The Oceans and the Carbon Cycle

Part B: Phytoplankton - The Ocean's Green Machines

What causes phytoplankton blooms?

In Part A, you learned that phytoplankton are responsible for bringing carbon dioxide from the atmosphere into the ocean's biological pump. Given enough sunlight, CO₂, and nutrients, populations of phytoplankton can reproduce explosively, doubling their numbers in just one day. The satellite image on the right shows a massive phytoplankton bloom phytoplankton bloom: when phytoplankton rapidly reproduce, producing high concentrations of phytoplankton in the water. containing millions to billions of individual phytoplankton all drawing down atmospheric CO₂ to use for photosynthesis.

Large phytoplankton blooms impact climate in two important ways:

• higher amounts of carbon dioxide are removed from the atmosphere into the oceanic biological pump. Removing greenhouse gas molecules from the atmosphere mitigates the warming effect of CO₂ fossil fuel emissions; and
• higher amounts of carbon drawn into the biological pump eventually move down into deep ocean currents and sediments. Carbon stored in deep ocean currents is there for time scales of hundreds to thousands of years. The carbon that makes its way to sea floor sediments is stored for time scales of millions to billions of years.

Environmental factors that limit the size, longevity and timing of phytoplankton blooms will also limit the efficiency of the oceanic biological pump. Sunlight and nutrients are the most important ingredients for a phytoplankton bloom to occur. When nutrients and sunlight are plentiful, microscopic phytoplankton reproduce quickly. Some blooms are so massive that they tint the water and can be seen from space. The phytoplankton bloom at the top of the page is a excellent example.

There are many biotic and abiotic environmental variables factors that influence the formation of phytoplankton blooms. The most important ones include:

• sunlight—needed for photosynthesis to occur;
• availability of critical nutrients - nitrogen(N), phosphorus(P), iron(Fe), Sulfur(S) and B vitamins;
• water temperature, density and salinity;
• mixing of the upper layer of the ocean- helps to mix in the nutrients welling up from deeper layers;
• types of phytoplankton; and
• types and numbers of zooplankton grazing on the phytoplankton.

Begin exploring the importance of phytoplankton blooms by watching the NASA video "The Ocean's Green Machines."When you finish watching, answer the two Checking In questions that follow.

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Satellites and ocean color are used to study phytoplankton blooms and the oceanic biological pump

Because phytoplankton blooms and the oceanic biological pump are important to climate, scientists are interested in studying:

• the seasonal variability of phytoplankton blooms;
• the size, geographic location, and timing of phytoplankton blooms;
• the geographic zones where nutrients such as nitrogen are limited and where they are plentiful;
• how the ocean environment is changing year to year; and
• changing trends in phytoplankton populations over time.

Scientists use both in situ sea water samples and Ocean Color measurements from satellites such as Terra to monitor changes in size, location and timing of phytoplankton blooms and the impact of these changes on the Earth's system. You can easily observe ocean colors in this image above of a phytoplankton bloom off the coast of France. Ocean color is created when sunlight reflects off chlorophyll pigment molecules in the cells of phytoplankton floating in the upper surface of the ocean. Light reflected from sediments and dissolved organic material also contribute to ocean color. Different shades of the phytoplankton bloom depend on the types of species and the density of the phytoplankton population inside the bloom.

Ocean color chlorophyll data is used to determine the net primary productivity (NPP)net primary productivity (NPP): a measure of the amount of carbon dioxide taken in by phytoplankton via photosynthesis and converted into carbon compounds.] of the phytoplankton bloom. Net primary productivity tells scientists how much CO₂ carbon is being drawn down from the atmosphere by phytoplankton and moved into the oceanic biological pump.

Visualizations of ocean color can come in two formats: true-color (natural) images and false-color images.

Consider the two different images of the same phytoplankton bloom in the Bering Sea taken by SeaWiFS ( This site may be offline. ) on June 15th and 16th, 2000. Click to enlarge.

