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Terrestrial Umbrella - eutrophication and acidification of terrestrial ecosystems in the UK - AQ0806

Description
In upland areas of the UK located away from direct human disturbance through agriculture, industrial activities and urban pollution, atmospheric pollution poses one of the major threats to the chemical and biological quality of lakes and streams. One of the most important groups of pollutants is nitrogen (N) compounds, including oxidised forms of N called NOx, generated mainly by fossil fuel combustion especially in motor vehicles, and reduced forms of N (ammonia gas or dissolved ammonium compounds) generated mainly from agricultural activities and livestock. These nitrogen compounds may dissolve in rain or soilwater to form acids, or may be taken up as nutrients by plants and soil microbes in upland catchments, and then subsequently released in acid form associated with nitrate leaching at a later date. It is well established that nitrate leaching contributes to acidification of upland waters, with damage to aquatic ecosystems including plants, invertebrates and fish. However it has recently been suggested that nitrate leaching may also be associated with nutrient enrichment of upland waters that contain biological communities adapted to very low nutrient levels.

Issues of acidification, nutrient enrichment and biological controls on nitrate leaching are therefore intrinsically linked and need to be understood in order to determine current and future impacts of N deposition on water quality and ecological status of upland waters. The Freshwater Umbrella programme was specifically designed to tackle these scientifically challenging problems, with key objectives and methods to:

1. assess the importance of inorganic forms of N (i.e. nitrate and ammonium) as nutrients in upland lakes through a combination of literature review, lake sediment studies and experimental work;
2. improve understanding of nitrate leaching pathways and controls using complementary isotopic tracer and dual isotope approaches. The first approach allows N compounds to be marked and then tracked through the environment, while the second uses naturally occurring isotopic differences between atmospherically deposited nitrate that is leached unchanged from catchments and nitrate produced from ammonium by the microbial process of nitrification in soils;
3. develop models for predicting which nutrient (N or phosphorus) is the major control of biological productivity in upland waters from catchment characteristics and other nationally available datasets, including studies of catchment soils and mosses as possible controlling factors;
4. assess the impacts of climate change on effects of atmospheric pollutants in upland catchments through literature review and experimental work; and
5. review and develop models linking deposition loads to ecological effects (with a quantified biologically relevant threshold or “critical load”) for acidity in UK freshwaters and review the case for application of nutrient N critical loads.

Results from this contract have provided substantial evidence for adverse effects of N deposition on sensitive water bodies of the UK. Previous work under DEFRA contracts has already demonstrated the major importance of sulphur deposition in causing acidification but with the implementation of emission reduction measures in the UK and Europe the role of sulphur is declining. All sites still impacted by acidification are at least partly affected by N and more than half would exceed critical loads on the basis of N deposition alone, even following implementation of the EU National Emissions Ceiling Directive in 2010. Reductions in emissions targets for total N would be required to prevent critical load exceedance in the majority of sensitive freshwaters.

The role of N deposition in causing changes to lake nutrient cycles and productivity has been demonstrated through a review of the recent literature and through direct measurement of phytoplankton (microscopic, free-floating aquatic algae) responses to nutrient additions in laboratory studies. Growth and productivity are limited almost as frequently by N availability as by P in upland lakes but joint- or co-limitation of growth by both N and P together is the most common status. Independent evidence for historical changes in lake nutrient inputs and productivity has been found through studies of N stable isotopes in lake sediments. This approach may allow the role of anthropogenic N sources in lakes to be evaluated. The potential impacts of N deposition in increasing lake productivity are highly relevant to N emissions policy with respect to several international directives:

1. nutrient-poor lakes in the UK uplands designated under the EU Habitats Directive may be particularly sensitive to the effects of N deposition, with possible changes to phytoplankton species and productivity and changes to macrophyte flora;
2. these changes may be considered a deviation from the good ecological status required under the EU Water Framework Directive; and
3. there is a strong case for calculating nutrient N critical loads to feed into the integrated assessment modelling work under the UN-ECE Convention on Long-Range Transboundary Air Pollution, including the Gothenburg Protocol.

Confirmation that N deposition may lead to both acidification and nutrient enrichment highlights the increasing need to understand the processes that determine whether N deposition causes enhanced nitrate leaching over the short- or longer term. Results from this project using novel techniques in the UK have for the first time allowed the major controls on nitrate leaching to be separated and quantified at specific sites. Hydrological tracer experiments have proven that rapid flowpaths exist that can transport deposition through soils into surface waters in minutes to hours, but a large proportion of nitrate is still retained in soils. The nitrate dual isotope approach has further demonstrated that only 20-30% of leached nitrate is rapidly transported from NOx deposition; the remainder is generated within soils by microbial processes. Hence a fraction of leached nitrate will respond rapidly to changes in NOx deposition. However, a larger proportion of leached nitrate is generated in soils from N pools that may consist of accumulated NOx and reduced N deposition. Therefore a large proportion of nitrate leaching may respond very slowly to changes in N deposition. Understanding the balance of these nitrate sources is essential for predicting timescales of response to changes in N deposition and for calculating target loads to achieve given water quality targets within a specified timescale. This programme has shown that nitrate leaching models will also need to incorporate catchment scale attributes such as the carbon:nitrogen ratio of soil organic matter and moss biomass which explain part of the variation in leaching relative to N deposition.
Objective
Issues of acidification, nutrient enrichment and biological controls on nitrate leaching are therefore intrinsically linked and need to be understood in order to determine current and future impacts of N deposition on water quality and ecological status of upland waters. The Freshwater Umbrella programme was specifically designed to tackle these scientifically challenging problems. The key objectives may be summarised as:

1. continue to review and develop critical load models for acidity (S+N) in freshwaters (WP1);
2. assess the importance of inorganic N as a nutrient in upland lakes through a combination of literature review, palaeolimnological investigation and experimental work (WP2);
3. develop models for predicting nutrient limitation status of upland waters from catchment characteristics and other nationally available datasets (WP2);
4. review the case for application of nutrient N critical loads in the UK (WP2);
5. improve understanding of nitrate leaching pathways and controls using complementary isotopic tracer and dual isotope approaches (WP3);
6. assess the importance of organic soil C:N ratios and bryophyte biomass as controls on nitrate leaching
(WP3); and
7. assess the impacts of climate change on effects of atmospheric pollutants in upland catchments through literature review and experimental work (WP4).
Project Documents
• Final Report : Final report of the 2004–2007 research programme   (261k)
• Final Report - Annex : Final report of the 2004–2007 research programme Annex   (7681k)
Time-Scale and Cost
From: 2004

To: 2007

Cost: £1,089,945
Contractor / Funded Organisations
Centre for Ecology and Hydrology
Keywords
Air Pollution              
Air Quality              
Environmental Protection              
Pollution              
Fields of Study
Air Quality