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Freshwater ecosystems cover only ~1% of our planet, but play a critical role in its carbon cycle.
Freshwater covers only 1% of the Earth’s surface, and per unit area it produces similar amounts of biomass carbon as terrestrial ecosytems, making the contribution of inland water ecosystems (streams, rivers, lakes, reservoirs, wetlands and estuaries) to the global carbon cycle appear small. However, inland waters are carbon’s transit system from land to the oceans – and along the way, they disproportionately contribute to global carbon cycling through their high rates of carbon respiration and carbon sequestration. Globally, net carbon emissions by inland waters are comparable to net primary production in terrestrial and oceanic habitats, and the annual burial of carbon in freshwater sediments is comparable to sequestration on the vast ocean floor.
In order to place the role of Earth’s inland waters in the global carbon cycle, we have to look at the whole planet (Figure 1). The entire annual biospheric flux of carbon is just over 400 Pg (1 Pg=1015 g), with 212 Pg of carbon taken up on land and in oceans, and 206 Pg of carbon released. Anthropogenic emissions of carbon add around 10 Pg, of which about half (5 Pg) accumulates in the atmosphere each year. The remainder (5 Pg) is taken up by known (~80%) and unknown (~20%) terrestrial and oceanic sinks. Of the estimated 2.5 Pg of terrestrial organic carbon that enters freshwater systems per year, nearly half (~1.3 Pg) is respired by bacteria in inland freshwater and coastal water bodies. The bulk of the remaining carbon (~1.0 Pg) is exported to the oceans. This makes net carbon emissions from inland waters the same order of magnitude as the net uptake by each of the terrestrial and ocean components.
Figure 1. Schematic diagram showing the central role of inland waters in the global carbon cycle. All units are in Pg C y-1. Terrestrial ecosystems (LAND) take up more CO2 from the atmosphere than the other components of the biosphere, and they release 115 Pg C y-1 back to the atmosphere as CO2. Net Primary Production (NPP), the difference between primary production (uptake) and respiration (release) is about 5 Pg C y-1. This is the net terrestrial carbon sink and it helps to offset anthropogenic CO2 emissions (fossil fuels and biomass burning; brown and green arrows respectively). An amount of about half of the net terrestrial carbon sink is exported annually from land to inland waters where half again is respired and returned to the atmosphere as CO2. A smaller fraction is buried permanently in freshwater sediments, removing it from the carbon cycle. The remaining carbon is exported to oceans. The small differences between CO2 uptake and release on land and in the oceans makes the contribution of inland waters important in the global carbon budget. How the inland water carbon cycle responds to future climate change is an open question with considerable implications. This schematic diagram has been synthesized by the authors from original data reported in peer-reviewed literature listed under “Further Readings” at the end of this article (Cole et al. 2007, Battin et al. 2009, Tranvik et al. 2009 and Aufdenkampe et al. 2011).
Clearly, the freshwater carbon cycle is globally significant. Yet, freshwater carbon cycling is rarely incorporated into the global carbon budget. This exclusion means that we lack proper models to evaluate the responses of land-freshwater linkages and carbon cycling to global change, and the global carbon cycle is changing. Coupled carbon-cycle-climate models disagree on the trajectory of this change over the next 50 years, net terrestrial carbon uptake could double, or it could cease. This uncertainty is due to uncertainty in climate feedbacks on carbon storage. Among these uncertainties, the state of our understanding of carbon storage and transformation in inland waters is quite poor. Due to their now recognized enhanced role as a carbon processor (net carbon source as well as net carbon sink), what happens in the world’s rivers and streams, lakes, reservoirs, wetlands and flood plains will play a critical role in the biosphere’s feedback on future climate. This is especially of concern in the light of very recent observations that temperatures are rapidly rising in lakes all across the world in response to climate change. In recent decades, the surface waters of world’s lakes have been warming at an alarming rate of 0.34 °C per decade. In a warmer world with a growing human population, freshwater ecosystems in the 1% of Earth’s surface are coming under increasing stress as they heat up. Like a furnace, higher temperatures will intensify combustion reactions, increasing CO2 emissions. Internationally coordinated limnological studies should address these emerging issues.
Aufdenkampe, A. K. et al. Riverine coupling of biogeochemical cycles between land, oceans, and atmosphere. Frontiers in Ecology and the Environment 9, 53–60 (2011).
Battin, T. J. et al. The boundless carbon cycle. Nature Geoscience 2, 598–600 (2009).
Borges, A. V. et al. Globally significant greenhouse-gas emissions from African inland waters. Nature Geoscience 8, 637–642 (2015).
Butman, D. & Raymond, P. A. Significant efflux of carbon dioxide from streams and rivers in the United States. Nature Geoscience 4, 839–842 (2011).
Cole, J. J. et al. Plumbing the global carbon cycle: integrating inland waters into the terrestrial carbon budget. Ecosystems 10, 172–185 (2007).
Dean, W. E. & Gorham, E. Magnitude and significance of carbon burial in lakes, reservoirs, and peatlands. Geology 26, 535–538 (1998).
O’Reilly, C. M. et al. Rapid and highly variable warming of lake surface waters around the globe. Geophysical Research Letters (2015). http://onlinelibrary.wiley.com/doi/10.1002/2015GL066235/full
Tranvik, L. J. et al. Lakes and reservoirs as regulators of carbon cycling and climate. Limnology and Oceanography 54, 2298–2314 (2009).