As in the oceans, carbon dioxide from the atmosphere could throw off water chemistry in large freshwater bodies like the Great Lakes, putting the food web at risk.
But the science remains unsettled and, according to researchers, must be bolstered if we are to understand what increasing atmospheric carbon dioxide means for freshwater.
While most research on carbon dioxide (CO2) absorption from the atmosphere has focused on oceans and the resulting acidification, it is widely believed that CO2 levels also will rise in large freshwater lakes.
Nascent research suggests this could be a problem for the foundation of Great Lakes' food webs.
But little is known about long-term trends of pH levels (reduced pH leads to acidification) in the Great Lakes—which touch eight states and Canada and comprise almost 20 percent of the world's fresh surface water.
"We need to learn from the marine community, which has been thinking about this for about 10 years," said Galen McKinley, an associate professor at the University of Wisconsin-Madison's Department of Atmospheric and Oceanic Sciences.
Oceans act like a sink for carbon. Once CO2 enters, pH levels decrease and water becomes more acidic. The acidification and higher CO2 levels in oceans are predicted to alter fish behavior and development, and dramatically impact aquatic life such as oysters, clams and corals.
It's not as straightforward in freshwater, where several variables, from ecological changes to nutrient levels and water depth confound the calculations, said Cory David Suski, an associate professor at the University of Illinois at Urbana-Champaign's Department of Natural Resources and Environmental Sciences.
Many smaller freshwater lakes are net emitters of CO2, as they process carbon leaching from nearby watersheds.
"Most [freshwater] lakes, even larger ones, are much more directly influenced by watersheds than the oceans are," said Harvey Bootsma, an associate professor at the University of Wisconsin-Milwaukee's School of Freshwater Sciences. "Oceans are mostly driven by internal processes, because they're so huge."
Also, invasive species such as zebra mussels have caused irreversible changes to the Great Lakes and how carbon is cycled, said Jon Allan, director of the Michigan Department of Environmental Quality's Office of the Great Lakes.
The filter feeding zebra mussels, discovered in the basin in 1988, decrease alkalinity levels and calcium concentrations. Alkalinity in water acts like a buffer against changes to pH levels.
The lakes already have lower alkalinity levels than the open oceans, especially Lake Superior, which has an average alkalinity about 36 percent of that found on ocean surfaces.
The Great Lakes do have some similarities with oceans because of their size, and scientists believe watersheds have less influence than on smaller lakes. McKinley, Bootsma and colleagues this summer released a report that argued similarities between the Great Lakes and oceans warrant acidification concerns.
Assuming CO2 emissions remain at the current rate, they estimated that pH in the Great Lakes would decline about .29-.49 units over the next century—roughly the same rate as ocean acidification projections.
And what would lower pH levels—higher acidity—mean for the Great Lakes? Research indicates problems would start from the bottom of the food chain.
In oceans, the risk from acidification is highest for shelled "calcifying" organisms, because the minerals they use for shell building are expected to decrease as acidification occurs.
The Great Lakes don't have many calcifying organisms. But Bootsma said two species to keep an eye on are zebra and quagga mussels in the Great Lakes—both invasive species that are radically influencing the food webs and nutrients across the basin—because acidification could slow their shell building.
"Some people might say 'but they're invasive, it might be nice if their populations were impacted, and maybe it would be," Bootsma said. "The point is we want to understand if that'll happen or not. They're huge players in regards to food webs, and nutrient cycling."
In addition, phytoplankton, microscopic plants that live near the surface of water, seem to have reduced nutritional quality when more CO2 is introduced, according to Canadian researchers. This, in turn, could reduce the growth of the next level of the food chain, called zooplankton, which are tiny aquatic creatures, including things like small crustaceans and fish larvae.
Both types of plankton are the basis for healthy communities of fish and other species higher up on the food chain.
And scientists this year reported that a predator fish up the food chain—pink salmon—had stunted growth when exposed to elevated levels of CO2.
Scientists all agree there is a clear need for more acidification research in the Great Lakes—something NOAA called for in a 2010 report but that just hasn't happened.
Adding to the urgency is the continued rising of CO2 in the atmosphere: NASA recently reported the global concentration recently reached 400 parts per million for the first time in recorded history.
McKinley said a good start in understanding what this means for the Great Lakes would be bolstered pH measurements. They exist now but aren't designed to track long-term changes, she said.
"States like Washington have a whole panel, with detailed plans on how to deal with acidification. On the West Coast there's lots of thinking about how acidification will affect coastal resources," McKinley said.
"We should really be making it more of a priority here."