Scientists studying the global carbon cycle have primarily focused on quantifying storage and fluxes of the ocean and the terrestrial landscape, often to the exclusion of wetlands, lakes, rivers, and reservoirs. One of the first hints that aquatic systems may play a larger than expected role in regional and global-scale carbon dynamics was the observation of CO2 supersaturation in a world-wide survey of lakes, indicating that these environments were acting as points of CO2 emission to the atmosphere. But while this study demonstrated a role for lakes in the movement (flux) of C from one location to another, less well understood was the role of aquatic environments in storing carbon. Thus, NTL researchers undertook a project to estimate both terrestrial and aquatic carbon storage and fluxes to determine the role of lakes, streams, and wetlands in the C budget of Wisconsin’s ~6400 km2 Northern Highland Lake District.
Researchers were able to take advantage of over 20 years of detailed measurements of lake water column CO2 concentrations and a survey of over 100 randomly selected regional lakes to estimate lake CO2 exchange with the atmosphere. Consistent with the prior world-wide survey, this effort confirmed a consistent condition of lake supersaturation across the region and over time, with small lakes in particular serving as hot-spots of CO2 emissions to the atmosphere.
To estimate the amount of C stored in aquatic ecosystems, researchers analyzed sediment cores from NTL study lakes and developed new methods to estimate carbon stocks in peatlands using readily-available remote-sensing data. These efforts revealed substantial regional carbon storage in wetlands and lakes. In fact, although aquatic environments make up only about one-third of this northern landscape, they store over 80% of the organic carbon and far outweigh C storage in trees and forest soils. Lakes thus paradoxically function as both C sources (in the form of CO2 fluxes to the atmosphere) and C sinks (storing organic C in their sediments), and together with wetlands make up a massive long-term carbon pool. The sensitivity of fluxes into and out of these reservoirs is the subject of ongoing research at the NTL-LTER site.
Surface water carbon exchange estimates were integrated with a regional estimate of land-atmosphere CO2 exchange for forests and wetlands in partnership with researchers in the Ameriflux consortium. The collaborative effort led to a complete, spatially explicit landscape-scale regional C budget, one of the first ever published. The results highlight the dramatically different behavior of lakes as net exporters of CO2 to the atmosphere, vs. the forests, which are currently growing and thus function as a sink for atmospheric CO2. As a result, the larger landscape can be viewed as a heterogeneous mosaic of patches of varying C density and C flux intensity. Collectively, this regional budget emphasizes the major role played by aquatic ecosystems in carbon exchange and storage. Because surface waters almost invariably serve to remove/receive C from their terrestrial watersheds, any C budget that does not account for that loss will overestimate terrestrial C accumulation.