Although lakes cover less than 1% of the Earth’s surface, they are an essential component in the planet’s carbon cycle. But there are striking regional differences in the degree of carbon cycling that occurs in lakes, and the underlying causes aren’t well known. To improve understanding of these differences, an international research team has proposed a geographic framework to connect carbon processing at the ecosystem level with regional drivers such as climate, land cover, and human activity. Based on this framework, they’ve described two mechanisms that explain geographic differences in carbon cycling, providing new insight into the role of inland waters in the broader global carbon cycle.
The first mechanism proposes that regional differences in lake carbon cycling are linked to whether water color exceeds a threshold level. Where these levels fall in relation to this threshold affects ecosystem patterns, such as lake metabolic status. In Canada, for example, water color has been shown to dictate whether a lake achieves sufficient rates of photosynthesis to balance the respiration of its organisms.
The second mechanism focuses on threshold relationships arising from regional variations in energy and mass transfer to lakes. These factors can alter the coupling between nutrients, light and primary production. One likely cause for this variation are shifts in plant growth and fertilizer use on land, such as in the strong latitudinal gradients observed for these factors in Sweden and Denmark. When these gradients intersect, cross-regional differences in energy and mass transfer emerge.
Interestingly, looking at natural and human gradients at the global scale causes clear regional boundaries to emerge. For example, distinct regions are formed when overlaying global data on human population density, plant growth, and fertilizer application. Plotting the locations of prior studies on lake carbon cycling reveals that many such areas have not yet been evaluated. Future work in these regions may reveal the existence of novel environmental patterns.
Overall, the work argues that regional differences in lake carbon cycling are not simply the result of location, but rather that specific conditions promote the expression of distinct ecological and biogeochemical pathways. By uncovering the impact of manmade and natural gradients on these pathways, it may be possible to better predict how lake carbon cycling responds to various aspects of global change.