Introduction - The Biogeochemistry of Submerged Soils - page 2

40 °

20 °

0 ° Equator

20

°

Major Wetland Area
Area with Abundant
Wetlands

40 °
160 °

140 ° 120 ° 100 ° 80 ° 60 ° 40 ° 20 ° 0 ° 20 ° 40 ° 60 ° 80 ° 100 ° 120 ° 140 ° 160 °

Figure 1.1 Global distribution of wetlands (Mitsch and Gosselink, 2000). Reproduced
by permission of Wiley, New York

Table 1.1 Global extent of wetlands of different types
Area (Mha)

Polar Boreal Temperate Tropical Total

Bogs 21 104 42 20 187

Fens 54 62 32 — 148

Swamps — 1 10 102 113

Marshes — — 17 10 27

Floodplains — — 8 74 82

Shallow lakes — — 1 11 12

Ricefields — — 29 80 109

Total 75 167 139 297 678

Definitions of wetland types:
Bogs are raised peat-producing wetlands formed in wet climates where organic material has accumulated over
long periods. Because they are raised, water and nutrients are entirely derived from the atmosphere, and they are
therefore nutrient deficient and acid. Sphagnum moss is the main vegetation, though other types of vegetation are
also present in tropical regions.
Fens are peat-producing wetlands that receive water and nutrients through inflow from neighbouring land. They
are generally less acid than bogs and may be alkaline, and tend to be dominated by grasses and sedges. Because
of their better nutrient status they are generally more prolific than bogs.
Swamps are forested, freshwater wetlands on submerged soils in which little peat accumulates. This is the US
definition; elsewhere the term also includes non-forested wetlands with reeds. Swamps tend to form in warmer
climates.
Marshes are herbaceous freshwater, non-peat-producing wetlands dominated by grasses, sedges or reeds. The
distinction between swamps and marshes may be blurred.
Floodplains are periodically inundated areas along rivers or lakes. Their vegetative cover is variable.
Shallow lakes are open water bodies a few metres deep. Only considered foe temperate and tropical regions; in
polar and boreal regions it is difficult to separate shallow lakes from bogs and fens.
Ricefields exclude upland rice. The physical area is calculated from the sum of irrigated rice (51 Mha of which
47% is double- or triple-cropped with rice and 33% under rice-wheat rotation), rainfed lowland rice (54 Mha)
and deepwater rice (4 Mha). The riceland of China is taken to be all temperate.
Sources : Aselmann and Crutzen (1989); Huke and Huke (1997); IRRI (2002).