Acid Deposition :: Dunes, Shingle & Machair

[For Acid Deposition processes see overview link]

Effects and implications

  • Vascular plants on calcareous dunes will be relatively tolerant of acid deposition, although acid dunes are more sensitive.
  • Lichens and mosses will be sensitive to direct effects associated with acid deposition.

Overview: evidence, processes and main impacts

Soil acidification as a result of acid deposition has relatively little impact in UK dunes because sand dune soils are generally well-buffered, with the exception of the few acidic dune systems (UKREATE, 2000). However, in the Netherlands where the sand usually has a lower initial carbonate content, both acidification and eutrophication have resulted in a decline of rare pioneer species in dune slacks sensitive to acidification (Sival & Strijkstra-Kalk, 1999).

Sand dune habitats are one of the most natural remaining vegetation types in the UK, supporting over 70 nationally rare or red-data book species. In sand dunes, decalcification (in response to rainfall) reduces pH and this has the strongest influence upon forb diversity for this habitat. The majority of dune systems in the UK are calcareous, well buffered and low in heavy metals so should be tolerant of acid deposition. However, they are generally infertile and thus sensitive to N deposition (N dep-sand dunes link).

Machair systems represent former beaches and sand plains standing above the current adjacent beach. Machair sands owe their fertility largely to their high seashell content, up to 90%, and to fertilisation with seaweed as part of their traditional cultural management. There are no known studies of acid deposition impacts on machair. In principle a critical load may be set by comparison with other similar species-rich calcareous grassland types. In practice,  acid deposition rates in the main areas of machair on the western Isles of Scotland are rather small due to the absence of orographic enhancement.

Pollutant deposition type and risk areas

Type of acid deposition

Pollutant

Risk areas

Dry deposition

Gaseous

SO2

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

Gaseous

NOx

None expected unless near busy roads or combustion sources

Wet deposition

precipitation and occult

(cloud, mist)

H+, NO3-, SO42-

Small risk as areas are unaffected by orographic enhancement.

 

Acid dunes may be at risk

Indicators of acid deposition

  • Change in species composition
  • Increase in calcifuges species.

Evidence of species specific responses

Species

Response

Reference

Schoenus nigricans
(on dune slack in the Netherlands)

-ve

Sival and Strijkstra-Kalk, 1999

Critical Load/Level: 

Critical Load/ Level

No estimate available

References: 

Sival, F.P. ; Strijkstra-Kalk, M. 1999 Atmospheric deposition of acidifying and eutrophicating substances in dune slacks. Water Air and Soil Pollution 116 461-477