• The top image is a true-color image. The various ocean colors indicate the presence of different types and quantities of phytoplankton. For example, a milky white color indicates the presence of coccolithophores. Although the true color image gives a sense of how big the bloom is, it does not provide much information about the exact quantity of phytoplankton or how much carbon is being taken in through photosynthesis.
• The bottom image is a simulated, mathematically reconstructed "false-color" image using chlorophyll data measured by instruments on the SeaWiFS satellite. Note: Other data from shipboard in situ measurements may be incorporated into these types of false color images.
  

 

Phytoplankton blooms differ in size, location, and timing

To find answers to some of the questions above, you will examine static ocean color images and a time series ocean color animation created from ocean chlorophyll data.

The ocean color false image on the right has been generated from chlorophyll data taken by one of NASA's newest satellite - the Suomi NPP satellite. This image allows you to compare phytoplankton blooms in the summer in the northern hemisphere to summer in the southern hemisphere. Click to enlarge the image and take a few minutes to carefully examine the images for where phytoplankton populations thrive and where they don't thrive.

Next, watch NASA's SeaWiFS Biosphere Data over the North Atlantic time series animation showing a decade of of phytoplankton blooms. Hint: You can identify seasons by looking at the land vegetation. For example, land vegetation appears in the northern hemisphere during summer. Then answer the Checking In questions that follow:

 

The oceanic carbon cycle and nitrogen cycle have an interdependent relationship

Like land plants, phytoplankton need nitrogen and other nutrients to make important carbon compounds needed to grow and reproduce. For this reason, nitrogen and other nutrients have strong limiting effects on the growth, size, timing and longevity of phytoplankton blooms. When nutrients are plentiful, phytoplankton blooms appear. When the phytoplankton use up the nutrients, they die and sink and the phytoplankton bloom disappears.

Nitrogen gas, in the form of N₂, is very abundant in the atmosphere and dissolves in the sea surface water. However, marine organisms cannot use the N₂ form.

Click through the Wood's Hole Oceanographic Institute's (WHOI) nitrogen cycle interactive below to get a sense of the role of microbes in making N₂and other nitrogen compounds available to marine plants (phytoplankton) and food webs.

*This video replaces a Flash interactive for browsers/Android.

Trichodesmium—is a tiny organism with a big role in the nitrogen cycle

When you studied soil in Lab 5, you learned that mycorrhizal fungi and tiny soil microbes have big roles in the terrestrial nitrogen and carbon cycle by making nitrogen and other nutrients available to trees and plants. Without fungi and soil microbes, trees and other plants could not grow and store large amounts of carbon. Tiny organisms also play a big role in the oceanic nitrogen cycle and carbon cycle.

Use the WHOI interactive below to investigate the role of the tiny cyanobacterium Trichodesmium in the nitrogen and carbon cycle and then answer the discussion questions below:

*This video replaces a Flash interactive for browsers/Android.

Optional Extensions

Want to learn more about oceans, phytoplankton blooms and nutrients? Check out these resources:

• Research the latest research! New research on the carbon cycle, climate and the environment is on-going. You can use ScienceDaily and Phys.org to research recent research on the ocean carbon cycle by using combinations of the following tags: carbon cycle, oceans, oceans biological pump, phytoplankton, microbial loop and the nitrogen cycle. Here is an example: Scientists discover shifts in climate sensitive plankton over the past millennium.
• Use NOAA's View Data Exploration Tool' [link https://www.nnvl.noaa.gov/view/globaldata.html ']to investigate variables such as chlorophyll and nitrate data from NOAA's vast archives of satellites, climate models and observation tools. For example, you could track changes in nitrates and chlorophyll in the North Atlantic Phytoplankton Bloom over time. When you open the tool, make sure you take the Video Tour.
• Read more about phytoplankton:
• As Seasons Change, Will the Plankton? You will find links to other articles at the end of the article.
• Huge Phytoplankton Bloom Found Under the Arctic Ice.