Acid Deposition :: Fen, Marsh and Swamp

[For Acid Deposition processes see overview link]

Effects and implications

  • There is a paucity of data on acid deposition effects on this habitat type but it can be assumedthat where non vascular plants are present these might be sensitive, especially to N enrichment (see N deposition and Fen, Marsh and Swamp). Kooijman (1992) reported the loss of bryophyte species from rich fens in the Netherlands due to anthropogenic acidification and eutrophication.

Overview: evidence, processes and main impacts

Fens and marshes are characterised by a variety of vegetation types that represent their underlying geology and soil type. Fens represent more organic, peaty areas whereas marsh is found on mineral soils with a water table close to the surface, while in swamps the water table remains at or above the surface. Many are coastal or found in low lying areas, e.g. flood plains and lakesides. Some occur on calcareous soils while others are found on on acid, base poor soils, typically peats (fens) or organo-mineral soils and also impoverished poorly draining mineral soils (purple moor grass and rush pastures). This latter pasture type consists of a mosaic of plant communities, reflecting differences in water table. Acid and basic flushes (up wellings of ground water moving over or through the soil) can occur within these communities.

This wide ranging variation in vegetation types means responses to acid deposition have to be considered separately for the different ecosystem types and even within types. There are no specific studies of effects of acid deposition on these rather variable ecosystems . The process of acidification is largely dependent on the hydrology of fen ecosystems and the balance between rainfall and groundwater and/or surface water. Nutrient enrichment and polluted ground water represent the biggest threat, along with drainage / land use change and inappropriate or lack of management.

Upland variants are probably least likely to be affected by polluted water but these sites are likely to receive the largest atmospheric inputs and be the most sensitive to acid deposition. At high altitudes much of the wet deposition is concentrated through the seeder feeder scavenging effect and ion concentrations can be 10 times that in rainfall. Thus upland flushes, fens and swamps will be at risk from long-range transport of acid pollutants.

Pollutant type and risk

Type of acid deposition


Risk areas

Dry deposition



Significant reductions in sulphur emissions have successfully addressed by International control measures. Areas where exceedances could still occur are around industrial zones and port areas (due to shipping emissions).

 Dry deposition



None expected unless near busy roads or combustion sources

Wet deposition

precipitation and occult

(cloud, mist)

H+, NO3-, SO42-

High risk in the uplands from orographic enhancement


  • Increase in calcifuges species

Evidence of species specific responses




Scorpidium scorpioides


Kooijman 1992

Scorpidium cossonii


Kooijman 1992

Scorpidium revolvens


Kooijman 1992

Critical Load/Level: 
Critical Load/ Level

No estimate available

Kooijman, A.M. 1992 The decrease of rich fen Bryophytes in the Netherlands. Biological Conservation 59 139-143