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 drawing down carbon dioxide from the atmosphere 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 blooms and primary productivity.
In the satellite image of phytoplankton blooms in the Barents Sea at the top of this page, you can see brilliant blue and green colors in the ocean. Scientists refers to these colors as Ocean Color. Scientists measure ocean color to determine ocean primary productivity - the amount of carbon phytoplankton are removing from the atmosphere via photosynthesis. Satellites can measure ocean primary productivity by measuring the amount of green chlorophyll in the water.
The basic premise behind measuring ocean color to determine ocean primary productivity is as follows:
- Higher concentrations of green chlorophyll detected by satellites indicates that there are higher concentrations of phytoplankton.
- Higher concentrations of phytoplankton indicates higher rates of photosynthesis and thus higher rates of ocean primary productivity.
- Higher rates of primary productivity indicates that more carbon is being drawn down from the atmosphere by phytoplankton and moved into the oceanic biological pump.
Scientists use ocean color to track changes in the size, location and timing of phytoplankton blooms and primary production over different spatial and temporal scales. The data collected from these ocean color measurements help scientists to understand the impact of changes in the ocean carbon cycle on climate.
- 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 "false color" 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 primary productivity.
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. Which image type - true color or false color- gives more information about ocean primary productivity and the strength of the biological pump? Why?
- The seasonal variability of phytoplankton growth
- The geographic distribution of phytoplankton
- The relationship between phytoplankton and other ocean life, such as fish
- Identifying 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 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
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. Wind direction and speed, water temperature and topography all play a role in developing upwelling areas. Watch an animation of coastal upwelling animation of coastal upwelling and then answer the Stop and Think question below.
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.
2. Which processes carried out by microbes are the most important in moving N, P and Fe throughout the ocean's biological pump?
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 to biosynthesize their DNA, proteins and other important carbon compounds. Recent oceanographic research has identified that the tiny cyanobacterium Trichodesmium has a critical role in making nitrogen available to phytoplankton. To investigate Trichodesmium's role in the ocean's nitrogen cycle, you will use two interactives developed by the Wood's Hole Oceanographic Institute(WHOI). Then, you will answer the Checking in Questions and the Stop and Think questions that follow.
- First, go to the WHOI's interactive Trichodesmium:The little creatures that turn the wheels in the ocean. Click through the interactive to investigate the role of this tiny cyanobacteria in the oceanic nitrogen cycle.
- Next, enter The Nitrogen Cycle interactive. Click through the interactive to investigate the oceanic nitrogen cycle in greater detail. When you have finished, answer the Discussion questions below:
DiscussionWith a partner or your group, think about and discuss the following:
- What is Trichodesmium's role in the nitrogen cycle? How is Trichodesmium's role in the oceanic nitrogen cycle similar or different from the terrestrial nitrogen cycle?
- Which of the carbon cycle processes (photosynthesis, respiration, decomposition, biosynthesis and combustion) are critical processes in the nitrogen cycle?
- Why is the nitrogen cycle critical to moving carbon from the atmosphere down to long term storage in deep sea currents and sediments.
Stop and ThinkOverall, 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). When surface waters get warmer and sit on top of the water column, the mixing of nutrients from below slows down.
- In what ways could warmer ocean surface 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