Lake Superior's Chemical Imbalance? Element Cycles and Ultra-oligotrophy
Over the years, Minnesota Sea Grant and many researchers have said that Lake Superior is ultra-oligotrophic, that it's unproductive compared to most other lakes and that it is nutrient poor. It might be time to revise our thinking.
"Although the lake is incredibly beautiful and transparent, when we actually make the measurements, Lake Superior's productivity level is right within the oligotrophic range," said Robert Sterner, a Minnesota Sea Grant researcher and limnologist at the University of Minnesota. "I would argue that Lake Superior is not ultra-oligotrophic as defined by the technical literature."
Sea Grant's graduate fellow Jessica Van Der Werff recently earned a Masters Degree in Water Resources Science at the University of Minnesota Duluth. Working with the Department of Biology and Large Lakes Observatory's Stephanie Guildford and Bob Hecky, Van Der Werff studied phytoplankton in the deep chlorophyll layer of Lake Superior with the expectation that the primary producers could reveal clues to the nutrient dynamics of this cold and mysterious lake in which total phosphorus and chlorophyll concentrations have been declining as its nitrogen content has risen.
Van Der Werff's results indicate that the lake’s phytoplankton face extreme phosphorus deficiency if they live within about 30 meters of the surface in the epilimnion or metalimnion, especially in spring. "I didn't detect strong nutrient concentration gradients across the lake or with depth, but there appeared to be strong seasonality," said Van Der Werff. "As far as nutrient deficiency, the pattern I found with depth suggests that, in Lake Superior, nutrients are recycled near the surface as opposed to coming from a deepwater source."
After measuring primary productivity in the lake's offshore areas systematically for the first time, Sterner found the level of production to be low, but not unusually so. The vast inland sea remains unique, however. Sterner's findings, recently published in Inland Waters, suggest that Lake Superior is "extreme, unusual, or an end member among lakes" based on its high total nitrogen to total phosphorus ratio (up to 400:1) as well as its even more lopsided nitrate to phosphate ratio (about 13,500:1). Sterner recently completed a synthesis of 15 years' worth of research on the carbon, nitrogen and phosphorus cycles in Lake Superior. He wanted to assess how unusual Lake Superior‘s chemistry is compared to other large, nutrient-poor lakes around the world. These cycles are important in defining the basic functioning of the Lake Superior ecosystem, and help scientists and managers better understand what makes the waters so transparent and how the lake supports the growth of fish.
"It's true that the lake is unusual in terms of the imbalance between nitrate and phosphate in the lake," Sterner said. "But when it comes to the large quantity of nitrate, Lake Superior is one of quite a number of large oligotrophic lakes around the world that exhibit this phenomenon. It is unusual but not unique in this way."
Many environmental variables converge to form Lake Superior's chemical makeup. One variable is latitude. A more northern latitude means a shorter growing season and less carbon input into the lake's system from plants like algae. Another variable is geology. Granitic-rock covered by pine forests doesn't contribute much phosphorus to the lake. The third thing Sterner emphasized is the lake's homogeneity. "It's mostly deep, it's mostly cold and it's mostly offshore," Sterner said. "It lacks extensive nearshore mucky bays."
Sterner argues that this combination sets up a condition where the nitrogen cycle is unable to balance the phosphorous cycle. "Environments where nitrogen could be liberated to the atmosphere are rare to nonexistent in Lake Superior," Sterner said. "This is due in part to efficient recycling of organic carbon — because the lake is so deep, much of the carbon gets used by microorganisms before it can reach the bottom and serve as fuel for balancing the nitrate and phosphate cycles."
And whatever is going on has been happening steadily. To track the nitrate levels, Sterner compiled an unusually complete, century-long data record of nitrate concentrations by combing through U.S. and Canadian government publications. The database of 10,000 observations has been folded into a project called Eco Trends, (www.ecotrends.info) which is a data depository for long-term changes in ecosystems. The slow and steady rise of nitrate in Lake Superior may be its most unusual chemical feature. "I don't know of another example where a single parameter like nitrate has changed in a nearly linear way for 100 years," Sterner said.
Sterner credits his findings to support for Lake Superior monitoring. "There is no way I would have been able to lay down these relationships with a single project or short-term study," Sterner said. "The lake is just too big and access is too difficult. It shows the importance of sustaining research projects over time."
For details, read C:N:P Stoichiometry in Lake Superior: freshwater sea as end member, JR 598 offered on the journal reprints page.
By Marie Zhuikov