5. Water Chemistry
The potential consequences of enhanced diffusion of CO2 into the waters of the Great Lakes has not been evaluated in any systematic fashion. Ocean acidification, or the drop in pH seen when CO2 reacts with naturally occurring carbonate and bicarbonate ions in water, has been reported to cause a 0.002 per year decrease in pH in the open ocean. However, the chemistry of the Great Lakes is much different from that of the open ocean, making it harder to assess and predict changes in water chemistry due to climate change. Further, the Great Lakes integrate more variables than the open ocean, such as nutrient loading from agriculture and urban runoff, adding spatial and temporal complexity to predicting changes in water chemistry.
The Great Lakes are on average slightly basic with pH values ranging from 8.0 to 8.35 from 1983- 2009.67 When compared to the ocean, the Great Lakes have a lower buffering capacity against pH changes because they have a lower alkalinity. Alkalinity measures the ability of water to neutralize acids such as carbonic acid, a byproduct of CO2 dissolution into aqueous solution. The more alkaline a solution is, the more acid it can absorb without dramatically altering the pH. The average alkalinity of the Great Lakes ranges from 36% in Lake Superior to 95% in Lake Michigan relative to surface ocean alkalinity.68 Because of the lower alkalinity, the Great Lakes are predicted to respond more to changes in atmospheric CO2 than will the ocean. By 2090, under the Intergovernmental Panel on Climate Change (IPCC) emissions scenario A2, the pH of the Great Lakes may decline by 0.30 pH units, which represents a pH decrease of 0.004 per year, a rate twice that of the ocean.69
However, the Great Lakes differ from the ocean in many important ways that can affect pH. First, the deposition of acidic nitrogen and sulfur from the atmosphere or through agricultural and urban runoff can increase acidification by 10-50% or more over acidification rates expected from fossil fuel emissions alone. Second, in the fall and spring, the Great Lakes are relatively well mixed, meaning that lower pH surface waters will mix with the deeper waters in the Great Lakes serving to acidify the waters at depth. However, under stratified conditions in the summer and winter, the deeper waters of the Great Lakes naturally store CO2 produced by respiration; the CO2 is “vented” to the atmosphere during the mixing events of spring and fall. Net CO2 uptake and acidification rates of the Great Lakes will be a function of physical processes (mixing, temperature) and biological rates (photosynthesis, respiration). Most of these processes have not been well quantified, making it difficult to accurately predict net effects.
Relative to other freshwater lakes and rivers, the Great Lakes have a higher buffering capacity because of the surrounding geological terrain. All of the Great Lakes except Lake Superior contain a lot of carbonate rock, such as limestone and dolomite, in their drainage basin. Runoff from drainage basins with a high proportion of carbonate rock has a higher alkalinity than runoff from non-carbonate rock, such as granite and sandstone.70
In sum, the changes in water chemistry of the Great Lakes will not follow trends in either the open ocean or in smaller lakes and streams. This heightens the need to study and improve our understanding of the Great Lakes’ biogeochemical cycles in order to predict how increased fossil fuel emissions may affect water chemistry.
- Reduced calcification rates: As pH drops, the saturation states for aragonite and calcite, two forms of calcium carbonate, will decrease. This decreased saturation state means that it could be more energetically costly for calcifying animals, such as the zebra mussel, to biosynthesize their shell. The responses of zebra mussels and other calcifying animals in the Great Lakes needs to be further studied to determine physiological tolerances of native and non-native calcifying species to changes in pH.
- Non-linear biological responses: In general, freshwater plants and animals are thought to be more tolerant to slight changes in pH relative to oceanic plants and animals because of their evolutionarily derived adaptations to the highly variable conditions that can be naturally experienced in freshwater ecosystems. However, in a whole-lake acidification experiment in Wisconsin, when pH was systematically lowered from 6.1 to 4.7, monitored animal populations revealed non-linear responses to linear changes. Some animal populations, such as Daphnia, exhibited immediate declines, while other fish populations, such as yellow perch, increased but later decreased in population size.71 As yet, it is unknown how animal and plant populations in the Great Lakes will respond to changes in pH but there are sure to be some unexpected surprises.