Why carbon?

The presence of carbon, specifically carbon dioxide, is the prerequisite for life on Earth, including life in aquatic systems (Rheinheimer 1992). All living organisms require carbon as a food source. The significance of carbon cycling is that the processes involved form important links between the abiotic (non-living) and biotic (living) components of the particular ecosystem. This is in part due to the ability of carbon to form strong bonds to other non-metals such as hydrogen, nitrogen, oxygen, sulfur and the halogens (Zumdahl 1993). Under certain conditions metals such as iron and manganese, or nutrients such as phosphorus or nitrogen, can become adsorbed (or attached) to carbon molecules, temporarily rendering these unavailable to aquatic organisms. Many of these processes are mediated through the activities of micro-organisms which require carbon as a food source. For these reasons, fluxes or transformations of other aquatic components (e.g. nutrients) are almost always inevitably associated with fluxes and transformations of carbon. Therefore, the way carbon is stored and cycled through a system is a good indication of key wetland functions and processes.

Carbon exists in inorganic and organic forms. Organic forms typically consist of long chains or rings of carbon atoms which constitute a part of living matter (biota), or part of substances derived from living matter (e.g. decaying vegetation). Compounds such as the oxides of carbon (e.g. CO₂) and carbonates (CO₃) are considered to be inorganic substances (Zumdahl 1993). The cycling of carbon involves transitions of carbon between organic and inorganic states. These transitions are affected by, and themselves affect, a number of physico-chemical factors such as light, temperature, dissolved oxygen, pH, ionic composition of the water, as well as biological factors such as productivity.

In recent years much attention has been given to the impacts that human activities are having on carbon cycling. For instance, global climate change is predicted to alter the carbon balance of wetlands (e.g. Carroll and Crill 1997; Laine et al. 1996; Nykanen et al. 1998; Roden and Wetzel 1996; Siegel et al. 1995). Height of the water table is one of the governing influences on carbon balance as it determines whether aerobic or anaerobic processes dominate. Siegel et al. (1995) estimate that peatlands in the northern hemisphere alone now store 150-450 billion tonnes of carbon. Lowering of the water table would bring about the oxidation of the peats and result in the release of an appreciable amount of carbon dioxide (a greenhouse gas). However this would in part be offset by a reduction in methane production (another greenhouse gas) due to the shift from anaerobic to aerobic processes (e.g. Daulat and Clymo 1998; Laine et al. 1996; Nykkannen et al. 1998).

In addition to climatic changes, wetland- and groundwater levels have also been affected by changes in surrounding land-use and abstraction for agricultural and human use. For instance, wetlands on the Gnangara groundwater mound on the Swan Coastal Plain have undergone considerable water level fluctuations during the past 50 years (Froend et. al 1993). Hydrographs for the wetlands concerned clearly show the responses of the water table to changes in mean annual rainfall, to clearing of surrounding bushland (resulting in a rise of the water table), establishment of pine plantations and abstraction (resulting in a drop of the water table). A classic example of wetland response to such events in terms of carbon balance occurred at Lake Jandabup, where between 1997 and 1999 the water table dropped to unprecedented low levels, resulting in the acidification of the wetland (Sommer & Horwitz, in press).

Click on the links below to read about carbon cycling in wetlands.

Carbon pools, fluxes and processes  
Carbon sources
Size fractionation of organic matter
Composition of organic matter
Processes responsible for the fate of organic matter