SHALLOW LAKES AND THE GLOBAL CARBON CYCLE

Lakes represent less than 1% of the aquatic surface area of the Earth, yet are extremely important to carbon processing owing to their close proximity to terrestrial ecosystems and high rates of productivity. Global estimates indicate that nearly 3 Pg of terrestrial carbon are processed by freshwater aquatic systems annually, mostly in highly productive wetlands, oxbows, shallow lakes and rivers where about half of organic carbon coming into these freshwater systems is released into the atmosphere as CO₂, one-fifth is buried in the sediments and the rest is transported downstream, eventually to the oceans. But local estimates of these fluxes are quite variable owing to differences in hydrology, climate and productivity. We will discuss the role of these shallow lakes, focusing on Prairie Pothole lakes in central North America that bury large amounts of the terrestrial organic carbon that comes into them. Importantly in these lakes, the variability of internal carbon processing is strongly affected by biotic structure and human management of the lakes and surrounding landscapes. In lakes that were dominated by macrophytes, the organic matter degraded slower and facilitated anoxic and hypoxic conditions particularly under the ice. Our results suggest that managing shallow lakes for macrophyte, rather than phytoplankton biomass could increase the burial efficiency of organic carbon in freshwater ecosystems and facilitate CO₂ removal from the atmosphere.