Wetlands habitats

The Ecosystem Overviews provide a summary introduction to the main habitat types covered in APIS and the main air pollutant pressures to those habitats in the UK. In specific locations, e.g.close to a major source, other pollutants may be a concern for a habitat and the user should use the searches by location or by habitat/pollutant in these cases.


Wetland habitats divide into fens, swamps/reedbeds and grazing marsh (raised and blanket bogs are covered in theBogs overview). Due to the wide variation in these habitats (e.g. acid-alkaline, oligotrophic to eutrophic, topogenous or soligenous), they experience rather different sensitivities to nitrogen air pollutants, although similar responses to ozone are expected.

BAP habitats: Lowland Fens (priority); Reedbeds (priority); Upland flushes, fens and swamps (priority).

Main Pollutant Impacts

Nitrogen deposition

Fen ecosystems - can be 'ombrotrophic' (rain-fed) and are therefore especially sensitive to nitrogen deposition, as they derive all their nutrients from the atmosphere (see Bogs for more information).

Other types of minerotrophic fen are characterised by nutrient-poor, but base-rich conditions and have species-rich communities including small sedges and brown mosses. Excess nitrogen leads to preferential growth of grass and eventually tree species at the expense of the forming species. Coupled to these changes, the presence of less dominant associated bryophytes (Paulissen et al, 2004) and lichens may change accordingly. Other non-atmospheric sources are also important for wetland ecosystems including discharges to water from diffuse sources for example, nitrate leaching from agricultural land. In addition, urban and transport activities are further sources of pollution which can affect the a water quality of many catchments.

By contrast, grazing marshes may be less sensitive to atmospheric deposition, although there is much less information regarding the impacts on this habitat type. The most concern in such systems is often the species composition adjacent to ditches, and this may by differently sensitive to the main sward.


The impact of ozone on habitats is generally mediated through a primary impact on plants, either directly in the case of mosses and lichens or indirectly for higher plants, following stomatal uptake. As a result of this, plants with high stomatal conductances will receive higher ozone doses than those with low conductances. Ozone episodes often occur in periods with dry conditions, when plants will tend to close their stomata. Wetland habitats in the UK are less likely to experience water shortage; as a result, such habitats may be particularly prone to ozone impacts. Currently there is little available data on ozone impacts on wetlands. However, it is expected that responses will be similar to grassland ecosystems.

Further information and resources:

JNCC habitat pages


Paulissen, M.P.C.P.; Ven, P.J.M.; Dees, A.J.; Bobbink, R. 2004 Differential effects of nitrate and ammonium on three fen bryophyte species in relation to pollutant nitrogen input. New Phytologist 164 451-458