RE-EXAMINING THE EFFECTS OF EPISODIC ACIDITY ON MACROINVERTEBRATES IN SMALL CONIFER-FORESTED STREAMS IN IRELAND AND EMPIRICAL EVIDENCE FOR BIOLOGICAL RECOVERY Hugh B. Feeley and Mary Kelly-Quinn ABSTRACT Research throughout Europe has indicated the continuing reductions in acidic aerial deposition and associated improvements in freshwater pH. However, information on macroinvertebrate responses to these on-going changes is rare. During 2009, 2010 and 2011, 34 peat-dominated catchments in three geologically distinct acid-sensitive regions of Ireland were sampled for macroinvertebrates (6 occasions) and chemistry (3 seasons) to assess the effects of reduced sulphate and nitrate deposition in an ecoregion with historically low acidic deposition. Streams drained moorland catchments without forest, catchments with 20%50% (low) and catchments with forest cover and 50% (high) forest cover. Results indicate ecological improvements have occurred in many forested catchments when compared with findings in the early 1990s, especially on metamorphic and igneous geologies. However, episodically low pH ( B5.7) and high inorganic aluminium concentrations ( 150mgl 1 ), associated with storm-flow events, resulted in periods of ecological degradation. Nevertheless, the reductions in ecological health appeared seasonal, associated with antecedent hydrochemistry and climate. Interestingly, acid-sensitive Baetidae were present in the majority of forested streams in Ireland, owing to the recent improvements in water quality. However, caution must be advised as there are still many hydrochemical and ecological uncertainties in peat-soil forested catchments, owing to the high concentrations of dissolved organic carbon, climate change and changes to catchment management. INTRODUCTION Throughout Europe acidification is a common feature of many base-poor upland headwater streams and rivers. The acidification risk to water- ways has been largely influenced by the intercep- tion of atmospheric pollutants and acid deposition, and the capacity of catchments to buffer acid inputs. The quantity of atmospheric pollutants deposited at a given site is strongly influenced by the nature of the vegetation layer. Mature conifer forest canopies intercept aerosols and dust particles more efficiently (between 50% and 150% more) from the atmo- sphere than peatland vegetation or immature trees (Hornung et al. 1987; Fowler et al. 1989). This increased capture, the so-called scavenging effect, arises due to the turbulent air mixing above and within the forest canopy and is a function of the stand structure (Fowler et al. 1989). The atmo- spheric aerosols trapped in the forest canopy are subsequently flushed through to the forest floor and surface waters during rainfall events affecting asso- ciated waterways (Gee and Stoner 1988). In the early 1980s and 1990s, forest-mediated acidification was associated with scavenging of atmospheric sulphate and nitrates, resulting in reductions in the pH of streams, rivers and lakes (Allott et al. 1990; 1997; Kelly-Quinn et al. 1996; 1997). The inter- ception of atmospheric sulphate and nitrogen compounds directly influenced the magnitude of acidification in surface waters (e.g. Kelly-Quinn et al. 1996; Evans et al. 2008a; Ormerod et al. 1991). In Ireland, the processes driving acidification in lotic habitats have been predominantly episodic, associated with forested catchments, but also base- poor peat-soil moorland (non-forested) headwater catchments to a lesser extent (Kelly-Quinn et al. 1996; Feeley et al. 2013). Episodic acidification is reflected in changes in stream hydrochemistry with both an increase in stream water acidity and inorganic aluminium concentrations, and a decrease in calcium concen- trations, with increasing discharge (Howells 1990; Neal et al. 1992). Climatic conditions, such as the amount of rainfall, prevailing wind direction and air mass circulation patterns, may influence the Received 12 June 2014. Accepted 20 August 2014. Hugh B. Feeley (corresponding author; email: hughfeeley@gmail. com), Freshwater Biodiversity, Ecology and Fisheries Research Group, UCD School of Biology and Environmental Science, University College Dublin, Belfield, Dublin 4, Ireland, and Cardiff School of Biosciences, Sir Martin Evans Building, Cardiff University, Cardiff, CF10 3AX, United Kingdom; Mary Kelly- Quinn, Freshwater Biodiversity, Ecology and Fisheries Research Group, UCD School of Biology and Environmental Science, University College Dublin, Belfield, Dublin 4, Ireland. Cite as follows: Feeley, H.B. and Kelly-Quinn, M. 2014 Re-examining the effects of episodic acidity on macroinvertebrates in small conifer-forested streams in Ireland and empirical evidence for biological recovery. Biology and Environment: Proceedings of the Royal Irish Academy 2014. DOI: 10.3318/ BIOE.2014.187 DOI: 10.3318/BIOE.2014.18 BIOLOGY AND ENVIRONMENT: PROCEEDINGS OF THE ROYAL IRISH ACADEMY, VOL. 114B, NO. 3, 205218 (2014). # ROYAL IRISH ACADEMY 205