Defra - Department for Environment, Food and Rural Affairs.

Science Search

Science and Research Projects

Return to Science Search homepage   Return to Project List

Freshwater Umbrella, 2007-2010 - AQ0803

Throughout the 1980s and 1990s the major problem affecting upland lakes and streams in the UK was widely acknowledged to be acidification caused by deposition of oxidized sulphur compounds from fossil fuel combustion. During the 1990s, DEFRA (then DoE and subsequently DETR) funded a major programme of research into the biological effects of acid deposition and the development and application of critical load models which linked sulphur deposition loads to effects. A critical load is defined as “a quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge.” Critical loads for sulphur were used in the negotiation of international UN-ECE Protocols which were effective in greatly reducing emissions of sulphur compounds across Europe and led to major chemical recovery from acidification during the 1990s.

The importance of deposited nitrogen (N) as pollutant in upland waters emerged subsequently over the last decade. Recent work under the DEFRA Freshwater Umbrella programme has demonstrated that nitrate in upland waters contributes directly to acidification and within a decade is likely to be the major remaining cause of acidification in these systems, preventing achievement of acidity critical loads (restoration of waters to be suitable for healthy brown trout) and policy objectives under national and international directives such as the EU National Emissions Ceiling Directive and the UN-ECE Gothenburg Protocol. Critical load models for N suggest that the problem could deteriorate with increased nitrate leaching even if total N deposition levels are held constant. Phase 1 of the Freshwater Umbrella used novel dual isotope techniques to prove that up to a third of leached nitrate derives directly from oxidized N (NOx) deposition and that reductions in NOx deposition will benefit upland waters directly through reduced spring peaks in nitrate leaching. The remaining proportion of leached nitrate may ultimately derive from NOx or reduced N forms (ammonia and ammonium).

Furthermore, Freshwater Umbrella studies of upland lake sediments and phytoplankton have found unexpected but widespread evidence for nutrient effects of N deposition. Evidence from N isotopes stored in the sediments of many upland lakes, including the nationally important conservation site and Water Framework Directive (WFD) lake Wast Water, implies that anthropogenic N deposition has increasingly impacted lake nutrient cycles over the last century or so, while phytoplankton nutrient studies have shown that N deposition must have affected lake productivity. This type of upland lake has a unique assemblage of algae, plants and animals that have evolved to cope with very low nutrient levels and is a designated habitat for conservation importance under the EU Habitats Directive. While the overall ecosystem effects of additional nutrient N inputs are unknown, studies in similar lakes outside the UK have shown that algae and plant species composition may change in response to the extra nutrients, with the loss of some of the original species. These findings have major implications for the definition and achievement of the 'good ecological status' required under the WFD.

With DEFRA’s remit to determine emissions policy for NOx and reduced N it is essential to understand the nature of nutrient N impacts on upland lakes and streams, the severity of impacts in terms of continuing acidification and eutrophication of these systems, and just how widely current N impacts are distributed. Furthermore, the relationship between N deposition and nitrate leaching is complex because a large fraction of deposited N is used and stored in catchment plants and soils but may be released much later (days to years). Hence even where immediate damage is not apparent, stored N may cause problems in the future. It is therefore essential to understand how nitrate leaching as a function of N deposition may change into the future.

The new work proposed here aims to assess the relative importance of NOx and reduced N deposition in i) contributing to current and future nitrate leaching and hence to acidification and eutrophication of upland aquatic ecosystems adapted to low N availability, and ii) affecting lake nutrient cycles and changing productivity in naturally nutrient-poor systems. The proposed methods include a combination of:
a) site-specific studies using new isotopic tracer techniques, new nutrient assessment methods and new studies linking lake sediment changes to N deposition inputs and nutrient cycles, with experimental manipulations to determine interactions between nitrate leaching and acidification, and
b) development of upscaling and catchment modelling methods for the prediction of eutrophication and acidification effects of N deposition at regional and national scales.

