The Oceans: Carbon Sink or Source?
Part B: Phytoplankton: Nature's Green Machines
Other colors in the scene may come from sediment or other species of phytoplankton, particularly diatoms. The Barents Sea usually witnesses two major bloom seasons each year, with diatoms peaking in May and June, then giving way to coccolithophores as certain nutrients run out and waters grow warmer and more layered (stratified). Credit: NASA
In Part A, you learned that the ability of the oceans to absorb and sequester a lot of carbon is dependent on phytoplanktontiny one-celled photosynthetic organisms that float in the upper surface of ocean. The satellite image above shows a massive phytoplankton bloom in the Barent's Sea, off the coast of Norway. This phytoplankton bloom contains billions - trillions of individual phytoplankton taking up carbon dioxide into the ocean's biological pump. Given appropriate conditions, populations of phytoplankton can reproduce explosively.
- What conditions are necessary to create phytoplankton blooms? What conditions limit phytoplankton blooms?
- What can satellite images tell us about the location and strength of phytoplankton blooms and the biological pump?
Begin by watching a NASA video The Ocean's Green Machines. When you finish watching, answer the two Checking In questions below.
Ocean Color: Measuring and visualizing phytoplankton and primary productivity.
The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) instrument satellite sensor monitored "ocean color" for ten years before it became decommissioned. Consider the two different images of the same phytoplankton bloom in the Bering Sea taken by SeaWiFS on June 15th and 16th, 2000.
- The top image is a natural(true) color image. Various ocean colors indicate the presence of different types and quantities of phytoplankton. Although the natural 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, reconstructed image using chlorophyll data (and other relevant data) measured by SeaWiFS. Chlorophyll is the green, photosynthetic pigment in both land plants and phytoplankton and is used to calculate phytoplankton's net primary productivity(NPP) - the amount of carbon taken in by photosynthesis into the ocean's biological pump.
With a partner or your class, compare and contrast the two images.
1. What information or "story" does each image tell you about this phytoplankton bloom?
2. Where do you think phytoplankton's NPP and the ocean's biological pump would be the strongest?.....the weakest? What makes you think so?
- The seasonal variability of phytoplankton growth
- The geographic distribution of phytoplankton
- The relationship between phytoplankton and other ocean life, such as fish
- Indentifying geographic zones where nutrients are limited and where they are bountiful
- How the ocean environment is changing year to year
- SeaWiFS Biosphere Data over the North Atlantic on YouTube
- NASA visualization app for IPAD. Scroll down to "Super Blooms"
- Super Blooms
Stop and Think
- How do seasons affect the size and distribution of phytoplankton blooms?
- Where and when is primary production and the draw-down of carbon into the biological pump the strongest?
What causes phytoplankton blooms?Sunlight and nutrients are the most important ingredients for a phytoplankton bloom to occur. When nutrients and sunlight are plentiful, microscopic phytoplankton reproduce quickly, developing huge population explosions. 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 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) and B vitamins
- water temperature, density and salinity, mixing
- types of phytoplankton
- types and numbers of zooplankton grazing on the phytoplankton
1. What would some immediate and some long term effects of a phytoplankton bloom on the ocean's biological pump and the carbon cycle?
2. Phytoplankton need light to photosynthesize. What changes are happening to Arctic sea ice that provided the necessary light for the surprising under-ice phytoplankton bloom to occur?
Why are nutrients important to the ocean's biological pump?
Stop and Think
Examine the image of chlorophyll and nitrogen(nitrate) concentration on the above right. Click to enlarge.
3. What evidence from these images tells you that nitrogen is a limiting factor on phytoplankton primary production? List all the evidence that you can. Note: Nitrogen measurements are made from oceanographic ships and ocean buoys. Chlorophyll measurements are derived mostly from satellite data.
Where do the nutrients essential for phytoplankton growth come from?
Nutrients come into the oceans from a variety of sources. For example, soil erosion and fertilizer used on farms bring nitrogen and phosphorus compounds to rivers where they wash down into the ocean. Much of the iron (Fe) that phytoplankton need is blown in from deserts far away.
