There are two major pathways by which organic matter can enter a wetland. It can originate from within the wetland basin, in which case it is referred to as autochthonous; or it can originate from outside of the wetland basin and carried into it by water and wind, in which case it is termed allochthonous (eg. Hutchinson 1957; Wetzel 1983). The relative proportions of each in any given wetland depends on a variety of factors including landscape features (eg. vegetation, the presence of flowing surface waters, topography), catchment hydrology and nutrient status. The distinction between autochthonous and allochthonous organic matter is made due to the different chemical composition, and hence different detrital degradability, of terrestrial and aquatic vegetation. The classification into allochthonous or autochthonous, however, is not always clear-cut. For example, in Australia many wetlands support trees (eg. Melaleuca spp.) that clearly grow within the wetland basin and hence would be considered autochthonous. However, since the detritus they produce more closely resembles that of terrestrial vegetation (eg. more lignified and containing aromatic oils), it is more appropriate to treat this type of detritus as allochthonous. The quality, or type of organic matter present (ie. allochthonous or authochthonous) represents the major driving influence on wetland metabolism.
Aquatic plants and algae, and also some microorganisms such as cyanobacteria (blue-green algae) use solar energy to "fix" carbon from atmospheric CO2, or from CO2 dissolved in the water, through the process of photosynthesis. Such organisms are referred to as autotrophs, or primary producers. A number of algae and aquatic vascular plants are also able to utilise bicarbonate (HCO3-) in the synthesis of organic matter when CO2 concentrations are very low (Wetzel 1983). Photosynthesis takes place during sunlight hours, when CO2 is consumed and O2 released, while in the absence of light O2 is taken up, and CO2 is released. This latter process is called respiration. Plant and algal respiration, along with respiration of other organisms such as invertebrates and microorganisms exert a biological oxygen demand (BOD) on the system. In productive lakes this demand can exceed the availability of oxygen, particularly in the hypolimnion (the water mass at the bottom of the lake) and sediments, leading to oxygen depletion and anaerobic conditions.
Not all algae are obligate autotrophs. Many are heterotrophic (consumer organisms) in that they are able to take up dissolved organic compounds in the absence of light. However, Wetzel (1983) suggests that carbon assimilation via this process is insignificant compared to the amount assimilated in photoautotrophy. Furthermore, these algae are usually out-competed by bacteria feeding on the same substrates. Not all autotrophs are photoautotrophs (photosynthesizers). Chemoautotrophs are organisms (anaerobic bacteria) that obtain the energy to fix carbon from simple inorganic reactions. However, as the CO2 assimilated by these bacteria was produced by other bacteria during the process of decomposition, Wetzel (1983) considers chemoautotrophs to be secondary, rather than primary, producers. In the least chemoautotrophy can be regarded as a secondary process since energy is derived from re-mineralization of organic substances (NH3, NO2, H2S, H2, CH4) (Rheinheimer 1992).
Allochthonous organic matter can enter the system as large particles (eg. leaves from trees falling into the wetland, or wind-borne fragments of grasses or herbaceous vegetation), as fine particles that have been partially processed, either mechanically or biologically, or as dissolved material (Saunders 1980). The main mechanisms by which allochthonous organic matter can enter a wetland are surface run-off (including stormwater drainage), groundwater, shore erosion, litter fall, municipal and industrial wastes, and atmospheric precipitation (Saunders 1980). In many wetlands, notably in northern Europe and North America peat bogs either marginal to the lakes or within the drainage basin, constitute the most important external source of organic matter (Hutchinson 1957). Such wetlands are typically darkly stained as the bulk of this type of allochthonous material is in dissolved form. On the Swan Coastal Plain, many of the wetlands are groundwater fed and it is suspected that dissolved organic matter (DOM) from wooded areas within the catchment reach the wetlands via groundwater (REF). Hagedorn et al. (2000) found that in a forested catchment in Switzerland, aquic (wet) mineral soils represented a major source of DOM for groundwater. They believe the reason for this is that these soils are not able to retain DOM under reducing conditions due to the reductive dissolution of iron and manganese oxides.
Organic matter from external sources is much more resistant to degradation than autochthonous matter due to the higher lignin content of terrestrial plants, but also because much of the more easily degradable (labile) compounds have already been consumed ("biologically stripped") before reaching the wetland. Hence, there is often a much greater proportion of allochthonous organic matter in the wetland basin than there is autochthonous (Wetzel 1983).
