Acid Deposition :: Coastal and Floodplain Grazing Marsh

[For Acid Deposition processes see overview ]

Effects and implications

  • Effects are likely to be small as these habitats are generally brackish and alkaline.

Overview: evidence, processes and main impacts

Coastal and floodplain grazing marsh form some of the last remaining unimproved grasslands. They lie inland of saltmarshes (which are inundated by tides on a regular basis). Grazing marshes are only periodically inundated by the sea and often have a network of dykes and shallow lagoons, and ditches to maintain the water levels.

Impacts on the ditches and pools have been studied - a study of 16 coastal grazing marshes in the Greater Thames estuary (Samuels and Mason 1997) showed that most sites were brackish and alkaline (pH 7.9–8.7) but, following re-wetting after drying out the pH in some shallow lagoons decreased sharply (minimum pH 2.7). Low pH was recorded at some sites which would appear to be the result of rainwater acidifying when it comes into contact with dry sediments, rich in sulphates. However, acidity in the rainfall was not measured.

These habitats are more at risk from eutrophication via agricultural run-off, resulting in a loss of aquatic vegetation. See APIS record on Nitrogen Deposition :: Coastal and Floodplain Grazing Marsh

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



Coastal grazing marshes close to large combustion sources and majorroad networks

Wet deposition

precipitation and occult

(cloud, mist)

H+, NO3-SO42-

Coastal grazing marshes are unlikely to be subject to high concentration episodes due to their lower rainfall than upland habitats.


None available

Evidence of species specific responses

None available


Samuels, A. J., and C. F. Mason. 1997. Ecology of eutrophic waterbodies in a coastal grazing marsh. Hydrobiology 346: 203–214.

Critical Load/Level: 

Critical Load/ Level

No estimate available