Terrestrial-aquatic linkages in Prairie Pothole lakes in alternative stable states

Background/Question/Methods

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.

Results/Conclusions

We have been examining the carbon inputs and outputs to shallow lakes in western Minnesota that bury and transform large quantities of the terrestrial organic carbon that comes into them. Importantly in these lakes, the variability of internal carbon processing is affected by biotic structure and human management of the lakes and surrounding landscapes. In lakes dominated by macrophytes, the organic matter degraded slower and facilitated anoxic and hypoxic conditions particularly under the ice, yet burial rates of organic carbon were similar to lakes where phytoplankton were the dominant plants. Burial rates were high and varied from 56-96 g/m2/yr on average. The biggest difference between the carbon budgets in the two lake types was in dissolved organic matter degradation (DOM) rates with DOM in macrophyte-dominated systems turning over 3 times slower than in phytoplankton-dominated systems. These results suggest that macrophyte-dominated lakes may be an important source of recalcitrant DOM into the regional groundwater.