What is an upland lake?
Upland lakes in the UK are defined as those situated beyond the limit of enclosed agricultural land and occupying predominantly moorland or heathland landscapes. Some have afforested catchments. They are mainly ultra-oligotrophic headwaters of low alkalinity situated on hard, slow-weathering bedrock. The most sensitive ones, those with the lowest natural alkalinity, have been acidified as a result of the effects of acid rain over the last two centuries.
Why are upland lakes important?
Upland lakes and streams provide a range of services:
- They provide drinking water supply and purification, power generation and flood amelioration;
- They contribute significantly to the biological and genetic diversity of UK’s native flora and fauna. Many upland waters are located in areas with national or international conservation designations, such as Sites of Special Scientific Interest, National Nature Reserves, RAMSAR sites, International Biosphere Reserves, Special Areas of Conservation (EU Habitats Directive) and Special Protection Areas (EU Birds Directive);
- A large number are Biodiversity Action Plan priority habitats (e.g. rivers and streams, oligotrophic and dystrophic lakes), host priority species (e.g. otter, freshwater pearl mussel, common scoter) and support a substantial proportion of important salmonid fish populations including sensitive spring salmon stocks;
- They are of cultural and commercial importance for their recreational services, especially tourism associated with angling and watersports; and
- They are central to the landscape quality of many National Parks and Areas of Outstanding Natural Beauty.
Many poorly buffered upland lakes have been acidified as a result of their exposure to
sulphur and nitrogen deposition since the beginning of the industrial revolution over 150 years ago. However, following measures to reduce the emission of sulphur dioxide and N oxides from fossil fuel combustion sources in the UK and more widely in Europe, acid deposition has decreased significantly since the 1990s.
Acidified upland lakes are beginning to recover chemically and, to a lesser extent, biologically. However, nitrogen deposition remains relatively high in some parts of the UK and where soils are saturated by nitrogen, for example in the Southern Pennines, nitrate leaching occurs. Non-marine sulphate concentrations also remain well above target reference levels in these regions as sulphate continues to be released by catchment soils.
Except for the north-west of Scotland, upland lakes in the UK have been and continue to be exposed to high levels of nitrogen deposition not only causing acidification but potentially causing eutrophication. The extent of eutrophication from nitrogen deposition in upland lakes is not yet fully known, but there is evidence from palaeolimnological studies that upland tarns in the Cumbrian Lake District have been enriched since the 1920s.
Toxic substance contamination
Upland lakes have been contaminated by toxic heavy metals (such as mercury and lead) from the combustion of fossil fuels since the beginning of the 19th century and by persistent organic compounds (including DDT, PCBs and PDFEs) from a wide range of industrial processes since the mid 20th century. The emission of most of these substances are now effectively controlled and have declined sharply over recent decades but their legacy remains in catchment soils being gradually released into surface waters where they accumulate in the food chain. There is concern that projected increases in the intensity of winter precipitation and a consequent increase in organic soil erosion may exacerbate this process.
Land use and land management change
Land-use pressures on upland lakes include the impact of large-scale afforestation with non-native coniferous tree species in the latter half of the 20th century, the subsidised land drainage of wet organic soils and changes in livestock grazing practices and stocking densities. Afforestation of catchments of surface waters sensitive to acidification has exacerbated problems of acidification through the scavenging effects of forest canopies in areas of high acid deposition, and land drainage has invariably caused accelerated soil erosion leading to a decrease in water transparency and an increase in sediment accumulation rates.
There is little evidence so far that changes to stocking densities or management of heather moorland for grouse shooting has any significant negative effect on water chemistry, but these may cause soil erosion, especially of organic soils, leading to increased turbidity and deterioration of habitat for trout spawning.
Tree planting is now controlled in catchments sensitive to acidification and land drains in upland blanket peats are being blocked off. However, new and second rotation forestry and the expansion of agricultural production further upslope with climate change have the potential to alter water quality in future and there is concern locally that the expansion of renewable hydro-power and wind farm activities might impact negatively on water quality.
So far there is little strong evidence that changes to upland lakes have occurred as a result of anthropogenic climate change. However, projected changes in air temperature, precipitation and wind strength are likely to alter the physical, chemical and biological characteristics of upland lakes. Effects include increasing water temperature, increased and more variable inflow stream-water discharge, changes to hydrochemistry, especially greater base-flow dilution, and changes in the life-cycles and geographical ranges of aquatic plants and animals.
