Okay fellow ocean science heads, it's quiz time. Without resorting to textbooks, Wikipedia, or Google, answer me this:
What produces most of the Earth's oxygen? a) Forests;
b) Plants, phytoplankton, and other photosynthesizers in the ocean; or
c) Both equally.
In the interest of transparency, this is a real student survey question posed by a teacher who is testing the proposed
ocean literacy principles. This post is a distillation of an ongoing and lively discussion taking place on
Scuttlebutt, the
National Marine Educator's Association list serve. I modified the teacher's question only slightly.
So what's your answer? Did you select "b"? If so, you would have answered correctly from the viewpoint of the teacher who designed the test. Her justification is based on Ocean Literacy Principle 4: The Ocean Makes Earth Habitable, subsection a) Most of the oxygen in the atmosphere originally came from the activities of photosynthetic organisms in the ocean. In addition, Ocean Literacy Principle 6: The Oceans and Humans are Inextricably Interconnected reinforces this concept with subsection a) Nearly all Earth's oxygen comes from the ocean.
But our intrepid teacher was hoping to find some quantitative figure to back-up these ocean literacy principles. As she commented on Scuttlebutt, "
Textbooks generally say about 2/3 of the oxygen comes from the ocean. The Smithonian's Ocean Hall, on a pillar at the entrance, says about half of our oxygen comes from the ocean."
So who's correct?
A Holt high school biology textbook (© 2004) on my bookshelf by Johnson and Raven (California Science Standards Edition) states on p. 378 that, "
Photosynthetic plankton that form the base of the ocean food chain account for about 40 percent of all the photosynthesis on Earth."
Back on Scuttlebutt, marine science educators began offering suggested quantitative figures for oceanic oxygen production ranging from
nearly half of all oxygen produced to wide ranges of 20% to 70% over the years.
This apparent Gordian Knot of quantifying oceanic primary productivity was unceremoniously severed by the appearance of a quote from
Dr. Bob Stewart in the comment thread. Dr. Stewart, a professor of physical oceanography and expert ocean science educator at Texas A&M University provided the
coup de grâce in true Texan fashion:
"
Plants and phytoplankton both produce no oxygen if averaged over their lifetimes."
What?! No oxygen over their lifetime? What could he possibly mean? What about Ocean Literacy Principle 4: The Ocean Makes Earth Habitable, subsection a) Most of the oxygen in the atmosphere originally came from the activities of photosynthetic organisms in the ocean and Ocean Literacy Principle 6: The Oceans and Humans are Inextricably Interconnected, subsection a) Nearly all Earth's oxygen comes from the ocean?
Dr Stewart elaborates:
The answer is somewhat complicated. And it depends somewhat on what you are trying to understand.
On average, plants or plankton produce no net oxygen. The oxygen they produce during their life is used up when they die and decay away, or when they are eaten by animals and the animals die. Oxygen in the air comes from the burial of carbon in sediments in the ocean. Plants on land, and phytoplankton in the ocean both produce oxygen, but when they die the oxygen is used up when they decay away, and no oxygen is left over. However, if the carbon in their bodies is buried, it cannot decay and use up oxygen, so some oxygen is left in the atmosphere.
[Land] plants and phytoplankton produce oxygen at about the same rate. But it is mostly dead phytoplankton that end up in sediments.
As best we can estimate, the net primary productivity on land is about equal to the net primary productivity in the ocean. A comparison by area or type of organism does not make much sense. Photosynthesis is highly variable. Trees may photosynthesize for a while, and then less if there is not enough water. Phytoplankton cannot be compared with trees. They grow so fast, and double in numbers so fast, they cannot be compared with trees which grow much more slowly. We cannot compare by weight because the tree has much mass (wood) that weighs a lot but doesn't photosynthesize. Plankton weigh far less, but produce far more oxygen per kg than land plants. The standing crop of plants on land weighs far more than the standing crop of phytoplankton in the ocean. If we compare by area, some areas at some times are highly productive, but nearby, or a few days later, the productivity is far less. Even global averages are hard to calculate.
So the simple answer is, plants and phytoplankton both produce no oxygen if averaged over their lifetimes, where lifetime is from beginning to the time all carbohydrates have decayed away. For example, forests use as much oxygen as they produce, so their net production is zero.
I know this goes against intuition, but I have discussed this issue carefully with experts, and we all agree, oxygen in the atmosphere comes from the burial of reduced carbon, mostly in the ocean.
You could have heard a pin drop (or was that the sound of ocean educator jaws dropping) as those words were digested across the list serve community. This was one of those defining moments you hope to experience in your lifetime as a scientist or science educator. A moment of genuine cognitive dissonance where preconceptions are confronted with a new way of sense-making. It's a moment with tremendous opportunities for leaps in learning.
One commenter immediately responded that if Dr Stewart is right, "
and I have no reason to doubt what he says on the topic, there is going to be a lot of stomach upset among publishers and marine educators." The commenter goes on to explain that, "
we clearly need to adjust the Ocean Literacy statement previously cited. The source of the oxygen would still be the ocean, but the mechanism would be different from what has been generally accepted."