A major source of nutrients for phytoplankton is a process called upwelling. When upwelling occurs, deep cold waters rise to the surface bringing along nutrients originally from dead bits of decomposed matter, feces and urine that sank down to the deep ocean and entered the deep ocean currents. These nutrients act like fertilizer, stimulating phytoplankton growth which in turn, stimulates food webs. Upwelling occurs in open ocean and along coastlines as the image above right indicates. You can watch an animation of coastal upwelling here. Wind direction and speed, water temperature and topography all play a role in developing upwelling areas.
Stop and Think4. Explain why areas of upwelling and river deltas contribute to a stronger biological pump.
Nutrients, microbes and the biological pump.
1. Identify and describe all the ways that the nutrients(N, Fe and P) move into and around the ocean's biological pump.
- N and Fe enter the ocean from the atmosphere. Note: Atmospheric N2 gas dissolves in the ocean whereas particles of Fe are blown in from other areas of the world - such as deserts.
- N, P, Fe and silica(Si) move up from the bottom(benthos) in "upwelling currents". Note: As ocean microbes decompose dead organisms, the N, Fe, P and Si in those decomposed pieces sink down to the benthos.
- N, P and Fe run-off from the land into the ocean. Note: Agricultural fertilizer contains N, P and Fe. This fertilizer runs off from the fields into the river systems and eventually makes it way from the river systems into coastlines. This large influx of nutrient fertilizer into oceans can cause "dead zones" in coastal waters.
Trichodesmium- A tiny but important player in the ocean's nitrogen cycle.Like land plants, phytoplankton cannot take-in and use nitrogen in the form of N2. Instead, phytoplankton rely on microbes to convert N2 into other types of nitrogen-based molecules that phytoplankton can take-up and use. Recent oceanographic research has identified that the cyanobacterium Trichodesmium has a critical role in making nitrogen available to phytoplankton. To explore Trichodesmium's role in the ocean's nitrogen cycle, you will use one of two interactives developed by Wood's Hole Oceanographic Institute(WHOI).
- Go to Wood's Hole Oceanographic Institutes Interactives
- Scroll down to the "Interactive Illustrations" section. Note: If you see the word (ALL), click on this to bring up all of the interactive illustrations.
- Choose either the Trichodesmium interactive (less complex) or The Nitrogen Cycle interactive (more complex and comprehensive).
- Investigate Trichodesmium's role in the nitrogen cycle by clicking through the interactive. When you have finished, answer the Discussion questions below:
Stop and Think
5. What is Trichodesmium's role in the nitrogen cycle? How does this role affect the ocean's biological pump?
DiscussionWith a partner or your group, think about and discuss the following:
- Why are ocean microbes and nutrients critical to moving carbon from the atmosphere down to long term storage in deep sea currents and sediments.
- Overall, the world's oceans are warmer now than at any point in the last 50 years. The change is most obvious in the top layer of the ocean, which has grown much warmer since the late 1800s. This top layer is now getting warmer at a rate of 0.2°F per decade. Oceans are expected to continue getting warmer—both in the top layer and in deeper waters.(EPA) The upwelling of nutrients is slowed down when warmer water sits on top of the water column.
- In what ways would warmer ocean water change the ocean's biological pump?
- In turn, how would these changes affect the global carbon cycle?
- Have you ever wondered how NASA visualizations like the one on the right are created? If you have the time, watch this SciFri video Creating Earth
- Read the other articles on Phytoplankton and Ocean Research in the NASA Earth Observatory features collection. You can access these articles here
- Grow your own phytoplankton and use it to design an experiment on variables that affect phytoplankton growth. Here is one of several videos that tells you How to grow phytoplankton.
- Carry out a case study on predicting the Spring Phytoplankton Bloom in the Gulf of Maine. When is Dinner Served? Predicting the Spring Phytoplankton Bloom in the Gulf of Maine