Mires persist independently of a parent
water body, being fed by groundwater,
overland runoff or precipitation. They, too, can be divided into two types,
minerotrophic (or rheotrophic) mires and ombrotrophic mires, often referred to as
fens and bogs, respectively.
Fens are valley mires, receiving water
and nutrients from groundwater and runoff
from the surrounding catchment. They are generally nutrient rich, unless fed by very
pure groundwater.
Bogs, in contrast, occur above the water
table and are fed solely by rainwater and
aerial deposition, so are nutrient-poor.
Both bogs and fens are normally permanently
waterlogged, ensuring reduced rates
of decomposition and allowing partially decayed plant matter to accumulate in the
form of peat; they are thus sometimes collectively known as peatlands. Bogs, which
have acidic surface water, generally support vegetation characterised by mosses
of the genus Sphagnum; if the pH rises above 4.5, fen vegetation, characterised by
sedges (Carex) and woody plants, will take over.
