Peatlands and water

Hydrological processes strongly control the form and function of peatlands, since the flow of water, dissolved minerals and nutrients dictates the biodiversity and nature of the plant community (Bridgham and Richardson 1993; Zoltai and Vitt 1995) and production and decomposition dynamics (Moore et al. 2002) that result in the accumulation of peat. Vegetation and peat, in turn influence the hydrology through their effect on surface flow (Quinton and Hayashi 2005), groundwater (Siegel and Glaser 1987) and evapotranspiration (Lafleur et al. 2005). The functional links between plants, peat and water constitute a strongly coupled eco-hydrological feedback system.

There are many approaches to peatland classification (Moore 1984), reflecting in part the closely coupled plant–soil–water system. Because of the fundamental link between peat formation and hydrological conditions, it may be argued that hydrological status is a primary criterion in the classification of peatlands. The most basic distinction between two main mire types, bogs and fens, is based on water source. Fens receive minerogenous (rich in dissolved minerals) surface- and groundwater inputs in addition to precipitation. Bogs are peatlands that are entirely ombrogenous (fed only by precipitation) and often develop over fens (Kuhry et al. 1993), as the accumulating peat raises the elevation of the surface and the water table, isolating them from minerogenous water inputs. Further subdivisions of fen and bog are commonly identified through their hydrogeomorphic setting (Brinson 1993; Charman 2002), which is arguably a proxy for water source and water-table regime. In North America, minerogeneous systems are commonly further divided into swamps and fens (NWWG 1997). This distinction is drawn based on vegetation cover (swamps are forested while fens support graminoid and brown moss species), but, in fact, because of the strong plant–water coupling this reflects varying hydrological status with swamps characterised by a strongly variable water table (that allows tree growth), while fen systems have stable high water tables. In maritime climates, such as that of the British Isles and in Newfoundland, very high rainfall supports the development of ‘blanket’ bogs particularly in upland areas. Blanket bogs are laterally extensive, blanketing the pre-existing topography. Consequently, they typically include both true bog and areas where upslope drainage is a significant component of the water budget (although that drainage in intact mires may be nutrient-poor waters sourced from upslope bogs).