Acid deposition :: Rivers and Streams

Effects and implications

• The acidification of rivers and streams by acid deposition has been shown to influence aquatic biota at all levels of the food chain, from primary producers, such as aquatic algae and macrophytes, to macroinvertebrates, fish and even water birds (Ormerod and Tyler 1989).
• Acidification leads to an overall reduction in species biodiversity as well as functional diversity (Layer et al., 2010; Mulholland et al., 1986; Jenkins et al., 2013) In some areas acidification has led to major declines in yields of salmonid fisheries including the Galloway region of southwest Scotland (Harriman et al., 1987).
• Aquatic animals (invertebrates and fish) are vulnerable to increased aluminium, hydrogen ion and heavy metal toxicity (Stockdale et al., 2014) and changes in food availability and quality.
• Growth of some plants may be affected by the reduced availability of dissolved inorganic carbon (DIC – required for photosynthesis), macronutrients such as phosphorus, and changes in inter-specific competition.
• Water pH  is often identified as the chemical variable which explains the greatest amount of variance in macroinvertebrate species data in upland regions (Larsen et al., 1996; Murphy et al., 2013). Several molluscs, amphipod and mayfly taxa are confined to the less acidic end of the spectrum, whereas more tolerant species, including several stoneflies and chironomids, are often present throughout much of the range. Few acid tolerant species are solely restricted to acidic sites.
• The acidity of acidified streams tends to increase at times of high rainfall as a result of a proportionally smaller contribution to runoff from relatively well buffered groundwater (i.e. water that has interacted with mineral bearing rock) and increased export of organic matter from soil horizons near the surface in the form of organic acids. Acidity is also accentuated at sites within a few tens of kilometres of the coast following the deposition of seasalt during winter storms. These “acid episodes” can be harmful to acid sensitive biota and more extreme events have resulted in mass mortality of salmonids (Kroglund et al., 2008; Teien et al., 2004).

Overview: evidence, processes and main impacts

Acid deposition on acid sensitive catchments (mostly overlying rock types with low weathering rates, such as granites, sandstones and schists) can result in chronic acidification of runoff into drainage waters, and particularly headwater streams. Deposited compounds of sulphur and nitrogen, in addition to hydrochloric acid (Evans et al, 2011), derived predominantly from the burning of fossil fuels, contributed to a progressive loss in buffering capacity of catchment soils and consequent reductions in pH and increases in inorganic aluminium in these waters from around the onset of the industrial revolution until the 1970s-80s. The main source of evidence for the timing and magnitude of acidification has come from the palaeoecological analysis of acid sensitive upland lakes as these systems leave an historical archive of environmental change in their sediments. These records tend to show reductions in water pH from the mid-1800s, and it is assumed that the acidity of running waters in the same regions will have followed similar trajectories.

Assessments of the UK Upland Acid Waters Monitoring Network (UWMN; formerly known as the UK Acid Waters Monitoring Network AWMN) data (e.g. Monteith et al., 2014), and supporting palaeoecological evidence (Kreiser et al., 1990), suggest that UWMN surface waters with managed coniferous forest catchments experience higher acidic loads, through enhanced pollutant interception, and are more acidified than nearby sites without forests. Fowler et al., 1989 estimated that afforestation of moorland within Kielder Forest had increased sulphur and nitrogen inputs by 30% and 90% respectively. 

The ability of surface waters to withstand acid deposition is determined by the calculation of Acid Neutralising Capacity (ANC). ANC represents the balance between base cations and strong acid anions, is relatively easy to model (in comparison with pH). It has been shown to be a robust predictor of damage to salmonid populations in Norwegian lakes (Lien et al., 1996). A threshold for healthy brown trout populations of ANC 20 µeq L^-1 (or ANC20) has since been adopted internationally as the standard for protection of acid sensitive waters. Relationships with ANC have also been shown for macroinvertebrates (Raddum et al., 1995), and diatoms (e.g. Sickman et al., 2013). A negative ANC (i.e. a excess of acid anions over base cations) implies elevated concentrations of acid cations, i.e. hydrogen and aluminium ions, and hence acidic water. For water with a positive ANC, the excess of base cations may be accounted for by organic anions (i.e. dissolved organic carbon - DOC), bicarbonate and (at higher values) carbonate.

Inorganic aluminium concentration (often referred to as labile aluminium to distinguish it from organically bound, and therefore less biologically available aluminium) is not routinely measured by most environmental laboratories. However, where data are available it is often found to be a strong predictor of the assemblage in spatial studies in addition to assessments of temporal variation in monitoring studies (e.g. Monteith et al., 2005). Malcolm et al. (2014)found inorganic aluminium concentration to be the most sensitive overall indicator of acid impacts on brown trout populations across the UWMN.

Salmonids are most sensitive to acid waters during hatching, fry and smelting stages. Low pH has been shown to impair the regulation of ions (and particularly sodium) across cell membranes, while elevated levels of inorganic aluminium impair gill function.

Pollutant deposition type and risk

Indicators of stream acidification

• Detectable levels of inorganic aluminium (concentrations greater than circa 10 µg L^-1 rarely encountered in non-acidified upland streams)
• Negative or low Acid Neutralising Capacity
• Low diatom diversity, dominated by acid tolerant Eunotia taxa.
• Low diversity of aquatic mosses – bryophyte assemblage dominated by liverworts
• Low macroinvertebrate diversity with mayfly and mollusc species largely absent
• Salmonids at low density or absent

Examples of species specific responses

What factors modify acid deposition impacts?

• Buffering capacity of catchment soils and waters
• Soil thickness and presence of wetlands
• Catchment slope – steeper? slopes resulting in less opportunity for neutralisation.  

Evidence of Recovery

Data from the UWMN allows the assessment of the efficacy of recent large reductions in acidic emissions on the chemistry and ecology of acid sensitive UK surface waters. Papers in a recently published special issue summarising this work (Battarbee et al., 2014; Monteith et al., 2014) demonstrate that sulphate (the primary acidifying anion) concentrations have fallen substantially in streams across the UK over the past 2 decades (1988 – 2008). pH and Acid Neutralising Capacity have risen, and concentrations of inorganic aluminium have fallen sharply in the most acidic streams. Levels of nitrate (a secondary acidifying anion) show relatively little change and appear to be controlled primarily by inter-annual variations in climate. Despite the chemical improvements there has been relatively little biological recovery to date (Murphy et al., 2014). This is possibly partly as a result of the continued occurrence of acid episodes, the continued depleted base status of catchment soils, biogeographic limitations on the dispersal of acid sensitive species that may have been lost during acidification, and internal ecological inertia to recolonisation resulting from acid tolerant generalist taxa having taken over functional niches previously occupied by acid sensitive taxa (Ledger et al., 2005; Monteith et al., 2005).