Fen Management Handbook - Scottish Natural Heritage

Area of acute nutrient
enrichment on a northwest
Wales (Lleyn) fen
resulting from focussing
of farm-yard runoff
(P.Jones).
4.2.4 Sources within the fen – internal nutrient enrichment
Fens are generally considered to be relatively nutrient-poor habitats as the majority
of nutrients are retained within the developing peat layers. However, changes in the
fen environment can lead to release of nutrients from these stores, especially in the
later successional phases of fen development when the drier conditions and lower
soil water levels can result in the release of the nutrients which have accumulated
within the soil.
The release of additional nutrients via processes within a fen is often termed
‘internal nutrient enrichment’. Factors driving these changes might include increased
nitrogen and phosphorus input from atmospheric inputs, and falling or fluctuating
water levels due to drainage, abstraction or changes in precipitation.
4.3 Factors influencing the nutrient regime of fen habitats
The nutrient regime of a fen is affected by its relationship with the wider landscape.
Influential factors include:
– Geology – bedrock and drift type fundamentally affects substrate and water
nutrient content, and therefore affects the type of fen habitat that develops.
– Geomorphology – the topography will affect the rate and direction of water flow
and thus to an extent its chemistry on entering a fen.
– Catchment hydrology – this will dictate the source of the water, its chemistry
on entering the fen, along with the supply of nutrients via the volume and
timing of water inputs. This includes both atmospheric (rain, fog and other
precipitation) and terrestrial (ground, surface and sub-surface water) inputs.
65