Over the first 10 years of monitoring few parts of the UK experienced significant reductions in acid deposition. However, the acidity of UK UWMN lakes and streams was still found to vary significantly as a result of the sensitivity of sites to variations in climate at temporal scales ranging from episodic (reflecting short-term climatic extremes), to seasonal (in response to seasonal variations in rainfall, terrestrial primary productivity and generation of seasalt aerosol), to decadal (as a result of the dependence of UK weather on long-term hemispheric-level oscillations in climate).
Unusually acidic conditions were experienced during winter months over the first four years of monitoring. These were linked to periods of high rainfall (when the combination of acidic surface runoff and sub-surficial flow dominated over more alkaline groundwater) and periods of high sea-salt deposition (when marine cations temporarily displaced hydrogen and aluminium ions from soil exchange sites). Both factors are positively related to the state of the North Atlantic Oscillation (NAO), a measure of the dominant atmospheric pressure gradient over the North Atlantic Ocean (Hurrell 1995). The NAO was in an unusually persistent positive phase between 1989 and 1993, and its effects exacerbated water acidity in many sites, particularly those close to the west coast of the British mainland where seasalt deposition can be substantial and the relationship between the NAO and rainfall is particularly strong. Seasalt effects on acidity were most pronounced in the more anthropogenically acidified sites due to a greater displacement of acid cations (i.e. H+ or Al3+) by marine base cations in the more acidified systems.
Links with the NAO were also demonstrated for nitrate (NO3-) concentrations in UK UWMN sites. Nitrate concentrations show strong seasonality with the highest levels tending to occur in the early spring at the end of periods of soil freezing and before significant terrestrial biological uptake of N as temperatures begin to rise. However, Monteith et al (2000) found that the size of the spring peak NO3- concentration was inversely related to the winter (December to March) NAO Index, with the highest concentrations observed following the coldest and driest winters when easterly air flows were most dominant. They proposed that effects of soil freezing on plant roots may have increased the availability of mineralised N, while low temperatures would limit the amount of terrestrial uptake. At sites where NO3- represents a significant proportion of the total acid anion load, such as Lochnagar, the increase in NO3- during a period of negative NAO in the mid 1990s out-stripped the reduction in non-marine sulphate (xSO42-) resulting in temporary re-acidification.
In recent decades the NAO has shown approximately decadal scale variability, and Evans et al. (2001) argued, therefore, that a similar "naturally" induced periodicity should be expected in the acidity of UK UWMN lakes and streams undergoing longer term recovery from the effects of acid deposition. Over the longer term, future climate change can be expected to affect the fundamental catchment processes that determine susceptibility to acidification and NO3- leaching. Higher temperatures may increase alkalinity by increasing weathering, although this effect is expected to be small in poorly buffered catchments (Wright & Dillon, 2008). Warming could also accelerate biological activity in general, including plant productivity, soil respiration and N mineralisation (Rustad et al., 2001). However, any increase in the incidence of drought in summer (during which previously reduced sulphides in organic soils are re-oxidised and released as SO42-), or waterlogging during wetter winters, could offset the effects of increased temperatures. Given the uncertainty in General Circulation Model predictions with respect to the relative impacts on temperature and rainfall, future impacts of climate change on acidity levels are very difficult to predict.
All lakes and streams in the UK UWMN are now equipped to measure water temperature but further investment is urgently required to extend flow/water level monitoring to all sites.
Key questions associated with climate change, that should be addressed by future research, include:
- What effect will climate change have on atmospheric processing and deposition of pollutants?
- How will climate change influence the hydrological routing of pollutants within freshwater catchments?
- What are the implications of any climatically driven change in DOC concentrations (e.g. through effects of higher temperatures on NPP and organic matter decomposition) on the transport of atmospheric contaminants and the acidity of surface waters?
- Will climate change affect the long-term response of surface water to changing S and N deposition?
- What are the consequences of climate change for the mineralization of soil N and the export of acidifying NO3- to surface waters?
- What are the likely ecological effects of changes in the magnitude, frequency and seasonal timing of extreme flows and seasalt events?
- What are the physiological and ecological consequences of surface water warming and change in flows for the recovery from acidification of acid sensitive biota?
- How will climate change affect setting restoration targets for surface waters recovering from acidification?