UK DOC long-term trends inan international perspective Don Monteith CLAD 21st October 2009
Long term trendsČerné Lake – Bohemian Forest Secchi disc depth (m) • Vrba et al. 2003. STOTEN, 310, 73–85.
Long term trendsAlsterålven - SwedenFosberg, C. 1992. Hydrobiologia 229: 51-58, 1992.
Large trends in the chemical climateover the UK over last 2 decades (AWMN data)Non-marine SO42- concentration Cl- concentration Plots by Gavin Simpson UCL.
DOC trends over 2 decades at AWMN sites Plots by Gavin Simpson UCL.
Monteith, Stoddard et al. 2007, Nature 450, 537-541
UK outliersonly region where model unable to explain the regional trend
Explanations for UK’s deviant tendency• Sulphate deposition trend underestimated, due to early 70 retention & later desorption of seasalt derived SO42-? 65 60• Two sources of Cl in some regions 55 (seasalt and industrial) resulting in dual effects? 50 -10 -5 0 5 10 15 20 25 30 35 Yellow circles proportional to Nature model outlier size• Non-linear responses to declining deposition – most outliers are for sites showing largest %ΔDOC trends?
On site-by-site basis SO42- and Cl- variables are capable of completely explaining long term DOC trends in AWMN lakes 14 RLGH NAGAR CHON 12 TINKER GRANN SCOAT 10 BURMOOR LLAGI BLUE Raw data 8 DOC (mg/l) 6 4 2 0 1988 1988 1989 1990 1992 1992 1993 1994 1996 1996 1997 1998 2000 2000 2001 2002 2004 2004 2005 2006 2008 8 RLGH_SRES NAG_SRES CHON_SRES TINK_SRES1 6 GRANN_RESI1 SCOAT_SRES1 BURN_SRES1deposition 4 LLAGI_SRES1 BLUE_SRES1 standardised residuals (from dep model) model 2 standardised 0 residuals -2 -4 -6 [DOC] = α-(log [SO4]) + β-(log [Cl]) + c
Upland Water Myths• Evans et al.. (2005) have shown that decreases in SO4 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
• 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?•