Climate change will also have indirect effects on upland freshwaters through alterations to catchment biogeochemistry affecting carbon and nitrogen cycles in particular, increase in sea-salt rich storm events, especially in winter, that can create acid episodes and limit recovery from acidification, and increases in soil erosion that can accelerate the remobilisation of toxic substances from catchment soils.
There is a wide range of legislative instruments relevant to upland lakes with respect to air quality, water quality and biodiversity.
The UK is signatory to UNECE Protocols and EU Directives concerned with implementing policies to reduce long range transboundary air pollutants and their effects. These are:
- UNECE Oslo Protocol on Further Reduction of Sulphur Emissions (Second Sulphur Protocol) 1994;
- UNECE Gothenburg Protocol to Abate Acidification, Eutrophicationand Ground-level Ozone, 1999;
- EU National Emission Ceilings Directive (NECD) (1999);
All three require emissions of acid gases to be reduced to levels that do not exceed their respective critical loads to natural ecosystems including surface waters. There is no statutory duty for the UK to monitor the chemical and biological response of acidified upland waters to changes in acid deposition although the Acid (now Uplands) Waters Monitoring Network established in 1988 performs that function.
- EU Water Framework Directive
The UK has a duty to implement the requirements of the Water Framework Directive by restoring poor quality surface waters to good or high ecological status. However most upland lakes are too small (< 50 ha) to be included in the WFD as individual water bodies. The WFD hence is a poor instrument for managing these systems.
- EU Environmental Quality Standards (EQS) Directive (2008/105/EC)
This Directive, previously classed as a WFD Daughter Directive, now has independent status. It is concerned with toxic substances and thereby relevant to problems of trace metal and POP deposition in the Uplands.
- Forest and Water Guidelines, fourth edition, 2003 (fifth edition in preparation)
Proposals to afforest the catchments of upland lakes are required to follow these guidelines which are designed to protect upland lakes sensitive to acidification from the effects of tree planting. Permission to plant is not given when and where proposed planting leads to the exceedance of critical loads for acidity.
- EU Directive on the Conservation of Natural Habitats and of Wild Fauna and Flora: The Habitats Directive (1992)
The Habitats Directive is designed to establish special areas of conservation (SACs) and prevent the deterioration of natural habitats and the habitats of species. Most upland waters belong to the designated SAC habitat type: “Oligotrophic to mesotrophic standing waters with vegetation of the Littorelletea uniflorae and/or of the Isoeto-Nanojuncetea”
- UK Wildlife and Countryside Act (1981) and related legislation
The Wildlife and Countryside Act provides for the designation of Sites of Special Scientific Interest. Several upland lakes have been so designated either as individual water bodies or as part of larger sites.
The most serious problem for upland lakes has been and is acidification. However not all upland lakes are either acidified or sensitive to acidification. Acidification status for lakes can be assessed in several ways:
- Critical load exceedance. Put simply the sensitivity of a lake to acidification is proportional to the lake’s capacity to neutralise acidity. This acid neutralising capacity (ANC) can be calculated from measurements of the cation and anion concentrations in lake water, but the simplest is to measure the calcium concentration of the lake water. Research has shown that lakes in the UK with calcium levels greater than ca. 100 µeq/l have adequate capacity to neutralise acid deposition and have not therefore been acidified (ref). Whether a lake with calcium levels below 100 µeq/l has been acidified depends on whether the combined deposition of acidity from sulphur and nitrogen compounds has exceeded the lake’s neutralising capacity or critical load. This is the “critical load exceedance”, a value that can be calculated using information on the water chemistry of the lake together with data on acid deposition modelled for the site (ref Curtis). Exceedance values across the UK uplands are decreasing as S and N deposition declines as a result of air pollution control measures (ref ROTAP). The current state of play (for 2016) can be obtained from the UK National Focal Centre for Critical Loads Mapping and Modelling at CEH Bangor (web ref).
- Diatom acidification metric. Following the advent of the EU Water Framework Directive the Environment Agency has developed a benthic diatom-based tool to assess the acidity status of lakes. It requires a knowledge of the composition of diatoms from a benthic sample in the lake together with values for lake-water calcium concentration and total organic carbon. The tool then compares the observed flora with the flora expected from a non-acidified reference with similar Ca and TOC (cf. Juggins et al. STOTEN 2016).
- Palaeoecology. The most reliable method of assessment involves the reconstruction of the acidification history of the lake using diatom analysis of dated sediment cores (Refs). The technique enables the timing and extent of acidification as well as the pre-acidification characteristics of an acidified lake to be assessed (Battarbee et al. 2014).
