaching the seafloor, carbon is consumed by organisms living in the mud with any left over carbon becoming buried in the sediments. The latter portion can turn into hydrocarbons, which if it subjected to the proper combination of pressure and temperature.   These hydrocarbons are then developed into the fossil fuels and extracted as oil and gas to  drive the world's economy.

The burning of fossil fuel releases large quantities of CO2 into the atmosphere, which may accumulate over time or can be incorporated into plant material or be absorbed back into the sea.  Much scientific effort is being expended to better understand how much CO2 is absorbed by the ocean and held in the deep waters of the global conveyor belt (more on this in chapters 9 & 10 of your book - Where the Water Goes and Turning off the Currents - and in later expeditions.

Tiny plants in the ocean called phytoplankton, take carbon dioxide out of the atmosphere and circulates it through the ocean. Some scientists say global warming might warm the oceans, change the circulation of minerals in the water and reduce the amount of phytoplankton. That means more carbon dioxide would remain in the atmosphere. 

So the "biological pump" cycles carbon in the ocean, as carbon is first absorbed from the atmosphere and then seawater, and converted to organic matter through photosynthesis, mainly by phytoplankton. Phytoplankton are then incorporated into larger organisms (consumers), through foraging and predator-prey relationships. Once these organisms die, carbon, and a host of other nutrients, are reabsorbed in the deep ocean as the decaying remains on dead organisms are decomposed by bacteria. The dissolved carbon, as well as other nutrients, in the mid- and deep waters are brought back up to the surface waters by a process called upwelling, which we will learn more about later in the class. The biological pump is presently diagrammatically in the image below, which shows that carbon and other biolimiting nutrients, are continuously cycled from the surface waters to the deep waters and back again.

CO2 Biological Pump

Image Courtesy of Hannes Grobe,
Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany

Here is another way to visualize the carbon cycle

Diagram from NEPTUNE Project

Take some time and review these diagrams as the biological pump or the carbon cycle
is foundation to the health of the marine ecosystem.

©Copyright 2017
March 13, 2017

Send to Don Reed

Department of Geology
San Jose State University

What are the life-sustaining conditions in the ocean?