wise use of mires and peatlands - Peatland Ecology Research Group

MIRES AND PEATLANDS29Figure 2/3: Hydraulic characteristics related to depth in different types of mire withsubstantial water flow. ———— = lowest water level (lasting for short durations only).sloping mires (sloopy fens, Hangmoore), andmost spring mires, which are fed by rainwater,surface run-off, and groundwaterrespectively.A third type of water flow mire is the acrotelmmire 38 , which accumulates organic materialthat combines a large storage coefficient(many large pores) with a limited ability todecay. The latter characteristic leads to a slowreduction in the pore size when subject toaerobic decay. As the deeper, older materialhas been prone to oxidation for longer, adistinct gradient in hydraulic conductivitydevelops in the upper part of the peat (Figure2/3c). In times of water shortage, the waterlevel drops into a less permeable range andrun off is retarded. Evapotranspiration stillleads to water losses, but because of the largestorage coefficient of the peat resulting fromits relatively large pores, the water level dropsonly to a small extent. In this way, the deeperpeat layers are continuously waterlogged,even under fluctuating water supply. Globallythe raised bog is the only acrotelm mire typeso far identified. In the raised bogs of thenorthern hemisphere, a handful of Sphagnumspecies 39 combine a limited ability to decay 40with favourable external (nutrient-poor andacidic) conditions. The world-widedistribution of raised bogs illustrates theeffectiveness of this peat formation strategy.The peat formation characteristics mentionedabove can be combined with a classificationbased on the origin of water (see Table 2/2).The catchment area to a large extentdetermines the quantitative and qualitativecharacteristics of the input water. Underequal climatic, geological andgeomorphologic conditions the amount,duration and frequency of water supplyincrease from(a) ombrogenous – stemming solely fromprecipitation water; to(b) soligenous – stemming fromprecipitation water and surficial runoff;to(c) lithogenous – also stemming from deepgroundwater.Nutrient and base richness usually increasesimilarly. A tight correlation between quantityand quality of water and its origin is, however,not possible over larger areas. A continuouswater supply not only occurs under springfedconditions: it can also be found in areaswith very frequent rainfall. Ombrogenouswater may show large differences in chemicalcomposition 41 . When the substrate is inert,lithogenous water can to a large extent have