In addition to concerns about the acidification threats posed by atmospheric deposition of nitrogen (N), work funded by Defra and NERC has demonstrated that nitrogen also poses an ecological threat to upland lakes where algal productivity had previously been assumed to be limited largely by the availability of phosphorus (P). These studies suggest that there are currently approximately equal proportions of lakes where algal productivity is limited by P or N or both nutrients (i.e. co-limited lakes). In N and co-limited systems, N deposition will have significant effects on nutrient balances and cycles, with associated ecological change. This is a key consideration when assessing ecological response to atmospherically deposited N. To address the increasing concerns over N as a nutrient source, measurements of TN and TP were introduced in 1995 and the Freshwater Umbrella research programme began a study of nutrient N in UK acid waters in 2004. Results from this research confirm the importance of N and the need to further modify the UK UWMN to assess its likely effects (e.g. either by bioassay techniques and/or the introduction of routine monitoring of chlorophyll a concentration).
Palaeoecological analysis of sediment records from many lakes in the boreal and sub-arctic zones reveal significant ecological changes over the past 200 years, following several thousand years of relative stability (e.g Wolfe et al., 2001; 2006; Pla et al, 2009). Stable isotope analysis of N has shown widespread incidence of declining δ15N throughout the industrial period that is consistent with increasing inputs of isotopically light N deposition from anthropogenic sources. These patterns indicate that N deposition affects sediment organic matter δ15N, and suggest that deposited N is biologically utilised in lakes and that sediment δ15N may provide a historical surrogate for total N deposition. Application of this isotopic approach to UK sites, including all UK UWMN lakes, has found many similar patterns of recently declining sediment δ15N (e.g. at Lochnagar, Round Loch of Glenhead, Scoat Tarn), while the newest UK UWMN lake site Loch Coire Fionnaraich has provided additional palaeolimnological evidence of ecological changes not linked to acidification and most probably due to N deposition effects (Pla et al., 2009; Simpson & Anderson, 2009).
A more direct approach for measuring the effects of N deposition developed from work first undertaken by the NERC Global Nitrogen Enrichment Programme on phytoplankton bioassays in lakes (Maberly et al., 2002). This study was carried out at 30 upland lakes including many UK UWMN sites, selected for their pre-existing monitoring data, and demonstrated that N limitation of phytoplankton production is almost as common as P limitation, while co-limitation by both N and P is the most common condition. While this study was one of the first to challenge the prevailing view that oligotrophic lakes tend to be P limited, the importance of N limitation in these lakes is now widely accepted (e.g. Bergstrom et al., 2005; Bergstrom & Jannson, 2006; Elser et al., 2007). While the current Freshwater Umbrella Programme has already found evidence of N limitation in many lakes (including more UK UWMN sites) and has tested new methods for bioassaying stream sites as well, these techniques are not yet incorporated into the suite of standard monitoring procedures for UK UWMN sites. While bioassay methods could feasibly be routinely adopted across the UK UWMN, more work is required to determine the ecological impacts of shifting nutrient balances and increased phytoplankton production. The UK UWMN lakes in particular provide ideal sites for developing this work given the pre-existing lake sediment core and sediment trap monitoring data.
The key questions associated with the impacts of nutrient N are;
- What is the spatial extent of eutrophication in UK upland lakes and streams which may be attributed to or exacerbated by N deposition?
- Does N deposition influence upland surface water quality, ecology and biodiversity through its influence on terrestrial primary productivity and generation of organic carbon?
- How successful have N emission reduction policies been to date in reducing or preventing a eutrophication of sensitive freshwaters in the UK?
- Which forms of N deposition lead to NO3- leaching into surface waters?
- Since only a small proportion of deposited N currently leaks into surface waters, which factors control leaching and how do they change through time, i.e. will the major increases in NO3- leaching predicted by mass-balance models occur over policy-relevant timescales (years to decades)?
- How will future reductions in NOx or ammonia emissions affect NO3- leaching and hence the eutrophication status of surface waters, and over what timescales?
- Will eutrophication associated with elevated NO3- in upland surface waters recovering from acidification prevent the re-establishment of communities and ecological function characteristic of pre-industrial (reference) times?