Results will contribute to critical loads and dynamic modelling work on acidification, and the development of new critical loads for nutrient N in fresh waters. Taken together, these approaches will greatly improve the understanding of both the processes controlling leaching of deposited N and its impacts on sensitive upland lakes and streams, and provide DEFRA with a stronger science base from which to underpin key air quality policy decisions on N emissions with respect to UN-ECE Protocols and EU Directives.

The overall objectives of the proposal are to employ a combination of intensive, site-specific studies with wider, upscaled measurements at the maximum feasible number of sites to provide:
1) the greatest possible scientific insights into processes controlling leaching of deposited N species and their ecological impacts, both for model development and linking N sources to effects, and 2) the most generally representative and hence upscalable, policy relevant datasets for assessing the spatial extent of the N related acidification and eutrophication problems in UK upland waters.

The extent of our ability to meet either of these general objectives, but in particular b) (spatial representation and upscaling), is linked largely to available resources and would be greatly enhanced by provision of level funding to include the optional Work Packages described at the end of this proposal. While financial constraints (a DEFRA imposed reduction from 2007 prices) required a prioritisation of work most likely to meet DEFRA objectives, the optional Work Packages all include work most efficiently pursued in parallel with the main programme. The work described in the main programme represents a minimum viable number of study sites or samples in many cases; the scientific quality of the results obtained and their statistical robustness would be substantially improved by the optional Work Packages. Furthermore, opportunities for directly linking the outputs of the three related DEFRA Umbrellas (Freshwater, Terrestrial and Critical Loads & Dynamic Modelling - CLDM) would be provided by funding option 3 (soil and vegetation surveys) which would generate key data enabling co-located modelling activities of all the Umbrellas at the same catchments. Given the fundamental links between terrestrial and aquatic biogeochemistry of N and the related impacts of N deposition, such datasets would add great value to the outputs from all three Umbrellas.


1.1 update the national freshwaters critical loads dataset to incorporate new data arising within this proposal and elsewhere, including critical load exceedances assessed for new deposition datasets / scenarios and application of any model modifications arising from new understanding of N dynamics and sensitivity analysis of model parameters;

1.2 calculate credible confidence intervals for critical loads based on sensitivity / uncertainty analysis;

1.3 classify lake types / analogues for upscaling and use in the CLDM contract “MAGIC library” approach (MAGIC: Model of Acidification of Groundwaters in Catchments);

2.1 determine how nutrient limitation of phytoplankton growth in upland lakes varies seasonally in lakes of differing nutrient status;

2.2 develop methods for nutrient bioassay to assess N effects in upland streams;

2.3 develop empirical catchment models to predict NO3- leaching and hence N limitation;

2.4 assess macrophyte δ15N as an indicator of N limitation status;

3.1 determine how N species (NO3- and NH4+) determine lake sediment δ15N and whether there is a seasonal variation in this process;

3.2 determine the relationship between the isotopic signature (δ15N) of deposited N species and sedimenting organic matter at the annual timescale;

3.3 determine whether other Acid Waters Monitoring Network (AWMN) lakes not yet analysed show similar patterns of declining lake sediment δ15N and hence possibly similar changes in lake nutrient status;

4.1 determine the relative contributions of hydrological (NOx derived) and microbial NO3- in the maximum number of sites possible using the dual isotope approach, to increase the range of characterised sites for modelling and upscaling;

4.2 measure the isotopic signature of microbially produced soilwater NO3- directly for comparison with theoretical assumptions used previously;

5.1 assess whether changing soil acidity in response to reduced sulphur deposition is affecting NO3- leaching processes.

Each of these aims is to be achieved through corresponding Tasks collected by theme into five Work Packages. The Work Packages, Tasks, linkages between Tasks and their relationship to the two primary objectives of the proposal are shown schematically in Figure 2.

The outline of the proposed work is presented below, with scientific approaches detailed by Task, followed by the work plan and GANNT.
Time-Scale and Cost
From: 2007

To: 2011

Cost: £675,931
Contractor / Funded Organisations
Air Pollution              
Environmental Protection              
Fields of Study
Air Quality