The Oceans: Carbon Sink or Source?
Part B: Phytoplankton: Ocean's Green Machines
- 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 CO2fossil fuel emissions.
- 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
- types and numbers of zooplankton grazing on the phytoplankton
What causes phytoplankton blooms?
Using satellites and ocean color 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 bountiful
- How the ocean environment is changing year to year
- 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) 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.
Consider the two different images of the same phytoplankton bloom in the Bering Sea taken by Seawifs 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.
With a partner or your class, compare and contrast the two images.
1. Where is the biological pump the strongest in this phytoplankton bloom? How do you know?
2. Which image type - true-color or false-color- is more useful to scientists in determining the amount of carbon moving from the atmosphere down into the biological pump? Explain why.
Using ocean color to monitor size, location, and timing of phytoplankton blooms
Because phytoplankton blooms and the oceanic biological pump are important to climate, scientists are interested in studying the following questions:
- What is the seasonal variability of phytoplankton blooms?
- How do phytoplankton blooms differ in size, population density, geographic location, and timing?
- Where are nutrients such as nitrogen limited and where they are bountiful?
- How is the ocean environment changing year to year?
- Are there changing trends in phytoplankton populations over time?
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.
What does this chlorophyll data tell scientists about the location and size of phytoplankton blooms?
- The highest concentration of phytoplankton exist in blooms along coastal areas. These are areas of coastal upwelling where cold water moves up from the deep ocean carrying dissolved CO2 and nutrients. Phytoplankton absorb the dissolved CO2 for photosynthesis and use the nutrients to make carbon-compounds necessary for life such as proteins and carbohydrates. Nutrients are also washed down from rivers into ocean coastal waters contributing to very dense phytoplankton blooms.
- The largest phytoplankton blooms exist in areas of cold, open ocean because cold water holds more dissolved CO2 than warm water. Nutrients are able to mix up into sea surface water from deep nutrient-rich water.
- Lowest concentrations of phytoplankton exist in areas indicated by blue ocean color. The surface water in these areas are nutrient-poor and warm. Unlike cold water, warm water holds less dissolved CO2 making less CO2 available for photosynthesis. Warm water sitting on top of nutrient-rich cold water prevents nutrients from mixing up into the top sea surface layer where phytoplankton live.
Note: You can view this time series animation at:
- SeaWiFS Biosphere Data over the North Atlantic on YouTube
- NASA visualization app for IPAD. Scroll down to "Super Blooms"
The oceanic carbon cycle and nitrogen cycle: 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 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.
Trichodesmium- A tiny organism with a BIG role!
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:
DiscussionWith a partner or your group, think about and discuss the following:
- Describe Trichodesmium's role in the nitrogen cycle and the formation of phytoplankton blooms.
- Why is the nitrogen cycle critical to moving carbon from the atmosphere into the oceanic biological pump?
- How could a small organism such as Trichodesmium impact climate?
Use NOAA's View Data Exploration Tool 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 articles 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. You can also can read and visualize this exciting discovery on the NASA Visualization App for IPad. Scroll down to "Secret Garden."