Acid deposition :: Standing Open Water and Canals

Effects and implications

• Effects of acid deposition on standing waters are largely confined to oligotrophic lakes which tend to occur in catchments underlain by lithologies (rock types) such as granites, sandstones and schists which have low weathering rates. In these systems, base cation supply from weathering may be insufficient to balance deposited acidity. Acidification is therefore rarely likely to affect mesotrophic to eutrophic lakes or canals as base cation supply is sufficient to buffer effects of acidification.
• The acidification of oligotrophic lakes 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.
• Acidification leads to an overall reduction in species biodiversity as well as functional diversity (K. 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.
• Acid episodes, driven either by high rainfall events or seasalt episodes, tend to have a less deleterious effect on water acidity in standing waters compared to streams (Acid deposition :: Rivers and Streams) as a result of the buffering effect of the larger volume of standing water, although depressions in pH and increased concentrations of inorganic aluminium can still be seen in smaller water bodies, including several within the UK Upland Waters Monitoring Network (UWMN; formerly known as the UK Acid Waters Monitoring Network AWMN).

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.

Data from the UWMN demonstrate that sulphate (the primary acidifying anion) concentrations have fallen substantially in lakes across the UK over the past 2 decades (1988 – 2008), pH and Acid Neutralising Capacity (ANC; see below) have risen, and concentrations of inorganic aluminium have fallen sharply in the most acidic lakes (Battarbee et al., 2014; Monteith et al., 2014).

Levels of nitrate (a secondary acidifying anion) show relatively little change and appear to be controlled primarily by inter-annual variations in climate.

Assessments of AWMN data (e.g. Monteith et al., 2014, and supporting palaeoecological evidence (Kreiser et al., 1990)) suggest that AWMN surface waters with managed coniferous forest catchments experience higher acidic loads, through enhanced pollutant interception, and are more acidified than nearby sites without forests. It has been estimated that afforestation of moorland within Kielder Forest increased sulphur and nitrogen inputs by 30% and 90% respectively (Fowler et al., 1989). 

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), and 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. excess 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.

A comparison of diatom species collected annually in lake sediment traps with the historical diatom record preserved in lake sediment cores, demonstrates that diatom communities in most lakes on the UWMN remain very different from  the communities that characterised the sites before they began to acidify around 200 years ago (Battarbee et al., 2014).

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). Inorganic aluminium concentration has been found to be the most sensitive overall indicator of acid impacts on brown trout populations across the UWMN (Malcolm et al., 2014).

Pollutant deposition type and risk

Indicators of lake 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 loving Tabellaria species.
• Low diversity of higher plants, and particularly elodeid species that extract inorganic carbon and nutrients from the water column rather than from sediments
• 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 – greater slopes resulting in less opportunity for neutralisation.  

Evidence of recovery

Despite the chemical improvements there has been relatively little biological recovery to date [10], possibly reflecting 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).

Some recovering acidified lakes on the UWMN have shown more marked declines in some acid-loving diatom species (i.e. unicellular siliceous algae), the appearance of some aquatic macrophyte species that do not occur in very acid water, and gradual increases in certain macroinvertebrate species (including caddis, stonefly and mollusc taxa) that are indicative of less acidic water. 

Recovery from acidification has been accompanied by a marked increase in the concentration of dissolved organic carbon (DOC) in lakes, which is thought to be a consequence of rising solubility of catchment soil organic matter in response to reduced acid deposition (Monteith et al., 2007; Evans et al., 2006).]. This is likely to have substantially reduced the transparency of some lakes, resulting in reductions in photic depth and potentially a reduction in whole lake productivity [16], while also providing additional protection to aquatic organisms from UVB radiation.