Several studies have highlighted an increase in DOC concentration in streams and lakes of UK upland catchments though the causal mechanisms controlling the increase have yet to be fully explained. This study, compiles a comprehensive data set of DOC concentration records for UK catchments to evaluate trends and test whether observed increases are ubiquitous over time and space. The study analysed monthly DOC time series from 198 sites, including 29 lakes, 8 water supply reservoirs and 161 rivers. The records vary in length from 8 to 42 years going back as far as 1961. Of the 198 sites, 153 (77%) show an upward trend in DOC concentration significant at the 95% level, the remaining 45 (23%) show no significant trend and no sites show a significant decrease in DOC concentration. The average annual increase in DOC concentration was 0.17 mg C/l/year. The dataset shows: (i) a spatial consistent upward trend in the DOC concentration independent of regional effects of rainfall, acid and nitrogen deposition, and local effects of land-use change; (ii) a temporally consistent increase in DOC concentration for period back as far as the 1960s; (iii) the increase in DOC concentration means an estimated DOC flux from the UK as 0.86 Mt C for the year 2002 and is increasing at 0.02 Mt C/year. Possible reasons for the increasing DOC concentration are discussed.

Use of critical metal loads in assessing the risk of metal inputsConcern about the input of metals to terrestrial ecosystems is related to: (i) the ecotoxicological impact on soil organisms and plants (e.g. Bringmark et al. 1998;Palmborg et al. 1998) and also on aquatic organisms due to runoff to surface water and (ii) the uptake via food chains into animal tissues and products, which may result in health effects on animals and humans (e.g. Clark 1989). Effects on soil organisms, including micro-organisms/macrofungi and soil fauna, such as nematodes and earthworms, are reduced species diversity, abundance and biomass and changes in microbe mediated processes (e.g. Bengtsson and Tranvik 1989;Giller et al. 1998;Vig et al. 2003). Effects on vascular plants are reduced development and growth of roots and shoots (toxicity symptoms), elevated concentrations of starch and total sugar, decreased nutrient contents in foliar tissues (physiological symptoms) and decreased enzymatic activity (biochemical symptoms) (e.g. Prasad 1995;Das et al. 1997). A review of these phytotoxic effects is given by Balsberg-Påhlsson (1989). Effects on aquatic organisms, including algae, crustacea and fish, include effects on gill function (e.g. Sola et al. 1995), nervous systems (e.g. Baatrup 1991), and growth and reproduction rates (e.g. Mance 1987). Environmental quality standards or critical limits (often also denoted as predicted No effect Concentrations or PNECs) for metals in soils and surface waters related to those effects serve as a guide in the environmental risk assessment process for those substances. 3Next to effects on soil organisms, metals may be transferred in food chains to cause effects on animals and humans (secondary poisoning). This may affect (i) humans by reducing food quality of crops and animal products and (ii) animal health through the accumulation in organs of cattle, birds and mammals (secondary poisoning). Heavy metal accumulation in food chains is specifically considered important with respect to cadmium (Cd) and mercury (Hg), and to a lesser extent for lead (Pb), for all of which no biological functions are known (e.g. Clark 1989). The only exception known is that Cd appears to be essential under Zn-limiting conditions for one marine diatom (Lee et al. 1995).One risk assessment approach, used successfully in international negotiations on the reduction of atmospheric deposition of pollutants, is to determine the maximum load of constant atmospheric pollution that causes no or tolerable damage ("long-term acceptable load" or "critical load"). A major advantage of this method is that it can be used to optimise the protection of the environment for a given international investment in pollution control by minimising the difference between present loads and critical loads on a regional scale. A major difficulty is the quantification of the relationship between atmospheric emission, deposition and environmental effects.The method to calculate critical loads of metals is based on the balance of all relevant metal flu...

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Article (refereed) -postprintTipping, E.; Chamberlain, P.M.; Froberg, M.; Hanson, P.J.; Jardine, P.M..

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