Upland Lake Restoration
- Alkalinity. Upland lakes in the UK naturally have low alkalinity (< 200 ueq/l) with pH values below 7, but rarely below pH 5.5. Consequently the restoration of acidified lakes (with historic pH often as low as pH 4.5), should aim for a minimum pH of 5.5 – 6.0 but not necessarily much higher. The target for brown water lakes, i.e. those with high dissolved organic carbon concentrations, is somewhat lower.
- Colour. The natural colour of water in upland lakes varies from very clear to very brown depending mainly on the abundance of organic soils in the catchment. However, as the concentration of DOC is also dependent on soil acidity, colour is expected to increase as lakes recover from acidification (Monteith et al. 2007).
- Productivity. Upland lakes are oligotrophic to ultra-oligotrophic. They should have very low productivity with almost undetectable amounts of N and P. Reference values of nitrate from sites unpolluted by atmospheric N deposition are <5 ueq/l. (ref). TP values should be less than 5 ug/l.
- Biodiversity. Upland waters lack biological diversity but as a minimum lakes should support plant and animal populations characteristic for low alkalinity waters including isoetid aquatic plants, a wide range of macroinvertebrates and brown trout.
- Turbidity. Turbidity from inwashed particulate matter should be very low in upland lakes. Rates of sediment accumulation prior to the twentieth century were rarely greater than 0.5 mm/yr.
- Recovery from acidification. As acidification of upland lakes is almost entirely due to “acid rain” there is little that can be done locally to mitigate the problem. However, over the last ?? years emissions of sulphur dioxide has fallen by ..?% and nitrogen oxides (NOx) by ?%. Emissions are set to continue to fall (ref) and one of the benefits of planned reductions in fossil fuel use to mitigate the increase in greenhouse gas concentrations will be a reduction in the emission of other pollutants, especially acidic gases. By and large the reduction in emissions has been followed by a reduction in acid deposition across the UK and by a reduction in non marine sulphate concentrations in upland lakes. However, so far there has been little reduction in nitrate concentrations in lakes and catchment soils remain contaminated by S and N as a legacy of decades of acid deposition. etc
- Controlling eutrophication.The primary cause of eutrophication in upland waters is atmospheric nitrogen deposition from fossil fuel combustion and agriculture. As N deposition decreases nitrate concentrations in surface waters will also decrease although in areas of the UK where soils are saturated by nitrogen the response will be delayed. However, catchment tree planting may be effective in sequestering N and thereby reduce N losses from catchment soils to surface waters. (Forest guidelines??).
- Controlling erosion.Erosion problems in the uplands that affect surface waters are associated with past management practices to drain peatland soils either to improve livestock grazing, enhance habitat for grouse rearing or to enable afforestation (cf Battarbee et al. Earth Surface Processes paper). Afforestation practices are now more effectively controlled by the Forestry Commission (Guidelines) and there is a major programme under the auspices of the IUCN Peatland Programme to restore blanket bogs by grip blocking and other techniques. By and large upland catchments are over-grazed, especially by sheep and there is evidence that the replacement of sheep by cattle can reduce erosion pressures. Heather burning when badly managed can also lead to erosion, and experiments are being conducted in the Pennines to assess whether cutting rather than burning is an effective alternative.
- Toxic substances.The contamination of upland lakes by toxic substances is almost entirely the result of long distance transported air pollutants, the emissions of which are controlled through EU EQS Directive. However, upland soils are strongly contaminated and pollutants continue to enter surface waters through both erosion and leaching processes (refs). Controlling soil erosion (see above) is the most effective method of reducing toxic pollution and food chain uptake.
- Climate change.There is little prospect of controlling lake water column temperature or lake level as a result of climate change. However, increased winter rainfall and an increase in frequency and intensity of storms is likely to lead to increased catchment soil erosion. It will therefore become increasingly important to protect soils from erosion in the uplands not only to control problems of turbidity and accelerated sediment accumulation but also to minimise surface water contamination by toxic substances (see above).
Monitoring the response
Describe what to expect and on what time-scale and outline the kind of monitoring needed to assess success, including the importance of before and after monitoring and the use of control sites. Link to case studies and papers
Prevention and Protection
Identifying reference sites
Bennion H, Kelly MG, Juggins S, et al. (2014) Assessment of ecological status in UK
lakes using benthic diatoms. Freshwater Science 33: 639-654