The scene may be familiar from natural history documentaries – a migrating herd of wildebeest attempting to cross a raging river, with many of the creatures drowning in the process – but what viewers do not generally notice is that large amounts of carbon are thereby transported from a grassland to an aquatic ecosystem. However, it did take the attention of Florian Altermatt and his team at Eawag and Zurich University: in a study supported by the Swiss National Science Foundation, these researchers conducted an extensive literature review aimed at quantifying the amounts of carbon moving between different ecosystems. First-author Isabelle Gounand (a postdoc in the Aquatic Ecology department) says: “Some spatial flows are larger than expected. In fact, for certain ecosystems, inputs of carbon from other ecosystems are similar to local fluxes.”
It is not only when wildebeests cross a river that large amounts of carbon flow from one ecosystem to another. But also when leaf litter falls in rivers, seaweed is washed ashore, or when thousands of mayfly larvae emerge from the water and take to the air over neighbouring terrestrial ecosystems. Altermatt’s group therefore reviewed scientific studies of cross-ecosystem carbon flows and, for comparison, also analysed data on local carbon fluxes within the recipient ecosystems
By combining the results in a global synthesis, the scientists were able to generate important new findings: in lakes, rivers and benthic marine ecosystems, the amount of carbon entering from neighbouring ecosystems can be equivalent to that which is locally fixed, respired, or released during decomposition. In forests or grasslands, the situation is different: here, because of the very high primary production occurring in terrestrial ecosystems, local carbon fluxes are two to three orders of magnitude greater than inputs from other ecosystems. Altermatt concludes: “When we study ecosystem processes or functioning, we need to pay more attention to carbon flows between connected ecosystems.” Strong spatial couplings mean that ecosystems are vulnerable to alterations in carbon flows. For example, if forest is converted to agricultural land, carbon inputs to surface waters are reduced, with changes to aquatic food webs and communities. Or, to return to the wildebeest: if their migration route is blocked by a new road, a source of carbon may thus be eliminated.
Isabelle Gounand, Chelsea J. Little, Eric Harvey, Florian Altermatt, Cross-ecosystem carbon flows connecting ecosystems worldwide, Nature Communications (open access): Doi: 10.1038/s41467-018-07238-2
Prof. Dr. Florian Altermatt
Institute for Evolutionary Biology and Environmental Studies
University of Zurich
Phone +41 44 635 47 00