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Unsolved Mystery: Lake Superior’s Missing Carbon

Jim Cotner in the lab aboard the Blue Heron.

Carbon cycle researcher Jim Cotner (on right) works in the lab aboard the University of Minnesota Duluth’s research vessel, the Blue Heron, on a chilly day. Photo courtesy of the Large Lakes Observatory.

If organic carbon were money and biochemical researchers were government auditors, Lake Superior would be in HUGE trouble. Lake Superior — a very big business — somehow annually spends much more than it makes without falling deeply into debt. Although this may be every consumer’s fantasy, such an imbalance is clearly unsustainable, whether in nature or an accounting book.

Even scientists who study the lake’s carbon cycle can only speculate about the unaccounted sources of carbon. Either something is “cooking the books” when it comes to Lake Superior’s carbon balance or the lake has secrets…or scientists have more work to do to get accurate figures.

Carbon is a chemical element that forms the basis of both diamonds and life. Discounting water, humans are about half carbon. The carbon in Lake Superior comes from sources around and within the lake, including plants.

Sea Grant-funded researcher Jim Cotner, associate professor with the Department of Ecology, Evolution, and Behavior at the University of Minnesota, and several colleagues are re-examining Lake Superior’s annual organic carbon income and expenses in hopes of making inroads into our understanding of food webs and climate dynamics.

“Evidence suggests that the carbon cycle is tightly linked to climate and even drives climate,” he said. “Everybody seems to care about the weather. Since humans are tinkering with climate, I think we should know how the carbon cycle operates.”

When biochemists like Cotner break Lake Superior’s carbon cycle down into to sources (income) and sinks (expenses), they often expresses the figures in teragrams (a trillion grams, or 1012) per year. To convert these enormous sums into figures that might be more relevant, pretend a teragram of organic carbon is $100. If this were the case, then Cotner and his collaborators published estimates suggesting that Lake Superior takes in $600 to $1,000 and spends between $1,300 and $4,000.

Even when Lake Superior’s “carbon auditors” don’t precisely agree on values (see table), they agree on three points:

  1. The largest and most obvious source of organic carbon is photosynthesis by algae.
  2. The largest and most obvious exit route for organic carbon is cellular “digestion” or respiration. This is the process where cells break down sugar or other organic compounds to release energy used for cellular work.
  3. Given the gap between sources and sinks (and more confidence in the values concerning the sinks), there must be sources of organic carbon that are not so obvious.
Derived from Cotner et al.* Derived from Urban et al.**
Sources of organic carbon
Photosynthesis by aquatic plants$500-800$200-670
Atmospheric deposition$10-40$2-10
Stuff from the watershed that washes in through runoff and streams$50-60$40-90
Shoreline erosion$2
Total$600-920$240-770

Sinks for organic carbon
Respiration$1300-3900$1300-8310
Burial$48-150$60-200
Outflow$10
Total$1348-4050$1320-8310

Imagining that a teragram of organic carbon is $100, Lake Superior is significantly overspending. The sources and sinks for organic carbon are listed and valued as currency rather than teragrams.

Confounding Factors

It is important to remember that Lake Superior is not a typical system. For one, it is oligotrophic relative to other lakes. Low temperatures and low light suppress productivity, and so possibly does a lack of debris and nutrients running off the land. For another, the lake’s watershed-to-surface area ratio (1.5) falls closer to an ocean (0.4) than to most lakes, which often have watersheds four to 100 times larger than their surface area.

Another confounder is that researchers are more certain about the figures cited for cellular respiration, or the carbon that leaves the lake, than they are about
calculations of primary production, or the carbon that enters the lake from rivers and runoff. Cotner suspects some of the unaccounted-for molecules of
carbon may be leaking through the thin membranes of algae. Possibly 13 percent of a phytoplankton’s manufactured organic carbon seeps out of the cells in average conditions; presumably more in a lake as cold and unproductive as Lake Superior, since the algae tend to have a smaller surface-to-volume ratio.

Also, time and space could confuse the organic carbon data collected over the last 30 years by dozens of researchers. The concentration of organic carbon could build throughout the lake in winter and spring, then bacterial respiration could draw it down in summer and fall. On a broader scale, organic carbon could have built in the cooler first half of the twentieth century and been consumed at faster rates in the second half.

Unexplored but intriguing ideas abound in this mystery. Perhaps pine pollen,
airborne and phosphorus-rich, could supply a significant pulse of organic
carbon. In a study conducted in smaller boreal lakes, pine pollen, which disintegrates in about 24 hours, was a major source of nutrients and dissolved organic carbon. Or perhaps tannic acid and other complex organic carbon molecules suspended in streams flow in under-tallied.

As humans brace for the consequences of escalated levels of carbon in the atmosphere on a global scale, Lake Superior’s organic carbon cycle also faces change. Lengthening summers, warmer winters, extra dust, additional nutrients, more sediment from larger rainstorms, and the other ramifications of human and global changes threaten to alter a fundamental cycle we have barely begun to understand.

For more information, read Organic Carbon Biogeochemistry of Lake Superior,
JR 517 offered on the journal reprints page.

*Cotner, J.B., B.A. Biddanda, W. Makino, and E. Stets (2004), Organic carbon biogeochemistry of Lake Superior, Aquatic Ecosystem Health & Management 7(4):451-464.

**Urban, N.R., M.T. Auer, S.A. Green, X. Lu, D.S. Apul, K.D. Powell, and L. Bub (2005), Carbon cycling in Lake Superior, Journal of Geophysical Research 110, C06S90.


By Sharon Moen
October 2006

Return to October 2006 Seiche



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