1. UK DOC long-term trends in an international perspective Don Monteith CLAD 21 st October 2009

2. Secchi disc depth (m) Long term trends Černé Lake – Bohemian Forest Vrba et al. 2003. STOTEN, 310, 73–85.

3. Long term trends Alsterålven - Sweden Fosberg, C. 1992. Hydrobiologia 229: 51-58, 1992.

4. Large trends in the chemical climate over the UK over last 2 decades (AWMN data) Non-marine SO 4 2- concentrationCl - concentration Plots by Gavin Simpson UCL.

5. DOC trends over 2 decades at AWMN sites Plots by Gavin Simpson UCL.

6. Monteith, Stoddard et al. 2007, Nature 450, 537-541

7. UK outliers only region where model unable to explain the regional trend

8. Explanations for UK’s deviant tendency Sulphate deposition trend underestimated, due to early retention & later desorption of seasalt derived SO 4 2- ? Two sources of Cl in some regions (seasalt and industrial) resulting in dual effects? Non-linear responses to declining deposition – most outliers are for sites showing largest %ΔDOC trends? Yellow circles proportional to Nature model outlier size

9. [DOC] = α-(log [SO 4 ]) + β-(log [Cl]) + c Raw data deposition model standardised residuals On site-by-site basis SO 4 2- and Cl - variables are capable of completely explaining long term DOC trends in AWMN lakes

10. Upland Water Myths Evans et al.. (2005) have shown that decreases in SO 4 deposition did not begin until the 1980s in the UK. –Evans et al. (2005) never said this, and S deposition has been falling since the mid 1970s Worrall et al. (2003) have shown that increases in DOC flux started in at least the 1960s. –Time series stretch back to 1960s but no indication of inflection until the mid 1970s Worrall and Burt (2007) present huge reductions in DOC at all sites within one operational region only - SW England (HMS data) - to illustrate diverging trends within UK. –Trends not supported by independent time series from same region. Data compromised by insufficiently documented changes in methodology.

11. UK DOC trends over last two decades are consistent with other regions of northern Europe and North America that have undergone large reductions in sulphate / and or chloride. Relationship suggests a control on DOC solubility by either/or acidity and ionic strength. Work to address this is ongoing. There are few, if any, examples of long-term DOC trends in the UK, where high quality sulphate and chloride concentration data are available, where these two variables cannot entirely explain the trend. Important to distinguish between this long-term driver and shorter term climatic and land-use drivers. The latter will set the boundaries for DOC variability as sulphur deposition fall background levels. Important to determine what is happening to DOC quality. Where land-use is largely unaltered, DOC is returning toward pre-acidification levels – but where soil base cation saturation is depleted, will it overshoot? Water treatment chlorination was only implemented after DOC had been suppressed by “acid rain”? Is it sustainable? What are the consequences for increasing solubility of DOC for the transport of industrial pollutant legacy compounds from catchments?