Acidic episodes retard the biological recovery of upland British streams from chronic acidification

Kowalik, Renata A.; Cooper, D.M.; Evans, C.D.; Ormerod, S.J.. 2007 Acidic episodes retard the biological recovery of upland British streams from chronic acidification. Global Change Biology, 13. 2439-2452. 10.1111/j.1365-2486.2007.0

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We tested two predictions required to support the hypothesis that anthropogenic acidic episodes might explain the poor biological response of upland British streams otherwise recovering from acidification: (i) that invertebrate assemblages should differ between episodic and well-buffered streams and (ii) these effects should differentiate between sites with episodes caused by anthropogenic acidification as opposed to base-cation dilution or sea-salt deposition. Chronic and episodically acidic streams were widespread, and episodes reflected acid titration more than dilution. Nonmarine sulphate (16–18% vs. 5–9%), and nitrate (4–6% vs. 1–2%) contributed more to anion loading during episodes in Wales than Scotland, and Welsh streams also had a larger proportion of total stream sulphate from nonmarine sources (64–66% vs. 35–46%). Sea-salts were rarely a major cause of episodic ANC or pH reduction during the events sampled. By contrast, streams with episodes driven by strong anthropogenic acids had lower pH (5.0±0.6) and more dissolved aluminium (288±271 μg L1) during events than where episodes were caused by dilution (pH 5.4±0.6; 116±110 μg Al L1) or where streams remained circumneutral (pH 6.7±1.0; 50±45 μg Al L1). Both biological predictions were supported: invertebrate assemblages differed among sites with different episode chemistry while several acid-sensitive species were absent only where episodes reflected anthropogenic acidification. We conclude that strong acid anions – dominantly nonmarine sulphate – still cause significant episodic acidification in acid-sensitive areas of Britain and may be a sufficient explanation for slow biological recovery in many locations.

Item Type: Publication - Article
Digital Object Identifier (DOI): 10.1111/j.1365-2486.2007.0
Programmes: CEH Programmes pre-2009 publications > Water
CEH Sections: Boorman (to September 2014)
ISSN: 1354-1013
Additional Information. Not used in RCUK Gateway to Research.: The definitive version is available at
Additional Keywords: acid deposition, biodiversity, episodic acidification, invertebrates, reversibility
NORA Subject Terms: Ecology and Environment
Date made live: 19 Oct 2007 09:22

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