On a more positive note, the same commenter celebrated the reminder that, "
Science is truly fascinating. 99.99% of us can believe we understand something and 0.01% of us can upset the apple cart."
But what about the poor ocean science teacher who opened this wet Pandora's Box? She still has a test to administer. Her reaction to the revelations was more pragmatic, "
But what do I do with the question on my ocean literacy survey? Right now it's:What produces most of the earth’s oxygen?
a) forests;
b) plants, phytoplankton, and other photosynthesizers in the ocean; or
c) both equally
Where the correct answer is "b". How should the question be written to accommodate what we've discussed today?"
Again, a plethora of answers came down the pike. There were calls to, "
Change the question to some other topic. Until we establish what "produces" means in the context of your question there will be no correct answer, or at least not one that the uninitiated will understand."
But there were also commenter's who were not so eager to give up ground so quickly, "
We are not going to sound particularly credible (even if we are) if we claim that oceanic plants are not producing any oxygen when what we mean is that there is no NET production of oxygen. Are we splitting hairs? I'll leave that to you, but when my 9th graders stick their noses into a sunlit tidepool and see oxygen bubbles forming on the lateral edges of surf grass, I can't very well tell them that no oxygen is being produced. . . or can I??"
George Matsumoto, Senior Education and Research Specialist with the Monterey Bay Aquarium Research Institute (MBARI) summed it up short and sweet, "
Ocean plants are producing oxygen!" He went on to explain that, "
Ocean protists (algae) and bacteria are producing even more oxygen. The oxygen in the atmosphere comes from net primary production - that is, the sequestration of these carbon sources into the sediments is what keeps the oxygen produced in the atmosphere - otherwise, it gets used up when the plants, protists, bacteria decompose."
Clear enough, but isn't attempting to explain sequestration of carbon sources in ocean sediments a little like strolling into a patch of quicksand with elementary or high school students? Does an ocean science literate person in our society (not a scientist or ocean education specialist) need to know this?
One of the longest-standing architects for the Ocean Literacy Principles, UC Berkeley Lawrence Hall of Science'
Craig Strang (a brilliant ocean science educator whom I have the honor of calling friend) offered some tempering thoughts, "
[Ocean Literacy] Concept (4.a.) was among the 3-4 most difficult to craft among the 44. We settled on a carefully worded statement that includes the words, "most" (not a %) and "originally came" (not currently produced") and "photosynthetic organisms" (not plants or plankton). We decided that an ocean literate person does NOT need to be able to quote the percentage of O2 that comes or came from the ocean (since none of us could!), nor did we think that an Ocean Literate person needs to know whether the ocean produces a net gain of O2 in the atmosphere or not."
A number of commenter's, and this reader, seemed to agree with Craig that it's probably not essential that an ocean literate person be able to quote the percentage of O2 that came from the ocean. Rather, one commenter wrote, "
the important part of that concept is the simple realization that photosynthetic organisms in the ocean played a big role in making the world we live in today." Yet another commenter suggested that "
the purpose of this ocean literacy concept is to start the dialog regarding the role of aquatic photosynthesis--that it is important for students to realize that aquatic photosynthetic organisms were and are an important source of atmospheric O2. If it gets to the point where learners attain a more informed understanding of all the sources and sinks of atmospheric O2, that's bonus."
Another commenter also told a story that I'd like to retell as I suspect it may likely resonate with your own experiences in elementary and high school science education. To the best of my memory, photosynthesis was taught in school from a terrestrial point of view only. It was only my obsessive love of ocean science as a geeky youth that I remember learning from one of my Jacques Cousteau books that phytoplankton produce oxygen we breathe. There was little attention paid in high school of the contribution of aquatic photosynthesis--and almost zero attention paid to ocean habitats and marine ecology as a whole. This was my formal science experience through the 70's and until I graduated high school in 1981 in Pennsylvania.
From my limited experiences in working within the formal and informal elementary and high school science education systems in the late 90's, I'd have to conclude that things have not changed much. And in a system that became increasingly geared towards
No Child Left Behind (or as we called it, No Child Left Untested), teachers in many public systems taught primarily to the tests and dropped or de-emphasized science education. Not that ocean science was a big part of standard science curriculum anyway.
I want to conclude that I don't see any of these interesting online discussions as evidence of flaws or weaknesses in developing ocean science literacy principles. I believe we absolutely must strive to have a scientifically literate citizenry. In so doing, we also need to remember that a lot of the science (be it physical, biological, or chemical) operates in our mostly ocean planet, inappropriately named "Earth."
The apparent ocean literacy paradox I describe above illuminates a sometimes messy but incredibly important process when scientific domain content knowledge doesn't mesh easily with pedagogic content knowledge. In other words, there's what we know scientifically, and there's what effective science educators know in transmitting that content to their students. The skill and science (and art) is in marrying the two for most effective and meaningful learning.
What are your thoughts?
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